[camera_android_camerax] Fixes crash on hot restart caused by stale observers

[raju-muliyashiya]

fixes Issue #176451

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Description

• Fixes a crash on hot restart where stale Observer instances from the previous run remained attached to LiveData.
• The fix ensures callbacks from these untracked observers are safely ignored instead of crashing app.

[gemini-code-assist]

Code Review

This pull request addresses a crash on hot restart caused by stale observers. The fix involves adding a check in ObserverProxyApi.java to verify that an observer instance is still tracked in the InstanceManager before its onChanged callback is invoked. This prevents calls to Dart for observers that no longer have a corresponding Dart object. A new test has been added in ObserverTest.java to confirm that callbacks for untracked observers are ignored, and an existing test was updated to be more explicit about testing the valid observer path. The package version and changelog have been updated accordingly. The changes are correct and well-tested. I have one suggestion to refactor the test code to improve maintainability.

[gemini-code-assist]

To improve maintainability and reduce code duplication, you can extract the common test setup into a method annotated with @org.junit.Before. This will make the tests cleaner and easier to read by separating setup logic from the test logic itself.

  private ObserverProxyApi mockApi;
  private TestProxyApiRegistrar registrar;
  private ObserverProxyApi.ObserverImpl<String> instance;

  @org.junit.Before
  public void setUp() {
    mockApi = mock(ObserverProxyApi.class);
    registrar = new TestProxyApiRegistrar();
    when(mockApi.getPigeonRegistrar()).thenReturn(registrar);
    instance = new ObserverProxyApi.ObserverImpl<>(mockApi);
  }

  @Test
  public void onChanged_makesExpectedCallToDartCallback() {
    // Add the observer to the instance manager to simulate normal operation
    registrar.getInstanceManager().addDartCreatedInstance(instance, 0);

    final String value = "result";
    instance.onChanged(value);

    verify(mockApi).onChanged(eq(instance), eq(value), any());
  }

  @Test
  public void onChanged_doesNotCallDartCallbackWhenObserverNotInInstanceManager() {
    final String value = "result";
    instance.onChanged(value);

    // Verify that the Dart callback is NOT invoked for stale observers
    verify(mockApi, never()).onChanged(any(), any(), any());
  }

[camsim99]

Thanks for this fix! I think this would definitely catch the issue and I'm open to submitting it. My only concern is that I think there's also a way to better address the root of the problem.

The root of the problem seems to be that the plugin does not handle hot restart appropriately. I see that on dispose, we remove the observers, but this won't get called on hot restart. I think we need to handle this case when the camera is re-initialized, likely in one of the createCameraWithSettings or initialize methods. Did you possibly look into this?

[raju-muliyashiya]

Some thing like this inside createCameraWithSettings?

if (processCameraProvider != null) {
  await processCameraProvider!.unbindAll();
}

if (liveCameraState != null) {
  await liveCameraState!.removeObservers();
  liveCameraState = null;
}

[bparrishMines]

I think a better solution for this would be to update pigeon to do this check. It seems like a common use case where a callback is made after a hot restart. I can create a quick PR for it.

ProxyApiRegistrar.ignoreCallsToDart is called on the native side when the plugin is detached to prevent this crash, but it looks like we need something that also works for hot restarts.

Also there is still a small chance that this crash can still happen with this fix if the Observer makes a call to Dart before it is cleared from the InstanceManager. But this has to do with the fact that plugins are not notified when there is a hot restart.