Mockito Stale Mock + Silent Try/Catch Test Failures

Problem

When a PR adds new methods to a service class (e.g., AnalyticsApiService), and those methods are called from existing code paths that are wrapped in try/catch blocks, tests fail with misleading symptoms. The test assertion shows Expected: non-empty, Actual: [] or Expected: true, Actual: false — but the actual root cause is a MissingStubError being silently caught.

Context / Trigger Conditions

• Symptom: Test expects data (non-empty list, specific values) but gets empty/default
• No obvious error: No MissingStubError in test output because production code catches it
• PR context: The PR added new methods to a @GenerateMocks-annotated service class
• CI also shows: "Generated files are out of date" (stale .g.dart / .mocks.dart)
• Existing tests were passing before the PR's changes

The Failure Chain

1. PR adds method `getBulkVideoViews()` to AnalyticsApiService
2. PR adds `_enrichVideosWithBulkStats()` to HomeFeedProvider.build()
3. _enrichVideosWithBulkStats calls getBulkVideoViews (new method)
4. Generated mock is stale — doesn't have getBulkVideoViews at all
5. Even if regenerated, test setUp doesn't stub getBulkVideoViews
6. Mock throws MissingStubError when unstubbed method is called
7. Provider's try/catch catches ALL exceptions (including MissingStubError)
8. Provider falls back to empty state → test sees [] instead of expected data

Solution

Step 1: Regenerate mocks

dart run build_runner build --delete-conflicting-outputs

This updates all .g.dart and .mocks.dart files to include new methods.

Step 2: Identify new method calls in modified code paths

Trace the code path from the test's entry point. Look for any method calls on mocked services that were added or modified by the PR. Pay special attention to methods called after the already-stubbed method (e.g., enrichment/post-processing steps).

Step 3: Add stubs for ALL methods in the call chain

// In test setUp():

// Existing stub (was already there)
when(mockService.getHomeFeed(
  pubkey: anyNamed('pubkey'),
  limit: anyNamed('limit'),
)).thenAnswer((_) async => HomeFeedResult(videos: mockVideos));

// NEW stubs needed for enrichment methods added by PR
when(mockService.getBulkVideoStats(
  argThat(isA<List<String>>()),  // non-nullable positional param
)).thenAnswer((_) async => <String, BulkVideoStatsEntry>{});

when(mockService.getBulkVideoViews(
  argThat(isA<List<String>>()),  // non-nullable positional param
  maxVideos: anyNamed('maxVideos'),
  maxConcurrent: anyNamed('maxConcurrent'),
)).thenAnswer((_) async => <String, int>{});

Step 4: Handle non-nullable parameters

Mockito's any returns null which fails for non-nullable params. Use:

• argThat(isA<List<String>>()) for non-nullable positional params
• anyNamed('paramName') works for named params with default values (but NOT for non-nullable named params without defaults)

Verification

After fixing:

1. Run the specific test file: flutter test test/path/to/test.dart
2. Confirm previously failing assertions now pass
3. Run full test suite to check for no regressions

Example

Before (failing):

Expected: non-empty
  Actual: []

  test/providers/home_feed_loading_fix_test.dart 253:9

Root cause: _enrichVideosWithBulkStats called getBulkVideoStats (unstubbed) → MissingStubError → caught by provider try/catch → empty fallback state.

Fix: Added stubs in setUp() for getBulkVideoStats and getBulkVideoViews.

After (passing): All 11 tests pass, including the 2 that were failing.

Notes

• Always regenerate mocks when a PR modifies @GenerateMocks-annotated classes. The .mocks.dart files must match the current interface.
• Check the full call chain, not just the primary method. A getHomeFeed stub is useless if the provider then calls enrichVideos which calls 2 more unstubbed methods.
• This pattern is invisible in test output. The MissingStubError is caught by production error handling. You'll never see it unless you add temporary print statements inside the catch block or use @GenerateNiceMocks (which returns defaults instead of throwing, potentially masking different bugs).
• Non-nullable params in Dart null-safety: Mockito any returns null at the type level. For non-nullable positional params, use argThat(isA<Type>()) which returns a properly typed matcher.

References

• Mockito Dart - Null Safety
• build_runner documentation