improve the iOS camera plugin thread model

[hellohuanlin]

Use case

This issue will be the main issue that tracks various threading problems in iOS camera plugin.

Proposal

We wrote up a formal proposal to improve the thread model, with detailed execution plan in the end. There are also a couple non-threading issues that we'd like to address in the spirit of "yak shaving".

Action Items

We need to fix these threading related issues:

• [x] Rename "dispatchQueue" to "sessionQueue"
• [x] Dispatch engine API calls on iOS main queue
• [x] Handle all SampleBuffer callbacks on the session queue
• [x] Write photo file in IO queue
• [x] Thread safety for state access
  • [x] Fix _dispatchQueue race condition which is causing a known crash
  • [x] Fix FLTSavePhotoDelegate's reference
  • [x] Fix _camera ivar
  • [x] Fix dispatch orientation change on the session queue (_deviceOrientation, _lockedCaptureOrientation, _isRecording)
• [x] Pixel buffer with queues

These are optional cleanups not related to threading:

• [ ] Handle system pressure for video writes
• [x] Remove selfReference https://github.com/flutter/plugins/pull/4780
• [x] Break up CameraPlugin.m
• [ ] Clean up properties and ivars

[jmagman]

I think this is already what https://github.com/flutter/flutter/issues/52578 is tracking. Can we close this, and you can just add a comment to that issue with a link to your proposal?

[hellohuanlin]

@jmagman Looks like #52578 only tracks the crash with calling engine API on the background thread. Though either way is fine for me.

[hellohuanlin]

Re-opened this issue to track various threading related tasks in the proposal

[hellohuanlin]

im closing this issue since all required action items are complete.

[github-actions[bot]]

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