Complete example in Github

Required dependencies

implementation 'com.android.support:appcompat-v7:26.1.0'
implementation 'com.google.android.gms:play-services-vision:11.6.0'
implementation 'com.google.zxing:core:3.2.1'

Required permissions in the manifest file.

Activities in the manifest file.

This code processes an existing QR code image and decode it to a string. When the button btn_read_qr_code in the layout is clicked, it reads a qr code image from the drawable folder, decode it, gets the string value and assign the string value to a text view.

btn_read_qr_code.setOnClickListener(object : View.OnClickListener {
    override fun onClick(v: View) {
        // Load the Image
        val myBitmap = BitmapFactory.decodeResource(applicationContext.resources, R.drawable.puppy)
        iv_qr_code_image.setImageBitmap(myBitmap)

        // Setup the Barcode Detector
        val detector = BarcodeDetector.Builder(applicationContext)
                .setBarcodeFormats(Barcode.DATA_MATRIX or Barcode.QR_CODE)
                .build()
        if (!detector.isOperational) {
            tv_qr_code_text_content.setText("Could not set up the detector!")
            return
        }

        // Detect the Barcode
        val frame = Frame.Builder().setBitmap(myBitmap).build()
        val barcodes = detector.detect(frame)

        // Decode the Barcode
        val thisCode = barcodes.valueAt(0)
        tv_qr_code_text_content.text = thisCode.rawValue
    }
})

This code takes the text from an EditText, generates a QR code bitmap, then sets an ImageView with this QR code bitmap. The end result is a QR code image.

btn_generate_qr.setOnClickListener({
    val textValue = et_qr_code_text.getText().toString()
    try {
        val bitmap = textToQRBitmap(textValue)
        iv_qr_code_image.setImageBitmap(bitmap)
    } catch (e: WriterException) {
        e.printStackTrace()
    }
})

@Throws(WriterException::class)
fun textToQRBitmap(Value: String): Bitmap? {
    val bitMatrix: BitMatrix
    try {
        bitMatrix = MultiFormatWriter().encode(Value, BarcodeFormat.QR_CODE, 500, 500, null)
    } catch (Illegalargumentexception: IllegalArgumentException) {
        return null
    }

    val matrixWidth = bitMatrix.width
    val matrixHeight = bitMatrix.height
    val pixels = IntArray(matrixWidth * matrixHeight)

    for (y in 0 until matrixHeight) {
        val offset = y * matrixWidth
        for (x in 0 until matrixWidth) {
            pixels[offset + x] = if (bitMatrix.get(x, y)) Color.BLACK else Color.WHITE
        }
    }
    val bitmap = Bitmap.createBitmap(matrixWidth, matrixHeight, Bitmap.Config.RGB_565)
    bitmap.setPixels(pixels, 0, 500, 0, 0, matrixWidth, matrixHeight)
    return bitmap
}

Scan QR Code

In MainActivity.kt, put te button listener code in the onCreate function, declare a request code(BARCODE_READER_REQUEST_CODE) and process the QR code result in the onActivityResult function.

private val BARCODE_READER_REQUEST_CODE = 1


btn_scan_qrcode.setOnClickListener(View.OnClickListener {
    val intent = Intent(applicationContext, BarcodeCaptureActivity::class.java)
    startActivityForResult(intent, BARCODE_READER_REQUEST_CODE)
})

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    if (requestCode == BARCODE_READER_REQUEST_CODE) {
        if (resultCode == CommonStatusCodes.SUCCESS) {
            if (data != null) {
                val barcode : Barcode = data.getParcelableExtra(BarcodeCaptureActivity.BarcodeObject)
                val p = barcode.cornerPoints
                tv_qr_code_text_content.setText(barcode.displayValue)
            } else
                tv_qr_code_text_content.setText(R.string.no_barcode_captured)
        } else
            Log.e(LOG_TAG, String.format(getString(R.string.barcode_error_format), CommonStatusCodes.getStatusCodeString(resultCode)))
    } else
        super.onActivityResult(requestCode, resultCode, data)
}

activity_main.xml

barcode_capture.xml

BarcodeCaptureActivity.java

/*
 * Copyright (C) The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * This file and all BarcodeXXX and CameraXXX files in this project edited by
 * Daniell Algar (included due to copyright reason)
 */

import android.Manifest;
import android.annotation.SuppressLint;
import android.app.AlertDialog;
import android.app.Dialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.pm.PackageManager;
import android.hardware.Camera;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.util.DisplayMetrics;
import android.util.Log;
import android.widget.Toast;

import com.example.qr_code_read_write_scan.R;
import com.example.qr_code_read_write_scan.camera.CameraSource;
import com.example.qr_code_read_write_scan.camera.CameraSourcePreview;
import com.google.android.gms.common.ConnectionResult;
import com.google.android.gms.common.GoogleApiAvailability;
import com.google.android.gms.common.api.CommonStatusCodes;
import com.google.android.gms.vision.MultiProcessor;
import com.google.android.gms.vision.barcode.Barcode;
import com.google.android.gms.vision.barcode.BarcodeDetector;
import java.io.IOException;

public final class BarcodeCaptureActivity extends AppCompatActivity
        implements BarcodeTracker.BarcodeGraphicTrackerCallback {

    private static final String TAG = "Barcode-reader";

    // Intent request code to handle updating play services if needed.
    private static final int RC_HANDLE_GMS = 9001;

    // Permission request codes need to be < 256
    private static final int RC_HANDLE_CAMERA_PERM = 2;

    // Constants used to pass extra data in the intent
    public static final String BarcodeObject = "Barcode";

    private CameraSource mCameraSource;
    private CameraSourcePreview mPreview;

    /**
     * Initializes the UI and creates the detector pipeline.
     */
    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        setContentView(R.layout.barcode_capture);

        mPreview = (CameraSourcePreview) findViewById(R.id.preview);

        boolean autoFocus = true;
        boolean useFlash = false;

        // Check for the camera permission before accessing the camera.  If the
        // permission is not granted yet, request permission.
        int rc = ActivityCompat.checkSelfPermission(this, Manifest.permission.CAMERA);
        if (rc == PackageManager.PERMISSION_GRANTED) {
            createCameraSource(autoFocus, useFlash);
        } else {
            requestCameraPermission();
        }
    }

    @Override
    public void onDetectedQrCode(Barcode barcode) {
        if (barcode != null) {
            Intent intent = new Intent();
            intent.putExtra(BarcodeObject, barcode);
            setResult(CommonStatusCodes.SUCCESS, intent);
            finish();
        }
    }

    // Handles the requesting of the camera permission.
    private void requestCameraPermission() {
        Log.w(TAG, "Camera permission is not granted. Requesting permission");

        final String[] permissions = new String[]{Manifest.permission.CAMERA};

        if (!ActivityCompat.shouldShowRequestPermissionRationale(this,
                Manifest.permission.CAMERA)) {
            ActivityCompat.requestPermissions(this, permissions, RC_HANDLE_CAMERA_PERM);
        }
    }

    /**
     * Creates and starts the camera.
     *
     * Suppressing InlinedApi since there is a check that the minimum version is met before using
     * the constant.
     */
    @SuppressLint("InlinedApi")
    private void createCameraSource(boolean autoFocus, boolean useFlash) {
        Context context = getApplicationContext();

        // A barcode detector is created to track barcodes.  An associated multi-processor instance
        // is set to receive the barcode detection results, track the barcodes, and maintain
        // graphics for each barcode on screen.  The factory is used by the multi-processor to
        // create a separate tracker instance for each barcode.
        BarcodeDetector barcodeDetector = new BarcodeDetector.Builder(context)
                .setBarcodeFormats(Barcode.ALL_FORMATS)
                .build();
        BarcodeTrackerFactory barcodeFactory = new BarcodeTrackerFactory(this);
        barcodeDetector.setProcessor(new MultiProcessor.Builder<>(barcodeFactory).build());

        if (!barcodeDetector.isOperational()) {
            // Note: The first time that an app using the barcode or face API is installed on a
            // device, GMS will download a native libraries to the device in order to do detection.
            // Usually this completes before the app is run for the first time.  But if that
            // download has not yet completed, then the above call will not detect any barcodes
            // and/or faces.
            //
            // isOperational() can be used to check if the required native libraries are currently
            // available.  The detectors will automatically become operational once the library
            // downloads complete on device.
            Log.w(TAG, "Detector dependencies are not yet available.");

            // Check for low storage.  If there is low storage, the native library will not be
            // downloaded, so detection will not become operational.
            IntentFilter lowstorageFilter = new IntentFilter(Intent.ACTION_DEVICE_STORAGE_LOW);
            boolean hasLowStorage = registerReceiver(null, lowstorageFilter) != null;

            if (hasLowStorage) {
                Toast.makeText(this, R.string.low_storage_error,
                        Toast.LENGTH_LONG).show();
                Log.w(TAG, getString(R.string.low_storage_error));
            }
        }

        // Creates and starts the camera.  Note that this uses a higher resolution in comparison
        // to other detection examples to enable the barcode detector to detect small barcodes
        // at long distances.
        DisplayMetrics metrics = new DisplayMetrics();
        getWindowManager().getDefaultDisplay().getMetrics(metrics);

        CameraSource.Builder builder = new CameraSource.Builder(getApplicationContext(), barcodeDetector)
                .setFacing(CameraSource.CAMERA_FACING_BACK)
                .setRequestedPreviewSize(metrics.widthPixels, metrics.heightPixels)
                .setRequestedFps(24.0f);

        // make sure that auto focus is an available option
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
            builder = builder.setFocusMode(
                    autoFocus ? Camera.Parameters.FOCUS_MODE_CONTINUOUS_PICTURE : null);
        }

        mCameraSource = builder
                .setFlashMode(useFlash ? Camera.Parameters.FLASH_MODE_TORCH : null)
                .build();
    }

    // Restarts the camera
    @Override
    protected void onResume() {
        super.onResume();
        startCameraSource();
    }

    // Stops the camera
    @Override
    protected void onPause() {
        super.onPause();
        if (mPreview != null) {
            mPreview.stop();
        }
    }

    /**
     * Releases the resources associated with the camera source, the associated detectors, and the
     * rest of the processing pipeline.
     */
    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (mPreview != null) {
            mPreview.release();
        }
    }

    /**
     * Callback for the result from requesting permissions. This method
     * is invoked for every call on {@link #requestPermissions(String[], int)}.
     * 

     * Note: It is possible that the permissions request interaction
     * with the user is interrupted. In this case you will receive empty permissions
     * and results arrays which should be treated as a cancellation.
     * 
     *
     * @param requestCode  The request code passed in {@link #requestPermissions(String[], int)}.
     * @param permissions  The requested permissions. Never null.
     * @param grantResults The grant results for the corresponding permissions
     *                     which is either {@link PackageManager#PERMISSION_GRANTED}
     *                     or {@link PackageManager#PERMISSION_DENIED}. Never null.
     * @see #requestPermissions(String[], int)
     */
    @Override
    public void onRequestPermissionsResult(int requestCode,
                                           @NonNull String[] permissions,
                                           @NonNull int[] grantResults) {
        if (requestCode != RC_HANDLE_CAMERA_PERM) {
            Log.d(TAG, "Got unexpected permission result: " + requestCode);
            super.onRequestPermissionsResult(requestCode, permissions, grantResults);
            return;
        }

        if (grantResults.length != 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            Log.d(TAG, "Camera permission granted - initialize the camera source");
            // we have permission, so create the camerasource
            boolean autoFocus = true;
            boolean useFlash = false;
            createCameraSource(autoFocus, useFlash);
            return;
        }

        Log.e(TAG, "Permission not granted: results len = " + grantResults.length +
                " Result code = " + (grantResults.length > 0 ? grantResults[0] : "(empty)"));

        DialogInterface.OnClickListener listener = new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int id) {
                finish();
            }
        };

        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Multitracker sample")
                .setMessage(R.string.no_camera_permission)
                .setPositiveButton(R.string.ok, listener)
                .show();
    }

    /**
     * Starts or restarts the camera source, if it exists.  If the camera source doesn't exist yet
     * (e.g., because onResume was called before the camera source was created), this will be called
     * again when the camera source is created.
     */
    private void startCameraSource() throws SecurityException {
        // check that the device has play services available.
        int code = GoogleApiAvailability.getInstance().isGooglePlayServicesAvailable(
                getApplicationContext());
        if (code != ConnectionResult.SUCCESS) {
            Dialog dlg =
                    GoogleApiAvailability.getInstance().getErrorDialog(this, code, RC_HANDLE_GMS);
            dlg.show();
        }

        if (mCameraSource != null) {
            try {
                mPreview.start(mCameraSource);
            } catch (IOException e) {
                Log.e(TAG, "Unable to start camera source.", e);
                mCameraSource.release();
                mCameraSource = null;
            }
        }
    }
}

BarcodeTracker.java

/*
 * Copyright (C) The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.content.Context;

import com.google.android.gms.vision.Tracker;
import com.google.android.gms.vision.barcode.Barcode;

class BarcodeTracker extends Tracker {
    private BarcodeGraphicTrackerCallback mListener;

    public interface BarcodeGraphicTrackerCallback {
        void onDetectedQrCode(Barcode barcode);
    }

    BarcodeTracker(Context listener) {
        mListener = (BarcodeGraphicTrackerCallback) listener;
    }

    @Override
    public void onNewItem(int id, Barcode item) {
        if (item.displayValue != null) {
            mListener.onDetectedQrCode(item);
        }
    }
}

BarcodeTrackerFactory.java

/*
 * Copyright (C) The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.content.Context;

import com.google.android.gms.vision.MultiProcessor;
import com.google.android.gms.vision.Tracker;
import com.google.android.gms.vision.barcode.Barcode;

/**
 * Factory for creating a tracker and associated graphic to be associated with a new barcode.  The
 * multi-processor uses this factory to create barcode trackers as needed -- one for each barcode.
 */
class BarcodeTrackerFactory implements MultiProcessor.Factory {
    private Context mContext;

    BarcodeTrackerFactory(Context context) {
        mContext = context;
    }

    @Override
    public Tracker create(Barcode barcode) {
        return new BarcodeTracker(mContext);
    }
}

CameraSourcePreview.java

/*
 * Copyright (C) The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.Manifest;
import android.content.Context;
import android.content.res.Configuration;
import android.support.annotation.RequiresPermission;
import android.util.AttributeSet;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.ViewGroup;
import com.google.android.gms.common.images.Size;
import java.io.IOException;

public class CameraSourcePreview extends ViewGroup {
    private static final String TAG = CameraSourcePreview.class.getSimpleName();

    private Context mContext;
    private SurfaceView mSurfaceView;
    private boolean mStartRequested;
    private boolean mSurfaceAvailable;
    private CameraSource mCameraSource;

    public CameraSourcePreview(Context context, AttributeSet attrs) {
        super(context, attrs);
        mContext = context;
        mStartRequested = false;
        mSurfaceAvailable = false;

        mSurfaceView = new SurfaceView(context);
        mSurfaceView.getHolder().addCallback(new SurfaceCallback());
        addView(mSurfaceView);
    }

    @RequiresPermission(Manifest.permission.CAMERA)
    public void start(CameraSource cameraSource) throws IOException, SecurityException {
        if (cameraSource == null) {
            stop();
        }

        mCameraSource = cameraSource;

        if (mCameraSource != null) {
            mStartRequested = true;
            startIfReady();
        }
    }

    public void stop() {
        if (mCameraSource != null) {
            mCameraSource.stop();
        }
    }

    public void release() {
        if (mCameraSource != null) {
            mCameraSource.release();
            mCameraSource = null;
        }
    }

    @RequiresPermission(Manifest.permission.CAMERA)
    private void startIfReady() throws IOException, SecurityException {
        if (mStartRequested && mSurfaceAvailable) {
            mCameraSource.start(mSurfaceView.getHolder());
            mStartRequested = false;
        }
    }

    private class SurfaceCallback implements SurfaceHolder.Callback {
        @Override
        public void surfaceCreated(SurfaceHolder surface) {
            mSurfaceAvailable = true;
            try {
                startIfReady();
            } catch (SecurityException se) {
                Log.e(TAG,"Do not have permission to start the camera", se);
            } catch (IOException e) {
                Log.e(TAG, "Could not start camera source.", e);
            }
        }

        @Override
        public void surfaceDestroyed(SurfaceHolder surface) {
            mSurfaceAvailable = false;
        }

        @Override
        public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
        }
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        int previewWidth = 320;
        int previewHeight = 240;
        if (mCameraSource != null) {
            Size size = mCameraSource.getPreviewSize();
            if (size != null) {
                previewWidth = size.getWidth();
                previewHeight = size.getHeight();
            }
        }

        // Swap width and height sizes when in portrait, since it will be rotated 90 degrees
        if (isPortraitMode()) {
            int tmp = previewWidth;
            previewWidth = previewHeight;
            previewHeight = tmp;
        }

        final int viewWidth = right - left;
        final int viewHeight = bottom - top;

        int childWidth;
        int childHeight;
        int childXOffset = 0;
        int childYOffset = 0;
        float widthRatio = (float) viewWidth / (float) previewWidth;
        float heightRatio = (float) viewHeight / (float) previewHeight;

        // To fill the view with the camera preview, while also preserving the correct aspect ratio,
        // it is usually necessary to slightly oversize the child and to crop off portions along one
        // of the dimensions.  We scale up based on the dimension requiring the most correction, and
        // compute a crop offset for the other dimension.
        if (widthRatio > heightRatio) {
            childWidth = viewWidth;
            childHeight = (int) ((float) previewHeight * widthRatio);
            childYOffset = (childHeight - viewHeight) / 2;
        } else {
            childWidth = (int) ((float) previewWidth * heightRatio);
            childHeight = viewHeight;
            childXOffset = (childWidth - viewWidth) / 2;
        }

        for (int i = 0; i < getChildCount(); ++i) {
            // One dimension will be cropped.  We shift child over or up by this offset and adjust
            // the size to maintain the proper aspect ratio.
            getChildAt(i).layout(
                    -1 * childXOffset, -1 * childYOffset,
                    childWidth - childXOffset, childHeight - childYOffset);
        }

        try {
            startIfReady();
        } catch (IOException e) {
            Log.e(TAG, "Could not start camera source.", e);
        } catch (SecurityException se) {
            Log.e(TAG, "Does not have permission to start the camera.", se);
        }
    }

    private boolean isPortraitMode() {
        int orientation = mContext.getResources().getConfiguration().orientation;
        if (orientation == Configuration.ORIENTATION_LANDSCAPE) {
            return false;
        }
        if (orientation == Configuration.ORIENTATION_PORTRAIT) {
            return true;
        }

        Log.d(TAG, "isPortraitMode returning false by default");
        return false;
    }
}

CameraSource.java

/*
 * Copyright (C) The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.Manifest;
import android.annotation.SuppressLint;
import android.annotation.TargetApi;
import android.content.Context;
import android.graphics.ImageFormat;
import android.graphics.SurfaceTexture;
import android.hardware.Camera;
import android.hardware.Camera.CameraInfo;
import android.os.Build;
import android.os.SystemClock;
import android.support.annotation.Nullable;
import android.support.annotation.RequiresPermission;
import android.support.annotation.StringDef;
import android.util.Log;
import android.view.Surface;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.WindowManager;

import com.google.android.gms.common.images.Size;
import com.google.android.gms.vision.Detector;
import com.google.android.gms.vision.Frame;

import java.io.IOException;
import java.lang.Thread.State;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

// Note: This requires Google Play Services 8.1 or higher, due to using indirect byte buffers for
// storing images.

/**
 * Manages the camera in conjunction with an underlying
 * {@link com.google.android.gms.vision.Detector}.  This receives preview frames from the camera at
 * a specified rate, sending those frames to the detector as fast as it is able to process those
 * frames.
 * 

 * This camera source makes a best effort to manage processing on preview frames as fast as
 * possible, while at the same time minimizing lag.  As such, frames may be dropped if the detector
 * is unable to keep up with the rate of frames generated by the camera.  You should use
 * {@link CameraSource.Builder#setRequestedFps(float)} to specify a frame rate that works well with
 * the capabilities of the camera hardware and the detector options that you have selected.  If CPU
 * utilization is higher than you'd like, then you may want to consider reducing FPS.  If the camera
 * preview or detector results are too "jerky", then you may want to consider increasing FPS.
 * 

 * The following Android permission is required to use the camera:
 * 

 * 
android.permissions.CAMERA
 * 
 */

public class CameraSource {
    @SuppressLint("InlinedApi")
    public static final int CAMERA_FACING_BACK = CameraInfo.CAMERA_FACING_BACK;
    @SuppressLint("InlinedApi")
    public static final int CAMERA_FACING_FRONT = CameraInfo.CAMERA_FACING_FRONT;

    private static final String TAG = "OpenCameraSource";

    /**
     * The dummy surface texture must be assigned a chosen name.  Since we never use an OpenGL
     * context, we can choose any ID we want here.
     */
    private static final int DUMMY_TEXTURE_NAME = 100;

    /**
     * If the absolute difference between a preview size aspect ratio and a picture size aspect
     * ratio is less than this tolerance, they are considered to be the same aspect ratio.
     */
    private static final float ASPECT_RATIO_TOLERANCE = 0.01f;

    @StringDef({
            Camera.Parameters.FOCUS_MODE_CONTINUOUS_PICTURE,
            Camera.Parameters.FOCUS_MODE_CONTINUOUS_VIDEO,
            Camera.Parameters.FOCUS_MODE_AUTO,
            Camera.Parameters.FOCUS_MODE_EDOF,
            Camera.Parameters.FOCUS_MODE_FIXED,
            Camera.Parameters.FOCUS_MODE_INFINITY,
            Camera.Parameters.FOCUS_MODE_MACRO
    })
    @Retention(RetentionPolicy.SOURCE)
    private @interface FocusMode {}

    @StringDef({
            Camera.Parameters.FLASH_MODE_ON,
            Camera.Parameters.FLASH_MODE_OFF,
            Camera.Parameters.FLASH_MODE_AUTO,
            Camera.Parameters.FLASH_MODE_RED_EYE,
            Camera.Parameters.FLASH_MODE_TORCH
    })
    @Retention(RetentionPolicy.SOURCE)
    private @interface FlashMode {}

    private Context mContext;

    private final Object mCameraLock = new Object();

    // Guarded by mCameraLock
    private Camera mCamera;

    private int mFacing = CAMERA_FACING_BACK;

    /**
     * Rotation of the device, and thus the associated preview images captured from the device.
     * See {@link Frame.Metadata#getRotation()}.
     */
    private int mRotation;

    private Size mPreviewSize;

    // These values may be requested by the caller.  Due to hardware limitations, we may need to
    // select close, but not exactly the same values for these.
    private float mRequestedFps = 30.0f;
    private int mRequestedPreviewWidth = 1024;
    private int mRequestedPreviewHeight = 768;


    private String mFocusMode = null;
    private String mFlashMode = null;

    // These instances need to be held onto to avoid GC of their underlying resources.  Even though
    // these aren't used outside of the method that creates them, they still must have hard
    // references maintained to them.
    private SurfaceView mDummySurfaceView;
    private SurfaceTexture mDummySurfaceTexture;

    /**
     * Dedicated thread and associated runnable for calling into the detector with frames, as the
     * frames become available from the camera.
     */
    private Thread mProcessingThread;
    private FrameProcessingRunnable mFrameProcessor;

    /**
     * Map to convert between a byte array, received from the camera, and its associated byte
     * buffer.  We use byte buffers internally because this is a more efficient way to call into
     * native code later (avoids a potential copy).
     */
    private Map mBytesToByteBuffer = new HashMap<>();

    //==============================================================================================
    // Builder
    //==============================================================================================

    /**
     * Builder for configuring and creating an associated camera source.
     */
    public static class Builder {
        private final Detector mDetector;
        private CameraSource mCameraSource = new CameraSource();

        /**
         * Creates a camera source builder with the supplied context and detector.  Camera preview
         * images will be streamed to the associated detector upon starting the camera source.
         */
        public Builder(Context context, Detector detector) {
            if (context == null) {
                throw new IllegalArgumentException("No context supplied.");
            }
            if (detector == null) {
                throw new IllegalArgumentException("No detector supplied.");
            }

            mDetector = detector;
            mCameraSource.mContext = context;
        }

        /**
         * Sets the requested frame rate in frames per second.  If the exact requested value is not
         * not available, the best matching available value is selected.   Default: 30.
         */
        public Builder setRequestedFps(float fps) {
            if (fps <= 0) {
                throw new IllegalArgumentException("Invalid fps: " + fps);
            }
            mCameraSource.mRequestedFps = fps;
            return this;
        }

        public Builder setFocusMode(@FocusMode String mode) {
            mCameraSource.mFocusMode = mode;
            return this;
        }

        public Builder setFlashMode(@FlashMode String mode) {
            mCameraSource.mFlashMode = mode;
            return this;
        }

        /**
         * Sets the desired width and height of the camera frames in pixels.  If the exact desired
         * values are not available options, the best matching available options are selected.
         * Also, we try to select a preview size which corresponds to the aspect ratio of an
         * associated full picture size, if applicable.  Default: 1024x768.
         */
        public Builder setRequestedPreviewSize(int width, int height) {
            // Restrict the requested range to something within the realm of possibility.  The
            // choice of 1000000 is a bit arbitrary -- intended to be well beyond resolutions that
            // devices can support.  We bound this to avoid int overflow in the code later.
            final int MAX = 1000000;
            if ((width <= 0) || (width > MAX) || (height <= 0) || (height > MAX)) {
                throw new IllegalArgumentException("Invalid preview size: " + width + "x" + height);
            }
            mCameraSource.mRequestedPreviewWidth = width;
            mCameraSource.mRequestedPreviewHeight = height;
            return this;
        }

        /**
         * Sets the camera to use (either {@link #CAMERA_FACING_BACK} or
         * {@link #CAMERA_FACING_FRONT}). Default: back facing.
         */
        public Builder setFacing(int facing) {
            if ((facing != CAMERA_FACING_BACK) && (facing != CAMERA_FACING_FRONT)) {
                throw new IllegalArgumentException("Invalid camera: " + facing);
            }
            mCameraSource.mFacing = facing;
            return this;
        }

        /**
         * Creates an instance of the camera source.
         */
        public CameraSource build() {
            mCameraSource.mFrameProcessor = mCameraSource.new FrameProcessingRunnable(mDetector);
            return mCameraSource;
        }
    }

    //==============================================================================================
    // Bridge Functionality for the Camera1 API
    //==============================================================================================

    /**
     * Callback interface used to signal the moment of actual image capture.
     */
    public interface ShutterCallback {
        /**
         * Called as near as possible to the moment when a photo is captured from the sensor. This
         * is a good opportunity to play a shutter sound or give other feedback of camera operation.
         * This may be some time after the photo was triggered, but some time before the actual data
         * is available.
         */
        void onShutter();
    }

    /**
     * Callback interface used to supply image data from a photo capture.
     */
    public interface PictureCallback {
        /**
         * Called when image data is available after a picture is taken.  The format of the data
         * is a jpeg binary.
         */
        void onPictureTaken(byte[] data);
    }

    /**
     * Callback interface used to notify on completion of camera auto focus.
     */
    public interface AutoFocusCallback {
        /**
         * Called when the camera auto focus completes.  If the camera
         * does not support auto-focus and autoFocus is called,
         * onAutoFocus will be called immediately with a fake value of
         * success set to true.
         * 

         * The auto-focus routine does not lock auto-exposure and auto-white
         * balance after it completes.
         *
         * @param success true if focus was successful, false if otherwise
         */
        void onAutoFocus(boolean success);
    }

    /**
     * Callback interface used to notify on auto focus start and stop.
     * 

     * 
This is only supported in continuous autofocus modes -- {@link
     * Camera.Parameters#FOCUS_MODE_CONTINUOUS_VIDEO} and {@link
     * Camera.Parameters#FOCUS_MODE_CONTINUOUS_PICTURE}. Applications can show
     * autofocus animation based on this.
     */
    public interface AutoFocusMoveCallback {
        /**
         * Called when the camera auto focus starts or stops.
         *
         * @param start true if focus starts to move, false if focus stops to move
         */
        void onAutoFocusMoving(boolean start);
    }

    //==============================================================================================
    // Public
    //==============================================================================================

    /**
     * Stops the camera and releases the resources of the camera and underlying detector.
     */
    public void release() {
        synchronized (mCameraLock) {
            stop();
            mFrameProcessor.release();
        }
    }

    /**
     * Opens the camera and starts sending preview frames to the underlying detector.  The preview
     * frames are not displayed.
     *
     * @throws IOException if the camera's preview texture or display could not be initialized
     */
    @RequiresPermission(Manifest.permission.CAMERA)
    public CameraSource start() throws IOException {
        synchronized (mCameraLock) {
            if (mCamera != null) {
                return this;
            }

            mCamera = createCamera();

            // SurfaceTexture was introduced in Honeycomb (11), so if we are running and
            // old version of Android. fall back to use SurfaceView.
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
                mDummySurfaceTexture = new SurfaceTexture(DUMMY_TEXTURE_NAME);
                mCamera.setPreviewTexture(mDummySurfaceTexture);
            } else {
                mDummySurfaceView = new SurfaceView(mContext);
                mCamera.setPreviewDisplay(mDummySurfaceView.getHolder());
            }
            mCamera.startPreview();

            mProcessingThread = new Thread(mFrameProcessor);
            mFrameProcessor.setActive(true);
            mProcessingThread.start();
        }
        return this;
    }

    /**
     * Opens the camera and starts sending preview frames to the underlying detector.  The supplied
     * surface holder is used for the preview so frames can be displayed to the user.
     *
     * @param surfaceHolder the surface holder to use for the preview frames
     * @throws IOException if the supplied surface holder could not be used as the preview display
     */
    @RequiresPermission(Manifest.permission.CAMERA)
    public CameraSource start(SurfaceHolder surfaceHolder) throws IOException {
        synchronized (mCameraLock) {
            if (mCamera != null) {
                return this;
            }

            mCamera = createCamera();
            mCamera.setPreviewDisplay(surfaceHolder);
            mCamera.startPreview();

            mProcessingThread = new Thread(mFrameProcessor);
            mFrameProcessor.setActive(true);
            mProcessingThread.start();
        }
        return this;
    }

    /**
     * Closes the camera and stops sending frames to the underlying frame detector.
     * 

     * This camera source may be restarted again by calling {@link #start()} or
     * {@link #start(SurfaceHolder)}.
     * 

     * Call {@link #release()} instead to completely shut down this camera source and release the
     * resources of the underlying detector.
     */
    public void stop() {
        synchronized (mCameraLock) {
            mFrameProcessor.setActive(false);
            if (mProcessingThread != null) {
                try {
                    // Wait for the thread to complete to ensure that we can't have multiple threads
                    // executing at the same time (i.e., which would happen if we called start too
                    // quickly after stop).
                    mProcessingThread.join();
                } catch (InterruptedException e) {
                    Log.d(TAG, "Frame processing thread interrupted on release.");
                }
                mProcessingThread = null;
            }

            // clear the buffer to prevent oom exceptions
            mBytesToByteBuffer.clear();

            if (mCamera != null) {
                mCamera.stopPreview();
                mCamera.setPreviewCallbackWithBuffer(null);
                try {
                    // We want to be compatible back to Gingerbread, but SurfaceTexture
                    // wasn't introduced until Honeycomb.  Since the interface cannot use a SurfaceTexture, if the
                    // developer wants to display a preview we must use a SurfaceHolder.  If the developer doesn't
                    // want to display a preview we use a SurfaceTexture if we are running at least Honeycomb.

                    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
                        mCamera.setPreviewTexture(null);

                    } else {
                        mCamera.setPreviewDisplay(null);
                    }
                } catch (Exception e) {
                    Log.e(TAG, "Failed to clear camera preview: " + e);
                }
                mCamera.release();
                mCamera = null;
            }
        }
    }

    /**
     * Returns the preview size that is currently in use by the underlying camera.
     */
    public Size getPreviewSize() {
        return mPreviewSize;
    }

    /**
     * Returns the selected camera; one of {@link #CAMERA_FACING_BACK} or
     * {@link #CAMERA_FACING_FRONT}.
     */
    public int getCameraFacing() {
        return mFacing;
    }

    public int doZoom(float scale) {
        synchronized (mCameraLock) {
            if (mCamera == null) {
                return 0;
            }
            int currentZoom = 0;
            int maxZoom;
            Camera.Parameters parameters = mCamera.getParameters();
            if (!parameters.isZoomSupported()) {
                Log.w(TAG, "Zoom is not supported on this device");
                return currentZoom;
            }
            maxZoom = parameters.getMaxZoom();

            currentZoom = parameters.getZoom() + 1;
            float newZoom;
            if (scale > 1) {
                newZoom = currentZoom + scale * (maxZoom / 10);
            } else {
                newZoom = currentZoom * scale;
            }
            currentZoom = Math.round(newZoom) - 1;
            if (currentZoom < 0) {
                currentZoom = 0;
            } else if (currentZoom > maxZoom) {
                currentZoom = maxZoom;
            }
            parameters.setZoom(currentZoom);
            mCamera.setParameters(parameters);
            return currentZoom;
        }
    }

    /**
     * Initiates taking a picture, which happens asynchronously.  The camera source should have been
     * activated previously with {@link #start()} or {@link #start(SurfaceHolder)}.  The camera
     * preview is suspended while the picture is being taken, but will resume once picture taking is
     * done.
     *
     * @param shutter the callback for image capture moment, or null
     * @param jpeg    the callback for JPEG image data, or null
     */
    public void takePicture(ShutterCallback shutter, PictureCallback jpeg) {
        synchronized (mCameraLock) {
            if (mCamera != null) {
                PictureStartCallback startCallback = new PictureStartCallback();
                startCallback.mDelegate = shutter;
                PictureDoneCallback doneCallback = new PictureDoneCallback();
                doneCallback.mDelegate = jpeg;
                mCamera.takePicture(startCallback, null, null, doneCallback);
            }
        }
    }

    /**
     * Gets the current focus mode setting.
     *
     * @return current focus mode. This value is null if the camera is not yet created. Applications should call {@link
     * #autoFocus(AutoFocusCallback)} to start the focus if focus
     * mode is FOCUS_MODE_AUTO or FOCUS_MODE_MACRO.
     * @see Camera.Parameters#FOCUS_MODE_AUTO
     * @see Camera.Parameters#FOCUS_MODE_INFINITY
     * @see Camera.Parameters#FOCUS_MODE_MACRO
     * @see Camera.Parameters#FOCUS_MODE_FIXED
     * @see Camera.Parameters#FOCUS_MODE_EDOF
     * @see Camera.Parameters#FOCUS_MODE_CONTINUOUS_VIDEO
     * @see Camera.Parameters#FOCUS_MODE_CONTINUOUS_PICTURE
     */
    @Nullable
    @FocusMode
    public String getFocusMode() {
        return mFocusMode;
    }

    /**
     * Sets the focus mode.
     *
     * @param mode the focus mode
     * @return {@code true} if the focus mode is set, {@code false} otherwise
     * @see #getFocusMode()
     */
    public boolean setFocusMode(@FocusMode String mode) {
        synchronized (mCameraLock) {
            if (mCamera != null && mode != null) {
                Camera.Parameters parameters = mCamera.getParameters();
                if (parameters.getSupportedFocusModes().contains(mode)) {
                    parameters.setFocusMode(mode);
                    mCamera.setParameters(parameters);
                    mFocusMode = mode;
                    return true;
                }
            }

            return false;
        }
    }

    /**
     * Gets the current flash mode setting.
     *
     * @return current flash mode. null if flash mode setting is not
     * supported or the camera is not yet created.
     * @see Camera.Parameters#FLASH_MODE_OFF
     * @see Camera.Parameters#FLASH_MODE_AUTO
     * @see Camera.Parameters#FLASH_MODE_ON
     * @see Camera.Parameters#FLASH_MODE_RED_EYE
     * @see Camera.Parameters#FLASH_MODE_TORCH
     */
    @Nullable
    @FlashMode
    public String getFlashMode() {
        return mFlashMode;
    }

    /**
     * Sets the flash mode.
     *
     * @param mode flash mode.
     * @return {@code true} if the flash mode is set, {@code false} otherwise
     * @see #getFlashMode()
     */
    public boolean setFlashMode(@FlashMode String mode) {
        synchronized (mCameraLock) {
            if (mCamera != null && mode != null) {
                Camera.Parameters parameters = mCamera.getParameters();
                if (parameters.getSupportedFlashModes().contains(mode)) {
                    parameters.setFlashMode(mode);
                    mCamera.setParameters(parameters);
                    mFlashMode = mode;
                    return true;
                }
            }

            return false;
        }
    }

    /**
     * Starts camera auto-focus and registers a callback function to run when
     * the camera is focused.  This method is only valid when preview is active
     * (between {@link #start()} or {@link #start(SurfaceHolder)} and before {@link #stop()} or {@link #release()}).
     * 

     * 
Callers should check
     * {@link #getFocusMode()} to determine if
     * this method should be called. If the camera does not support auto-focus,
     * it is a no-op and {@link AutoFocusCallback#onAutoFocus(boolean)}
     * callback will be called immediately.
     * 

     * 
If the current flash mode is not
     * {@link Camera.Parameters#FLASH_MODE_OFF}, flash may be
     * fired during auto-focus, depending on the driver and camera hardware.

     *
     * @param cb the callback to run
     * @see #cancelAutoFocus()
     */
    public void autoFocus(@Nullable AutoFocusCallback cb) {
        synchronized (mCameraLock) {
            if (mCamera != null) {
                CameraAutoFocusCallback autoFocusCallback = null;
                if (cb != null) {
                    autoFocusCallback = new CameraAutoFocusCallback();
                    autoFocusCallback.mDelegate = cb;
                }
                mCamera.autoFocus(autoFocusCallback);
            }
        }
    }

    /**
     * Cancels any auto-focus function in progress.
     * Whether or not auto-focus is currently in progress,
     * this function will return the focus position to the default.
     * If the camera does not support auto-focus, this is a no-op.
     *
     * @see #autoFocus(AutoFocusCallback)
     */
    public void cancelAutoFocus() {
        synchronized (mCameraLock) {
            if (mCamera != null) {
                mCamera.cancelAutoFocus();
            }
        }
    }

    /**
     * Sets camera auto-focus move callback.
     *
     * @param cb the callback to run
     * @return {@code true} if the operation is supported (i.e. from Jelly Bean), {@code false} otherwise
     */
    @TargetApi(Build.VERSION_CODES.JELLY_BEAN)
    public boolean setAutoFocusMoveCallback(@Nullable AutoFocusMoveCallback cb) {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN) {
            return false;
        }

        synchronized (mCameraLock) {
            if (mCamera != null) {
                CameraAutoFocusMoveCallback autoFocusMoveCallback = null;
                if (cb != null) {
                    autoFocusMoveCallback = new CameraAutoFocusMoveCallback();
                    autoFocusMoveCallback.mDelegate = cb;
                }
                mCamera.setAutoFocusMoveCallback(autoFocusMoveCallback);
            }
        }

        return true;
    }

    //==============================================================================================
    // Private
    //==============================================================================================

    /**
     * Only allow creation via the builder class.
     */
    private CameraSource() {
    }

    /**
     * Wraps the camera1 shutter callback so that the deprecated API isn't exposed.
     */
    private class PictureStartCallback implements Camera.ShutterCallback {
        private ShutterCallback mDelegate;

        @Override
        public void onShutter() {
            if (mDelegate != null) {
                mDelegate.onShutter();
            }
        }
    }

    /**
     * Wraps the final callback in the camera sequence, so that we can automatically turn the camera
     * preview back on after the picture has been taken.
     */
    private class PictureDoneCallback implements Camera.PictureCallback {
        private PictureCallback mDelegate;

        @Override
        public void onPictureTaken(byte[] data, Camera camera) {
            if (mDelegate != null) {
                mDelegate.onPictureTaken(data);
            }
            synchronized (mCameraLock) {
                if (mCamera != null) {
                    mCamera.startPreview();
                }
            }
        }
    }

    /**
     * Wraps the camera1 auto focus callback so that the deprecated API isn't exposed.
     */
    private class CameraAutoFocusCallback implements Camera.AutoFocusCallback {
        private AutoFocusCallback mDelegate;

        @Override
        public void onAutoFocus(boolean success, Camera camera) {
            if (mDelegate != null) {
                mDelegate.onAutoFocus(success);
            }
        }
    }

    /**
     * Wraps the camera1 auto focus move callback so that the deprecated API isn't exposed.
     */
    @TargetApi(Build.VERSION_CODES.JELLY_BEAN)
    private class CameraAutoFocusMoveCallback implements Camera.AutoFocusMoveCallback {
        private AutoFocusMoveCallback mDelegate;

        @Override
        public void onAutoFocusMoving(boolean start, Camera camera) {
            if (mDelegate != null) {
                mDelegate.onAutoFocusMoving(start);
            }
        }
    }

    /**
     * Opens the camera and applies the user settings.
     *
     * @throws RuntimeException if the method fails
     */
    @SuppressLint("InlinedApi")
    private Camera createCamera() {
        int requestedCameraId = getIdForRequestedCamera(mFacing);
        if (requestedCameraId == -1) {
            throw new RuntimeException("Could not find requested camera.");
        }
        Camera camera = Camera.open(requestedCameraId);

        SizePair sizePair = selectSizePair(camera, mRequestedPreviewWidth, mRequestedPreviewHeight);
        if (sizePair == null) {
            throw new RuntimeException("Could not find suitable preview size.");
        }
        Size pictureSize = sizePair.pictureSize();
        mPreviewSize = sizePair.previewSize();

        int[] previewFpsRange = selectPreviewFpsRange(camera, mRequestedFps);
        if (previewFpsRange == null) {
            throw new RuntimeException("Could not find suitable preview frames per second range.");
        }

        Camera.Parameters parameters = camera.getParameters();

        if (pictureSize != null) {
            parameters.setPictureSize(pictureSize.getWidth(), pictureSize.getHeight());
        }

        parameters.setPreviewSize(mPreviewSize.getWidth(), mPreviewSize.getHeight());
        parameters.setPreviewFpsRange(
                previewFpsRange[Camera.Parameters.PREVIEW_FPS_MIN_INDEX],
                previewFpsRange[Camera.Parameters.PREVIEW_FPS_MAX_INDEX]);
        parameters.setPreviewFormat(ImageFormat.NV21);

        setRotation(camera, parameters, requestedCameraId);

        if (mFocusMode != null) {
            if (parameters.getSupportedFocusModes().contains(
                    mFocusMode)) {
                parameters.setFocusMode(mFocusMode);
            } else {
                Log.i(TAG, "Camera focus mode: " + mFocusMode + " is not supported on this device.");
            }
        }

        // setting mFocusMode to the one set in the params
        mFocusMode = parameters.getFocusMode();

        if (mFlashMode != null) {
            if (parameters.getSupportedFlashModes().contains(
                    mFlashMode)) {
                parameters.setFlashMode(mFlashMode);
            } else {
                Log.i(TAG, "Camera flash mode: " + mFlashMode + " is not supported on this device.");
            }
        }

        // setting mFlashMode to the one set in the params
        mFlashMode = parameters.getFlashMode();

        camera.setParameters(parameters);

        // Four frame buffers are needed for working with the camera:
        //
        //   one for the frame that is currently being executed upon in doing detection
        //   one for the next pending frame to process immediately upon completing detection
        //   two for the frames that the camera uses to populate future preview images
        camera.setPreviewCallbackWithBuffer(new CameraPreviewCallback());
        camera.addCallbackBuffer(createPreviewBuffer(mPreviewSize));
        camera.addCallbackBuffer(createPreviewBuffer(mPreviewSize));
        camera.addCallbackBuffer(createPreviewBuffer(mPreviewSize));
        camera.addCallbackBuffer(createPreviewBuffer(mPreviewSize));

        return camera;
    }

    /**
     * Gets the id for the camera specified by the direction it is facing.  Returns -1 if no such
     * camera was found.
     *
     * @param facing the desired camera (front-facing or rear-facing)
     */
    private static int getIdForRequestedCamera(int facing) {
        CameraInfo cameraInfo = new CameraInfo();
        for (int i = 0; i < Camera.getNumberOfCameras(); ++i) {
            Camera.getCameraInfo(i, cameraInfo);
            if (cameraInfo.facing == facing) {
                return i;
            }
        }
        return -1;
    }

    /**
     * Selects the most suitable preview and picture size, given the desired width and height.
     * 

     * Even though we may only need the preview size, it's necessary to find both the preview
     * size and the picture size of the camera together, because these need to have the same aspect
     * ratio.  On some hardware, if you would only set the preview size, you will get a distorted
     * image.
     *
     * @param camera        the camera to select a preview size from
     * @param desiredWidth  the desired width of the camera preview frames
     * @param desiredHeight the desired height of the camera preview frames
     * @return the selected preview and picture size pair
     */
    private static SizePair selectSizePair(Camera camera, int desiredWidth, int desiredHeight) {
        List validPreviewSizes = generateValidPreviewSizeList(camera);

        // The method for selecting the best size is to minimize the sum of the differences between
        // the desired values and the actual values for width and height.  This is certainly not the
        // only way to select the best size, but it provides a decent tradeoff between using the
        // closest aspect ratio vs. using the closest pixel area.
        SizePair selectedPair = null;
        int minDiff = Integer.MAX_VALUE;
        for (SizePair sizePair : validPreviewSizes) {
            Size size = sizePair.previewSize();
            int diff = Math.abs(size.getWidth() - desiredWidth) +
                    Math.abs(size.getHeight() - desiredHeight);
            if (diff < minDiff) {
                selectedPair = sizePair;
                minDiff = diff;
            }
        }

        return selectedPair;
    }

    /**
     * Stores a preview size and a corresponding same-aspect-ratio picture size.  To avoid distorted
     * preview images on some devices, the picture size must be set to a size that is the same
     * aspect ratio as the preview size or the preview may end up being distorted.  If the picture
     * size is null, then there is no picture size with the same aspect ratio as the preview size.
     */
    private static class SizePair {
        private Size mPreview;
        private Size mPicture;

        public SizePair(Camera.Size previewSize,
                        Camera.Size pictureSize) {
            mPreview = new Size(previewSize.width, previewSize.height);
            if (pictureSize != null) {
                mPicture = new Size(pictureSize.width, pictureSize.height);
            }
        }

        public Size previewSize() {
            return mPreview;
        }

        @SuppressWarnings("unused")
        public Size pictureSize() {
            return mPicture;
        }
    }

    /**
     * Generates a list of acceptable preview sizes.  Preview sizes are not acceptable if there is
     * not a corresponding picture size of the same aspect ratio.  If there is a corresponding
     * picture size of the same aspect ratio, the picture size is paired up with the preview size.
     * 

     * This is necessary because even if we don't use still pictures, the still picture size must be
     * set to a size that is the same aspect ratio as the preview size we choose.  Otherwise, the
     * preview images may be distorted on some devices.
     */
    private static List generateValidPreviewSizeList(Camera camera) {
        Camera.Parameters parameters = camera.getParameters();
        List supportedPreviewSizes =
                parameters.getSupportedPreviewSizes();
        List supportedPictureSizes =
                parameters.getSupportedPictureSizes();
        List validPreviewSizes = new ArrayList<>();
        for (Camera.Size previewSize : supportedPreviewSizes) {
            float previewAspectRatio = (float) previewSize.width / (float) previewSize.height;

            // By looping through the picture sizes in order, we favor the higher resolutions.
            // We choose the highest resolution in order to support taking the full resolution
            // picture later.
            for (Camera.Size pictureSize : supportedPictureSizes) {
                float pictureAspectRatio = (float) pictureSize.width / (float) pictureSize.height;
                if (Math.abs(previewAspectRatio - pictureAspectRatio) < ASPECT_RATIO_TOLERANCE) {
                    validPreviewSizes.add(new SizePair(previewSize, pictureSize));
                    break;
                }
            }
        }

        // If there are no picture sizes with the same aspect ratio as any preview sizes, allow all
        // of the preview sizes and hope that the camera can handle it.  Probably unlikely, but we
        // still account for it.
        if (validPreviewSizes.size() == 0) {
            Log.w(TAG, "No preview sizes have a corresponding same-aspect-ratio picture size");
            for (Camera.Size previewSize : supportedPreviewSizes) {
                // The null picture size will let us know that we shouldn't set a picture size.
                validPreviewSizes.add(new SizePair(previewSize, null));
            }
        }

        return validPreviewSizes;
    }

    /**
     * Selects the most suitable preview frames per second range, given the desired frames per
     * second.
     *
     * @param camera            the camera to select a frames per second range from
     * @param desiredPreviewFps the desired frames per second for the camera preview frames
     * @return the selected preview frames per second range
     */
    private int[] selectPreviewFpsRange(Camera camera, float desiredPreviewFps) {
        // The camera API uses integers scaled by a factor of 1000 instead of floating-point frame
        // rates.
        int desiredPreviewFpsScaled = (int) (desiredPreviewFps * 1000.0f);

        // The method for selecting the best range is to minimize the sum of the differences between
        // the desired value and the upper and lower bounds of the range.  This may select a range
        // that the desired value is outside of, but this is often preferred.  For example, if the
        // desired frame rate is 29.97, the range (30, 30) is probably more desirable than the
        // range (15, 30).
        int[] selectedFpsRange = null;
        int minDiff = Integer.MAX_VALUE;
        List previewFpsRangeList = camera.getParameters().getSupportedPreviewFpsRange();
        for (int[] range : previewFpsRangeList) {
            int deltaMin = desiredPreviewFpsScaled - range[Camera.Parameters.PREVIEW_FPS_MIN_INDEX];
            int deltaMax = desiredPreviewFpsScaled - range[Camera.Parameters.PREVIEW_FPS_MAX_INDEX];
            int diff = Math.abs(deltaMin) + Math.abs(deltaMax);
            if (diff < minDiff) {
                selectedFpsRange = range;
                minDiff = diff;
            }
        }
        return selectedFpsRange;
    }

    /**
     * Calculates the correct rotation for the given camera id and sets the rotation in the
     * parameters.  It also sets the camera's display orientation and rotation.
     *
     * @param parameters the camera parameters for which to set the rotation
     * @param cameraId   the camera id to set rotation based on
     */
    private void setRotation(Camera camera, Camera.Parameters parameters, int cameraId) {
        WindowManager windowManager =
                (WindowManager) mContext.getSystemService(Context.WINDOW_SERVICE);
        int degrees = 0;
        int rotation = windowManager.getDefaultDisplay().getRotation();
        switch (rotation) {
            case Surface.ROTATION_0:
                degrees = 0;
                break;
            case Surface.ROTATION_90:
                degrees = 90;
                break;
            case Surface.ROTATION_180:
                degrees = 180;
                break;
            case Surface.ROTATION_270:
                degrees = 270;
                break;
            default:
                Log.e(TAG, "Bad rotation value: " + rotation);
        }

        CameraInfo cameraInfo = new CameraInfo();
        Camera.getCameraInfo(cameraId, cameraInfo);

        int angle;
        int displayAngle;
        if (cameraInfo.facing == CameraInfo.CAMERA_FACING_FRONT) {
            angle = (cameraInfo.orientation + degrees) % 360;
            displayAngle = (360 - angle); // compensate for it being mirrored
        } else {  // back-facing
            angle = (cameraInfo.orientation - degrees + 360) % 360;
            displayAngle = angle;
        }

        // This corresponds to the rotation constants in {@link Frame}.
        mRotation = angle / 90;

        camera.setDisplayOrientation(displayAngle);
        parameters.setRotation(angle);
    }

    /**
     * Creates one buffer for the camera preview callback.  The size of the buffer is based off of
     * the camera preview size and the format of the camera image.
     *
     * @return a new preview buffer of the appropriate size for the current camera settings
     */
    private byte[] createPreviewBuffer(Size previewSize) {
        int bitsPerPixel = ImageFormat.getBitsPerPixel(ImageFormat.NV21);
        long sizeInBits = previewSize.getHeight() * previewSize.getWidth() * bitsPerPixel;
        int bufferSize = (int) Math.ceil(sizeInBits / 8.0d) + 1;

        //
        // NOTICE: This code only works when using play services v. 8.1 or higher.
        //

        // Creating the byte array this way and wrapping it, as opposed to using .allocate(),
        // should guarantee that there will be an array to work with.
        byte[] byteArray = new byte[bufferSize];
        ByteBuffer buffer = ByteBuffer.wrap(byteArray);
        if (!buffer.hasArray() || (buffer.array() != byteArray)) {
            // I don't think that this will ever happen.  But if it does, then we wouldn't be
            // passing the preview content to the underlying detector later.
            throw new IllegalStateException("Failed to create valid buffer for camera source.");
        }

        mBytesToByteBuffer.put(byteArray, buffer);
        return byteArray;
    }

    //==============================================================================================
    // Frame processing
    //==============================================================================================

    /**
     * Called when the camera has a new preview frame.
     */
    private class CameraPreviewCallback implements Camera.PreviewCallback {
        @Override
        public void onPreviewFrame(byte[] data, Camera camera) {
            mFrameProcessor.setNextFrame(data, camera);
        }
    }

    /**
     * This runnable controls access to the underlying receiver, calling it to process frames when
     * available from the camera.  This is designed to run detection on frames as fast as possible
     * (i.e., without unnecessary context switching or waiting on the next frame).
     * 

     * While detection is running on a frame, new frames may be received from the camera.  As these
     * frames come in, the most recent frame is held onto as pending.  As soon as detection and its
     * associated processing are done for the previous frame, detection on the mostly recently
     * received frame will immediately start on the same thread.
     */
    private class FrameProcessingRunnable implements Runnable {
        private Detector mDetector;
        private long mStartTimeMillis = SystemClock.elapsedRealtime();

        // This lock guards all of the member variables below.
        private final Object mLock = new Object();
        private boolean mActive = true;

        // These pending variables hold the state associated with the new frame awaiting processing.
        private long mPendingTimeMillis;
        private int mPendingFrameId = 0;
        private ByteBuffer mPendingFrameData;

        FrameProcessingRunnable(Detector detector) {
            mDetector = detector;
        }

        /**
         * Releases the underlying receiver.  This is only safe to do after the associated thread
         * has completed, which is managed in camera source's release method above.
         */
        @SuppressLint("Assert")
        void release() {
            assert (mProcessingThread.getState() == State.TERMINATED);
            mDetector.release();
            mDetector = null;
        }

        /**
         * Marks the runnable as active/not active.  Signals any blocked threads to continue.
         */
        void setActive(boolean active) {
            synchronized (mLock) {
                mActive = active;
                mLock.notifyAll();
            }
        }

        /**
         * Sets the frame data received from the camera.  This adds the previous unused frame buffer
         * (if present) back to the camera, and keeps a pending reference to the frame data for
         * future use.
         */
        void setNextFrame(byte[] data, Camera camera) {
            synchronized (mLock) {
                if (mPendingFrameData != null) {
                    camera.addCallbackBuffer(mPendingFrameData.array());
                    mPendingFrameData = null;
                }

                if (!mBytesToByteBuffer.containsKey(data)) {
                    Log.d(TAG,
                            "Skipping frame.  Could not find ByteBuffer associated with the image " +
                                    "data from the camera.");
                    return;
                }

                // Timestamp and frame ID are maintained here, which will give downstream code some
                // idea of the timing of frames received and when frames were dropped along the way.
                mPendingTimeMillis = SystemClock.elapsedRealtime() - mStartTimeMillis;
                mPendingFrameId++;
                mPendingFrameData = mBytesToByteBuffer.get(data);

                // Notify the processor thread if it is waiting on the next frame (see below).
                mLock.notifyAll();
            }
        }

        /**
         * As long as the processing thread is active, this executes detection on frames
         * continuously.  The next pending frame is either immediately available or hasn't been
         * received yet.  Once it is available, we transfer the frame info to local variables and
         * run detection on that frame.  It immediately loops back for the next frame without
         * pausing.
         * 

         * If detection takes longer than the time in between new frames from the camera, this will
         * mean that this loop will run without ever waiting on a frame, avoiding any context
         * switching or frame acquisition time latency.
         * 

         * If you find that this is using more CPU than you'd like, you should probably decrease the
         * FPS setting above to allow for some idle time in between frames.
         */
        @Override
        public void run() {
            Frame outputFrame;
            ByteBuffer data;

            while (true) {
                synchronized (mLock) {
                    while (mActive && (mPendingFrameData == null)) {
                        try {
                            // Wait for the next frame to be received from the camera, since we
                            // don't have it yet.
                            mLock.wait();
                        } catch (InterruptedException e) {
                            Log.d(TAG, "Frame processing loop terminated.", e);
                            return;
                        }
                    }

                    if (!mActive) {
                        // Exit the loop once this camera source is stopped or released.  We check
                        // this here, immediately after the wait() above, to handle the case where
                        // setActive(false) had been called, triggering the termination of this
                        // loop.
                        return;
                    }

                    outputFrame = new Frame.Builder()
                            .setImageData(mPendingFrameData, mPreviewSize.getWidth(),
                                    mPreviewSize.getHeight(), ImageFormat.NV21)
                            .setId(mPendingFrameId)
                            .setTimestampMillis(mPendingTimeMillis)
                            .setRotation(mRotation)
                            .build();

                    // Hold onto the frame data locally, so that we can use this for detection
                    // below.  We need to clear mPendingFrameData to ensure that this buffer isn't
                    // recycled back to the camera before we are done using that data.
                    data = mPendingFrameData;
                    mPendingFrameData = null;
                }

                // The code below needs to run outside of synchronization, because this will allow
                // the camera to add pending frame(s) while we are running detection on the current
                // frame.

                try {
                    mDetector.receiveFrame(outputFrame);
                } catch (Throwable t) {
                    Log.e(TAG, "Exception thrown from receiver.", t);
                } finally {
                    mCamera.addCallbackBuffer(data.array());
                }
            }
        }
    }
}

References
https://codelabs.developers.google.com/codelabs/bar-codes/#0
https://www.varvet.com/blog/android-qr-code-reader-made-easy/
https://github.com/varvet/BarcodeReaderSample
https://github.com/googlesamples/android-vision
https://stackoverflow.com/a/30529128/2069407
https://stackoverflow.com/a/25283174/2069407