[camera_avfoundation] fix race condition when starting image stream on iOS

[js2702]

Fixes flutter/flutter#147702

It's not easy to reproduce this with the example application (as shown in the linked issue). We've been able to reproduce it in our own application which makes heavy usage of Platform channels and different native plugins. After digging into the code we notice that:

The Dart code that starts listening to the event channel was being executed before initializing the stream in the native side.

packages/packages/camera/camera_avfoundation/lib/src/avfoundation_camera.dart

Line 271 in 9744c9d

const EventChannel cameraEventChannel =

packages/packages/camera/camera_avfoundation/ios/camera_avfoundation/Sources/camera_avfoundation/FLTThreadSafeEventChannel.m

Line 31 in 9744c9d

[self.channel setStreamHandler:handler];

I don't have much experience with Objective-C, so not sure how to create unit tests for that if necessary but I believe this would be an adecuate solution. I've seen other methods in the plugin that use the same strategy passing the completion object.

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[js2702]

I'm not sure why but I'm getting a 400 error when signing the CLA (Which I thought I had already signed for other PR)

[hellohuanlin]

This will require unit test in order to land.

[js2702]

I'm looking for any example test but I don't see anything. As this doesn't affect the Dart part I don't see how to make a test for this, wouldn't this be test exempt?

Additionally, this bug in particular manifests as a race condition in the communication with Dart and native, requiring an end to end test, which I'm not seeing either.

[stuartmorgan]

As this doesn't affect the Dart part I don't see how to make a test for this, wouldn't this be test exempt?

There is no blanket test exemption for native code; if there were, almost nothing in plugins would be tested. Please see https://github.com/flutter/flutter/blob/master/docs/ecosystem/testing/Plugin-Tests.md

[js2702]

I added a test for the issue. Everything was working before this last Swift refactor. A few things:

1. Now these tests that were just changed in StreamingTests.swift (I did not change this file in the PR, as I'm not sure why this is needed now) fail because they are not async/await. Not sure why this happens as I don't see anything that would need async. These are the calls that fail:

https://github.com/SkillDevs/flutter_packages/blob/423c038bdb06ee8d75a2e6523e1ce688e0b555a0/packages/camera/camera_avfoundation/example/ios/RunnerTests/StreamingTests.swift#L74

2. In my tests, I'm not sure if there is a better way to expect the streamHandler is initialized instead of checking the boolean flag.
3. Also, in this completion call
   https://github.com/SkillDevs/flutter_packages/blob/423c038bdb06ee8d75a2e6523e1ce688e0b555a0/packages/camera/camera_avfoundation/ios/camera_avfoundation/Sources/camera_avfoundation/FLTCam.m#L1206
   Should the completion be at the beginning, on each early return, or it's fine where it is?

Again, not an expert in Swift/Objective-C.

Also, the test was developed in Master and it failed unless you added a delay before the expect.

[js2702]

Ended up adding the async/await because the tests pass with that, I guess it's because of the callback added to the function?