When you’re writing unit tests for your Android applications, you will often need to handle interactions with third party APIs. In the above image we’re using the Picasso Image Loading library in our classes. Picasso has a very clean API, so it’s relatively simple to work with; but with other libraries you may not be so fortunate. So what can we do to simplify our interactions with those APIs and also set ourselves up for more maintainable code in the future?

We can encapsulate the calls to Picasso (or whatever API) into another class that allows us to control the API and make assertions against it in our tests. So here’s an example:

Initial Code

// MyClass class

public void loadImageFromUrl(String imageUrl) {
    presenter.setCurrentPreviewImageUrl(imageUrl);
    if (showThumbImage) {
        Picasso.with(context)
               .load(imageUrl)
               .into(thumbImageView);
    }
}

So in this method, loadImageFromUrl, we want to make sure that our Presenter is being called and that when the showThumbImage variable is set to true, we call the method to load the image. We want to do this of course, without actually loading the image since we’re attempting to verify this behavior inside of a unit test.

For our use case we will need two new classes: ImageLoader and ImageLoaderImpl. The first will be an interface that we will use to expose the desired API, and the second will be a class that implements that interface and wraps the calls to the third party API.

Updated Code

// ImageLoader interface

public interface ImageLoader {

    void loadImage(String imageUrl, ImageView imageView);

}

// ImageLoaderImpl class

public class ImageLoaderImpl implements ImageLoader {
  
    // fields omitted

    @Override
    public void loadImage(String imageUrl, ImageView imageView) {
        Picasso.with(context).load(imageUrl).into(imageView);
    }
}

// loadImageFromUrl method

public void loadImageFromUrl(String imageUrl) {
    presenter.setCurrentPreviewImageUrl(imageUrl);
    if (showThumbImage) {
        imageLoader.loadImage(imageUrl, thumbImageView);
    }
}

Unit Test

// MyClass

@Mock private ImageLoader imageLoader;

@Test
public void shouldLoadImageWhenShowThumbIsTrue() throws Exception {
    myClass.setShowThumbImage(true);

    myClass.loadImageFromUrl("http://www.myimageurl.png");

    verify(imageLoader).loadImage(anyString(),
                                  any(ImageView.class));
}

@Test
public void shouldNotLoadImageWhenShowThumbIsFalse() throws Exception {
    myClass.setShowThumbImage(false);

    myClass.loadImageFromUrl("http://www.myimageurl.png");

    verifyZeroInteractions(imageLoader);
}