android-do-not-mock

Writing Tests Without Mocking View / Activity

Chromium's custom Error Prone DoNotMock check rejects mocks and spies of android.view.View, android.app.Activity and their subclasses (RecyclerView, FrameLayout, ListView, ImageButton, ComponentActivity, ...). Mocked framework classes return defaults for everything (0, null, false), silently diverge from real behaviour, and make tests assert on implementation details. The check lives in tools/android/errorprone_plugin/src/org/chromium/tools/errorprone/plugin/ChromiumDoNotMockChecker.java.

Mocking other types is fine: Window, ViewTreeObserver, WindowInsetsController, RecyclerView.Adapter, LayoutManager, InputMethodManager, AppTask, MenuItem, DragEvent, delegates, etc.

Decision order (prefer earlier options)

1. Real object, real state. new View(context), new FrameLayout(...), Robolectric.buildActivity(Activity.class).setup().get(). Assert on observable state, not on calls.
2. Built-in Robolectric shadow. Shadows.shadowOf(x) exposes recorded state for most things you'd otherwise verify().
3. Re-judge the assert. If the only reason for a subclass is to count or capture an internal framework call, ask whether the assert is meaningful to what the test verifies. Replace it with a behavioural check, or drop it if another assert already covers the behaviour.
4. Fake a non-View collaborator. Subclass/spy/mock a LayoutManager, Adapter, PackageManager (via shadow), Window, ViewTreeObserver.
5. Minimal custom subclass (last resort). Only when production calls a method whose result must be controlled and Robolectric can't set it. Keep it to a single override, private static, and comment why it's needed.

Never re-introduce @SuppressWarnings("DoNotMock") as the fix.

Getting a real Context / Activity

Activity activity = Robolectric.buildActivity(Activity.class).setup().get();
// or: ActivityController<Activity> c = Robolectric.buildActivity(Activity.class);
//     c.create();  ... c.start().resume();  ... c.pause().stop();
Context context = ApplicationProvider.getApplicationContext();