Android preloading images before it gets used

Use Picasso fetch method to fetch the image by url before the image is being used. This fetch can be run during the initialization of the app or anywhere before the image is being used.

Picasso.with(getApplicationContext()).load(url).fetch();

Use Picasso to load image from the cache by specifiying the OFFLINE policy for in newworkPolicy. If it failed to load it from offline mode, try again from the network.

Picasso
.with(getApplicationContext())
.load(url)
.networkPolicy(NetworkPolicy.OFFLINE)
.into(imageView, new Callback() {
    @Override
    public void onSuccess() {
        Log.d("Picasso", "Image loaded from cache>>>" + url);
    }

    @Override
    public void onError() {
        Log.d("Picasso", "Try again in ONLINE mode if load from cache is failed");
        Picasso.with(getApplicationContext()).load(url).into(imageView, new Callback() {
            @Override
            public void onSuccess() {
                Log.d("Picasso", "Image loaded from web>>>" + url);
            }

            @Override
            public void onError() {
                Log.d("Picasso", "Failed to load image online and offline, make sure you enabled INTERNET permission for your app and the url is correct>>>>>>>" + url);
            }
        });
    }
});

A complete sample app for using Picass to warm up the image first, aka pre loading the images before it gets used.

Make sure you enabled the INTERNET permission in your AndroidManifest file.

<uses-permission android:name="android.permission.INTERNET"/>

Include the Picasso library as one of the dependepcies in your app build gradle file.

compile 'com.squareup.picasso:picasso:2.5.2'

The activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<ScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
    <LinearLayout
        android:id="@+id/ll_image_container"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:orientation="vertical">
        <Button
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:textAllCaps="false"
            android:text="Load Images"
            android:onClick="loadImages"/>
    </LinearLayout>
</ScrollView>

The MainActivity.java

public class MainActivity extends AppCompatActivity {

    private LinearLayout llImageContainer;
    private String [] imgUrls= {
            "http://developer.android.com/images/activity_lifecycle.png",
            "http://developer.android.com/images/fundamentals/fragments.png",
            "http://developer.android.com/images/activity_fragment_lifecycle.png",
            "http://developer.android.com/images/fragment_lifecycle.png",
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        llImageContainer = (LinearLayout) findViewById(R.id.ll_image_container);

        preloadImages();
    }

    private void preloadImages() {
        for(String url: imgUrls) {
            Picasso.with(getApplicationContext()).load(url).fetch();
        }
    }

    public void loadImages(View v) {
        for(final String url: imgUrls) {
            final ImageView imageView = new ImageView(this);
            imageView.setLayoutParams(new LinearLayout.LayoutParams(LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT));
            llImageContainer.addView(imageView);

            Picasso.with(getApplicationContext()).load(url).networkPolicy(NetworkPolicy.OFFLINE).into(imageView, new Callback() {
                @Override
                public void onSuccess() {
                    Log.d("Picasso", "Image loaded from cache>>>" + url);
                }

                @Override
                public void onError() {
                    Log.d("Picasso", "Try again in ONLINE mode if load from cache is failed");
                    Picasso.with(getApplicationContext()).load(url).into(imageView, new Callback() {
                        @Override
                        public void onSuccess() {
                            Log.d("Picasso", "Image loaded from web>>>" + url);
                        }

                        @Override
                        public void onError() {
                            Log.d("Picasso", "Failed to load image online and offline, make sure you enabled INTERNET permission for your app and the url is correct>>>>>>>" + url);
                        }
                    });
                }
            });
        }
    }

    private void loadWithLogging() {
        Picasso picasso = new Picasso.Builder(getApplicationContext())
                .listener(new Picasso.Listener() {
                    @Override
                    public void onImageLoadFailed(Picasso picasso, Uri uri, Exception exception) {
                        exception.printStackTrace();
                    }
                }).build();

        picasso.load("http://developer.android.com/images/activity_lifecycle.png").into(new ImageView(this));
    }

}

Complete example in Github