Android robolectric unit test load bitmap from drawable

Question

I am trying to load a bitmap from a drawable in my resources, but it appears to be empty when the following code is run in unit tests, but not empty when run from a normal activity:

val bitmapImage = BitmapFactory.decodeResource(context!!.resources, R.drawable.my_image)
val bos = ByteArrayOutputStream()
val quality = 10
bitmapImage.compress(Bitmap.CompressFormat.JPEG, quality, bos)
val pixels = bos.toByteArray()

I really don't understand what's happening, as the same code produces different results whether I run it in the app or in a test...

EDIT

When running this in instrumented tests, everything works well. I would still like to understand why drawables are not obtained in unit test though.

Answer 1

From what I can tell this is caused by the Robolectric Shadow implementation of BitmapFactory which doesn't actually open a resource, but just sets that the Bitmap was "created" by that resource. It is a stub implementation that doesn't actually do any work.

When Robolectric sees an object that has a Robolectric Shadow implementation, it uses the Shadow implementation instead of the real object. For more detailed information on how shadows work see the Robolectric Shadows documentation.