[camera] Permissions request are not well-awaiting on iOS

[AlexV525]

Related to #96429. It's a serious bug that breaks lifecycle control flow on iOS. This issue could be introduced by flutter/plugins#4140.

Steps to Reproduce

1. Run the example of the camera plugin like the first install (uninstall first if it's installed).
2. Request for a camera instance.
3. Exception is thrown.

======== Exception caught by widgets library =======================================================
The following assertion was thrown building CameraPreview:
A CameraController was used after being disposed.

Once you have called dispose() on a CameraController, it can no longer be used.

Details

From the screenshot, we can recognize that the initialize method completes before permissions requests are done. This could be affected by the thread model changing of implementations on iOS.

Once the new controller has been given to the field, the lifecycle listener is triggered a bit later, which will dispose the controller in a short time, causing a race condition issue.

And after the app is resumed, the controller will initialize again.

Environment

camera: 0.9.4+6

[AlexV525]

/cc @renefloor @hellohuanlin

[hellohuanlin]

I did a bisect and verified that this was introduced by flutter/plugins#4140 (v0.9.4+3).

[hellohuanlin]

I did some initial investigation:

Comparing with apple's AVCam sample project, looks like we are missing the logic that explicitly requests permission. The current permission alert is triggered implicitly by some of the AVFoundation calls which does not have completion callbacks (e.g. AVCaptureDeviceInput::deviceInputWithDevice:error:). This happened to work fine previously because this call blocked the main thread (which wasn't ideal either), and flutter/plugins#4140 that moves this operation off the main thread introduced the regression.

So in conclusion, I think a good solution is to explicitly request permission with a completion callback like what Apple did in the sample project. I will re-visit this after other threading works are done in #96429

[AlexV525]

So in conclusion, I think a good solution is to explicitly request permission with a completion callback like what Apple did in the sample project. I will re-visit this after other threading works are done in #96429

Thanks for the clarification. Though I would suggest a fix for a higher priority since it breaks most cases with the lifecycle hook.

[hellohuanlin]

Update: I recently wrapped up the thread model effort (#96429), and will be working on this permission issue now.

[hellohuanlin]

Update: There are API changes that may involve other platforms. I wrote up a draft proposal. It's still in progress and there are more investigation to be done.

[AlexV525]

I wrote up a draft proposal.

The proposal looks detailed and fluent overall even it's incompleted, great work!

[hellohuanlin]

Update: Just finished investigating all the platforms (iOS/Android/Web/Windows) and finalized the proposal.