[video_player] Avoid blocking the main thread loading video count

[jmagman]

#4639 introduced playing mp3 files with video_player. The method that checks if the player is ready needs to handle zero-sized audio files, so it called [AVAsset tracksWithMediaType:AVMediaTypeVideo] on the main thread. However, if the tracks aren't loaded, that API will synchronously load them. Docs say: "Becomes callable without blocking when the key @"tracks" has been loaded". This caused the UI to become unresponsive until the video loads.

Instead, check that the tracks are loaded, and if not, load them, then re-schedule the call to make sure the player is ready.

Unfortunately the loadTracksWithMediaType:completionHandler: we really want only became available on iOS 15.

Added more tests for controlling both audio and video media types.

Fixes flutter/flutter#97356
Blocked by test audio file PR flutter/assets-for-api-docs#177

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[jmagman]

I will update this to https://flutter.github.io/assets-for-api-docs/assets/videos/rooster.mp3 once flutter/assets-for-api-docs#177 merges. Will also update the duration integer assert.

[stuartmorgan-g]

Why a separate repo instead of landing the mp3 here and using a raw GitHub URL?

[jmagman]

Checking binaries into git repos is generally frowned upon since it never gets removed from the git history. I think assets-for-api-doc is the side-repo other flutter org repos use to localize the sin to one place.

plugins/packages/video_player/video_player/example/lib/main.dart

Line 222 in eb6e912

'https://flutter.github.io/assets-for-api-docs/assets/videos/bee.mp4',

plugins/packages/video_player/video_player_web/example/integration_test/video_player_web_test.dart

Line 27 in 7686be7

'https://flutter.github.io/assets-for-api-docs/assets/videos/bee.mp4',

We can host them somewhere else, but we should host the videos and images all together.

[jmagman]

#4720

[jmagman]

See similar logic at

plugins/packages/video_player/video_player/ios/Classes/FLTVideoPlayerPlugin.m

Lines 220 to 221 in 411f09e

	if ([asset statusOfValueForKey:@"tracks" error:nil] == AVKeyValueStatusLoaded) {
	NSArray *tracks = [asset tracksWithMediaType:AVMediaTypeVideo];

plugins/packages/video_player/video_player/ios/Classes/FLTVideoPlayerPlugin.m

Line 254 in 411f09e

[asset loadValuesAsynchronouslyForKeys:@[ @"tracks" ] completionHandler:assetCompletionHandler];

[AlexV525]

Thanks @jmagman ! I think we should do more checks with threads in the future, contributors (including me) can accidently break the thread model or cause the main thread blocked easily.