[camera_avfoundation] handle interruptions and use single offset

[misos1]

Handle interruptions of capture sessions by doing "internal pause". Another possibility would be to remove audio input from the session but this would be too specific for interruptions caused by an incoming call or alarm. Or to stop video recording as the native ios camera app does but this would require some additional event.

Handle asynchronous error notifications posted about capture sessions. This can happen for example when there is an incoming phone call and the camera starts recording with audio. For this specific case would be enough to just call startRunning on capture sessions but that is too specific and would require ability to restart capture session from dart side as seems there is no good way how to test when such phone call or other such event ends, and trying to restart it for example every second inside FLTCam does not look like good idea. Instead the camera controller enters an error state and can be recovered by disposing and recreating which should be more generic.

Use single offset for both video and audio as separate offsets can cause unsynchronized video with audio. Seems audio timestamps tend to be ignored except for the first sample so prefer them for calculating offset to avoid desynchronization. Avoid errors when trying to append samples with timestamp less or equal to the previous sample. Fix a bug when after adjusting offset also last sample time needs to be updated to avoid adding past offsets multiple times.

• fixes [camera_avfoundation] Separate time offsets for video and audio can cause shifted audio flutter#149978
• fixes [camera]When is recording interrupted by alarm or phone call video is not synchronized with audio flutter#151253

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[misos1]

Mac_arm64 ios_platform_tests_shard_2 master
Testing failed:
	Type 'AVCaptureSession' has no member 'wasInterruptedNotification'
	Command SwiftCompile failed with a nonzero exit code
	Testing cancelled because the build failed.

I am not sure what this should mean. AVCaptureSession.wasInterruptedNotification should be there since ios 4 (I managed to run tests successfully locally on real device and also on simulator).

[hellohuanlin]

nit: can infer .zero
nit2: split into multiple lines

[hellohuanlin]

... to fix a bug where audio and video are out of sync after interrupted by phone cals.

[hellohuanlin]

when will run time error happen?

[misos1]

As is in the description "for example when there is an incoming phone call and the camera starts recording with audio". When startVideoRecording is called with enabled audio while there is an incoming call (does not need to be answered in my case, ringing is enough). Audio session then stops running so no more audio can be recorded after this happens until the session is started again or FLTCam recreated (seems the only possibility to recover from the dart side for now). Seems no reasonable detection of this state is possible from the dart side for now (sending error to dart side on this error notification enables such detection).

[hellohuanlin]

can you put some comments there?

[hellohuanlin]

this is moved to top right?

[misos1]

Yes.

[hellohuanlin]

is lastAppendedVideoSampleTime the same as lastSampleEndTime?

Can you add some doc on these 4 properties?

[misos1]

lastAppendedVideoSampleTime is related to this text from the description "Avoid errors when trying to append samples with timestamp less or equal to the previous sample.". appendPixelBuffer throws an error if a sample with specified sample time (PresentationTime) is appended after another sample with higher or equal time was already appended before (and maybe this should be end time rather than start time too).

[hellohuanlin]

am i reading it right that

(1) recordingTimeOffset covers videoTimeOffset and audioTimeOffset and

(2) lastSampleEndTime covers lastVideoSampleTime and lastAudioSampleTime?

[misos1]

Yes, lastAudioSampleTime was actually the end time of a sample so I named it accordingly. And lastVideoSampleTime was practically end time too, according to logic for offset calculation both before and after this PR, it just happens that video samples tend to not have duration which can be interpreted as zero duration in this case so end time is same as start time. For that logic to work right this needs to be end time of sample, not start time.

[hellohuanlin]

we didn't have this output. can this be just a local variable?

[misos1]

That output is either audio or video output depending on whether audio or video output should be used for offset adjusting.

[hellohuanlin]

is lastAppendedVideoSampleTime the same as lastSampleEndTime?

Can you add some doc on these 4 properties?

[stuartmorgan-g]

From triage: what's the status of this PR? It's not clear to me if it's waiting for updates, or waiting for re-review.

[misos1]

It is waiting for updates.

[misos1]

Test testImageStreamEventFormat is failing locally with XCTAssertEqual(camera.isStreamingImages, true) XCTAssertEqual failed: ("false") is not equal to ("true"). Seems waitForQueueRoundTrip is "waiting" for the wrong queue as isStreamingImages is changed on captureSessionQueue. But even with the right queue it cannot wait on something which was not already dispatched at the time waitForQueueRoundTrip was called, and the block where that variable changes may not be. There should be a better way to wait for some variable to change in xc test.

[hellohuanlin]

can you add comment on what "adjusting of recording offset" means and why we need to adjust the offset?

[misos1]

It should have been "adjusting of recording time offset" -> recordingTimeOffset which as mentioned is sum of all pauses in recording so adjusting it of course means that if there is another pause then add duration of that pause into that offset. So after that offset is applied then we get continuous recording.

[hellohuanlin]

can you add comment on why using the audio output as the outputForOffsetAdjusting

[misos1]

This is because seems that audio samples (except the first) cannot be really moved in time meaning that audioWriterInput.append accepts sample with presentation time but seems ignores that time (except for the first) and just puts next sample at time of end of previous sample so they are always continuous. I suspect that is because these audio samples are stored in a video file in a single big chunk with just single presentation time. So if audio times cannot really be adjusted then video times need to be flexed according to audio times. I cannot be really sure that this is how it works on every platform and device so in case individual audio samples can be adjusted somewhere then it should be irrelevant whether audio or video is used for such adjusting. Thus audio output is preferred for adjusting.

[hellohuanlin]

i may need some help understanding this change. Can you walk me thought this change?

[misos1]

That check was moved elsewhere (line 308).