mock-thumbnail-engine

Test code that uses cached_video_thumbnail hermetically

The package has one designed test seam, and it makes everything else unnecessary: ThumbnailEngine.forTesting(extractor:, directory:) builds a fully real engine (scheduler, disk cache, metrics) on top of an injected ThumbnailExtractor fake and a temp directory. The only thing faked is the native decode. Never mock the Pigeon channel, path_provider, or the engine class itself.

Step 1 - make the code under test injectable

ThumbnailEngine.instance has no setter, and a widget that hardcodes VideoThumbnailImage(source) resolves through the real singleton (-> MissingPluginException under flutter test). Do NOT work around that with ImageCache seeding tricks; fix the seam - it is a one-parameter change:

class MyVideoTile extends StatelessWidget {
  const MyVideoTile({super.key, required this.videoUrl, this.engine});
  final String videoUrl;
  final ThumbnailEngine? engine;   // test seam; null = production singleton

  @override
  Widget build(BuildContext context) => Image(
    image: VideoThumbnailImage(
      VideoSource.network(Uri.parse(videoUrl)),
      spec: const ThumbnailSpec(maxWidth: 320, maxHeight: 320),
      engine: engine,   // public named parameter, exists for exactly this
    ),
    fit: BoxFit.cover,
  );
}

Note: VideoThumbnailImage's engine: parameter is declared as a Dart private named parameter (this._engine) - it does not appear in some API listings, but callers pass it as engine:. Non-widget code should take a ThumbnailEngine engine constructor/function parameter the same way.

Step 2 - copy the fake extractor

Take fake_thumbnail_extractor.dart into the app's test/support/. Contract it honors (required of any fake): write real decodable image bytes into destPath (the engine commits that file into its cache and the provider really decodes it), return the true dimensions, throw ThumbnailException for failure cases - never return without producing the file.

Step 3 - build the engine per test

late Directory root;
late FakeThumbnailExtractor extractor;
late ThumbnailEngine engine;

setUp(() async {
  root = await Directory.systemTemp.createTemp('thumb_test');
  extractor = FakeThumbnailExtractor();
  engine = ThumbnailEngine.forTesting(
    extractor: extractor,
    directory: Directory('${root.path}/cache'),
  );
  PaintingBinding.instance.imageCache..clear()..clearLiveImages();
});
tearDown(() => root.delete(recursive: true));

forTesting is @visibleForTesting: the analyzer allows it in test/ and integration_test/ directories; if it warns elsewhere, the call is in the wrong place. Each test gets a fresh engine + directory - engines are cheap and isolated; never share cache dirs between tests.

Step 4 - write the tests

Engine-level (plain test, no pumping needed):

final thumb = await engine.getThumbnail(
  VideoSource.asset('assets/v.mp4'),
  spec: const ThumbnailSpec(maxWidth: 64, maxHeight: 64),
);
expect(thumb.wasCached, isFalse);
expect(extractor.callCount, 1);
// Second call: cache hit, no new extraction.

Widget-level (testWidgets): wrap the pipeline in tester.runAsync - real file IO and image decoding never complete inside the fake-async test zone:

await tester.runAsync(() async {
  await tester.pumpWidget(MaterialApp(
    home: MyVideoTile(videoUrl: url, engine: engine),
  ));
  // Let extraction + decode land; then settle frames.
  await Future<void>.delayed(const Duration(milliseconds: 50));
});
await tester.pump();
final raw = tester.widget<RawImage>(find.byType(RawImage));
expect(raw.image?.width, 2);   // the fake's 2x2 PNG

Behavior cases the fake supports: failWith = ThumbnailException(...) to drive errorBuilder paths; gated = true + release() to test loading states and cancellation; delay for latency; cancelled list to assert cancellation reached the extractor.

Sharp edges

• Assert against a 2x2 image, not 1x1: when a provider's request is cancelled after listener detach, the stream settles with an internal 1x1 transparent pixel - a 1x1 assertion can pass for the wrong reason. The bundled fake writes a 2x2 PNG deliberately.
• fakeAsync/FakeAsync does not play well with the engine's real file IO - disk cache writes are genuine async IO that fake clocks do not advance. Use real async (runAsync, pumpEventQueue); reserve fake clocks for pure scheduler-timing units, as the package's own tests do.
• The provider cancels on last-listener detach: a widget disposed before decode completes will record a cancellation, not a failure. Use the gated fake to make such races deterministic instead of sleeping.
• Widget-test assertions on metrics counters: prefer counters over timing percentiles; percentiles are null until samples exist.
• Do not construct ThumbnailEngine.instance in host tests at all; even though construction is currently inert, only the injected engine is guaranteed hermetic.

Failure handling

If forTesting or the engine: parameter is missing in the installed version, this skill predates or postdates that API - read the installed package source in .pub-cache (lib/src/engine.dart, lib/src/image_provider.dart) and adapt; say which version you found.

Example scenario

"CI fails with MissingPluginException in golden tests of our video feed." -> add the engine seam to the feed widget, inject ThumbnailEngine.forTesting with the bundled fake (instant 2x2 PNGs), runAsync around pumps, goldens now deterministic and offline.