Creating Custom Shadows

Custom shadows are a Robolectric feature that allows you to make targeted changes in the way Android functions under test. This could be anything from capturing simply that a method was called, to inserting code that interacts with test objects, to doing nothing at all.

Custom shadows allow you to include shadow functionality in only some of your test code, as opposed to adding or modifying a Shadow in Robolectric source. They also allow your shadow to refer to domain specific context, like domain objects in your test classes.

Writing a Custom Shadow

Custom shadows are structured much the same as normal shadow classes. They must include the @Implements(AndroidClassName.class) annotation on the class definition. You can use the normal shadow implementation options, such as shadowing instance methods using @Implementation or shadowing constructors using public void __constructor__(...).

@Implements(Bitmap.class)
public class MyShadowBitmap {
    @RealObject private Bitmap realBitmap;
    private int bitmapQuality = -1;

    @Implementation
    public boolean compress(Bitmap.CompressFormat format, int quality, OutputStream stream) {
      bitmapQuality = quality;
      return realBitmap.compress(format, quality, stream);
    }

    public int getQuality() {
      return bitmapQuality;
    }
  }
}

Using a Custom Shadow

Custom Shadows get hooked up to Robolectric using the @Config annotation on the test class or test method, using the shadows array attribute. To use the MyShadowBitmap class mentioned in the previous section, you would annotate the test in question with @Config(shadows={MyShadowBitmap.class}), and to include multiple custom shadows: @Config(shadows={MyShadowBitmap.class, MyOtherCustomShadow.class}). This causes Robolectric to recognize and use your custom shadow when executing code against the class you shadowed.

However, the Robolectric.shadowOf() method will not work with custom shadows, as it has to be implemented in Robolectric for each shadow class. You can instead use Robolectric.shadowOf_() and cast the return value to the custom Shadow class you implemented.

Also, if you choose to shadow an Android class that already is shadowed in Robolectric, you will replace the Robolectric shadow. You could try inheriting from the Robolectric shadow if you still need the base shadow functionality.