[video_player] fix: add missing isPlaybackLikelyToKeepUp check.

[unknown-undefined]

When watching a live stream, if the playback buffers but the AVPlayerItem is likely to keep up, it is necessary to recheck AVPlayerItem.isPlaybackLikelyToKeepUp. If isPlaybackLikelyToKeepUp, a bufferingEnd event should be immediately triggered. I am encountering an issue with my product where, when watching a live stream, if I seek to the latest time, it continuously bufferingStart and does not come to an bufferingEnd.

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[tarrinneal]

Seems you're still working on this, but I'll just reiterate what the bot already said. You'll need to add some tests to this pr before we can merge it.

[unknown-undefined]

Hi @tarrinneal , I've added the necessary test case and all tests have passed, both for replay and livestream.

[tarrinneal]

Seems good to me, I'll let @hellohuanlin do final review.

[hellohuanlin]

I'm not sure if isPlaybackLikelyToKeepUp is a good indicator for bufferingStart and bufferingEnd events. From the doc this flag is just a "prediction", which may not be correct.

[hellohuanlin]

did you hit this call? When playback buffer is empty, is it possible for isPlaybackLikelyToKeepUp to be true at all?

[unknown-undefined]

I have tested this with below code and through OBS livestreaming, and have found that it is possible for both the 'isPlaybackLikelyToKeepUp' flag to be true and for the playback buffer to be empty at the same time while watching a livestream.

import AVKit
import UIKit

class ViewController: UIViewController {
    var isPlaybackBufferEmptyListener: NSKeyValueObservation?
    var isPlaybackBufferFullListener: NSKeyValueObservation?
    var isPlaybackLikelyToKeepUpListener: NSKeyValueObservation?

    @IBAction func onClickPlay(_ sender: Any) {
        /// Test with a live stream URL, it's m3u8 has no end tag
        guard let url = URL(string: "Test with a live stream URL") else { return }

        // guard let url = URL(string: "https://flutter.github.io/assets-for-api-docs/assets/videos/hls/bee.m3u8") else { return }

        let item = AVPlayerItem(url: url)

        isPlaybackBufferEmptyListener?.invalidate()
        isPlaybackBufferEmptyListener = item.observe(\.isPlaybackBufferEmpty, options: [.initial, .new]) { _, change in
            guard let new = change.newValue else { return }
            print("isPlaybackBufferEmpty ---> \(new)")
        }

        isPlaybackBufferFullListener?.invalidate()
        isPlaybackBufferFullListener = item.observe(\.isPlaybackBufferFull, options: [.initial, .new]) { _, change in
            guard let new = change.newValue else { return }
            print("isPlaybackBufferFull ---> \(new)")
        }

        isPlaybackLikelyToKeepUpListener?.invalidate()
        isPlaybackLikelyToKeepUpListener = item.observe(\.isPlaybackLikelyToKeepUp, options: [.initial, .new]) { _, change in
            guard let new = change.newValue else { return }
            print("isPlaybackLikelyToKeepUp ---> \(new)")
        }

        let player = AVPlayer(playerItem: item)
        let controller = AVPlayerViewController()
        controller.player = player
        present(controller, animated: true) {
            player.play()
        }
    }
}

[hellohuanlin]

Could you post your log here? Thanks!

[unknown-undefined]

Yes. the the log,

isPlaybackBufferFull ---> false
isPlaybackLikelyToKeepUp ---> false
isPlaybackBufferFull ---> true
isPlaybackBufferFull ---> false
isPlaybackLikelyToKeepUp ---> true


>>>>>>> SEEK BEGIN >>>>>>>
isPlaybackLikelyToKeepUp ---> false
<<<<<<<< SEEK END <<<<<<<<
isPlaybackLikelyToKeepUp ---> true


>>>>>>> SEEK BEGIN >>>>>>>
isPlaybackLikelyToKeepUp ---> false
<<<<<<<< SEEK END <<<<<<<<
isPlaybackLikelyToKeepUp ---> true

Based on the aforementioned logs, it is evident that the isPlaybackBufferFull consistently remains false, while the isPlaybackLikelyToKeepUp can be either true or false during seeking. I have uploaded the testing project to AVPlayer_livestream. Please feel free to run it and review the provided logs.

[hellohuanlin]

i was asking the opposite situation - isPlaybackBufferEmpty

[unknown-undefined]

Sorry, @hellohuanlin, I realized that I missed including the first line of the log. Here's the revised version:

isPlaybackBufferEmpty ---> true
isPlaybackBufferFull ---> false
isPlaybackLikelyToKeepUp ---> false
isPlaybackBufferFull ---> true
isPlaybackBufferFull ---> false
isPlaybackLikelyToKeepUp ---> true


>>>>>>> SEEK BEGIN >>>>>>>
isPlaybackLikelyToKeepUp ---> false
<<<<<<<< SEEK END <<<<<<<<
isPlaybackLikelyToKeepUp ---> true

Please note that the isPlaybackBufferEmpty always remains true, while the isPlaybackLikelyToKeepUp can be either true or false during seeking.

[hellohuanlin]

this is surprising. i'd expect isPlaybackBufferEmpty to be false most of the time. Do you have some insight on this?

[unknown-undefined]

It seems like there is a bug in AVFoundation related to the isPlaybackBufferEmpty key-value observing (KVO) not being triggered when its value changes. As a workaround for now, we may check the isPlaybackLikelyToKeepUp property within the playbackBufferEmptyContext and playbackBufferFullContext. refer log:

[KVO] isPlaybackBufferEmpty ---> true
[KVO] isPlaybackBufferFull ---> false
[KVO] isPlaybackLikelyToKeepUp ---> false  (crrent isPlaybackBufferEmpty ---> true)
2023-05-25 13:26:31.528310+0900 Lab_AVPlayer_livestream[1757:465625] Metal API Validation Enabled
[KVO] isPlaybackBufferFull ---> true
[KVO] isPlaybackBufferFull ---> false
[KVO] isPlaybackLikelyToKeepUp ---> true  (crrent isPlaybackBufferEmpty ---> false)

I tested this behavior on iPadOS 16.5 using a real device. You can also check the provided demo AVPlayer_livestream for further reference.

[hellohuanlin]

This behavior needs to be heavily documented in our code (since it's not mentioned in apple's doc), otherwise it's too hard to understand in the future. Could you add some comments explaining why we need this workaround?

[unknown-undefined]

Hello @hellohuanlin Before I proceed with adding comments about the necessity of the isPlaybackLikelyToKeepUp check when observing isPlaybackBufferFull and isPlaybackBufferEmpty using KVO, I would like to seek clarification on the exact meaning of isBuffering within the video_player module. I have raised this question in detail here.

Based on my understanding, the evaluation of isBuffering relies on the Key-Value Observing (KVO) of isPlaybackBufferFull, isPlaybackBufferEmpty, and isPlaybackLikelyToKeepUp.

[hellohuanlin]

from the doc, when buffer is full, it's still possible for isPlaybackLikelyToKeepUp to return NO. Do we want to emit bufferingStart then?

[unknown-undefined]

Yes, that is correct. Since the KVO code related to 'playbackBufferEmptyContext' is already responsible for emitting the 'bufferingStart' event, there is no need to emit it here.

[hellohuanlin]

I meant, do we need an else statement here (similar to playbackBufferEmptyContext)? I am having some trouble understanding why these 2 are different.

[unknown-undefined]

I believe I may have overlooked adding the same logic to the playbackBufferEmptyContext. As stated in the doc the isPlaybackLikelyToKeepUp property is a Boolean value that indicates whether the item will likely play through without stalling. I have already pushed the new commit.

[unknown-undefined]

As stated in the doc the isPlaybackLikelyToKeepUp property is a Boolean value that indicates whether the item will likely play through without stalling. While the isBuffering property in the video_player plugin may be the only way to check if the video is currently stalled, it is important to note that it refers to a different aspect of playback than isPlaybackLikelyToKeepUp. So, it may be a separate topic altogether.

[unknown-undefined]

Based on the code in FLTVideoPlayerPlugin.m at line 313, the isPlaybackLikelyToKeepUp property is used to indicate the bufferingEnd event. Therefore, using isPlaybackLikelyToKeepUp as an indicator for both the bufferingStart and bufferingEnd events ensures consistent behavior.

[hellohuanlin]

Based on the code in FLTVideoPlayerPlugin.m at line 313, the isPlaybackLikelyToKeepUp property is used to indicate the bufferingEnd event.

The line may have changed. could you link here? Thanks!

[unknown-undefined]

Hello @hellohuanlin, the link FLTVideoPlayerPlugin.m#LL332C4-L339C4

[hellohuanlin]

Hello @hellohuanlin, the link FLTVideoPlayerPlugin.m#LL332C4-L339C4

I'm a bit confused here. Simply observing isPlaybackLikelyToKeepUp is not enough? Is it missing notification? If so, it seems like a bug in AVFoundation.

[unknown-undefined]

@hellohuanlin, based on my testing log, it appears that simply observing isPlaybackLikelyToKeepUp is sufficient for checking if playback is stalling or not. However, the Key-Value Observing (KVO) for isPlaybackBufferEmpty or isPlaybackBufferFull does not always work correctly. It's a bug. There is a related issue: https://developer.apple.com/forums/thread/669541

Here is the log snippet from my test:

[KVO] isPlaybackBufferEmpty ---> true
[KVO] isPlaybackBufferFull ---> false
[KVO] isPlaybackLikelyToKeepUp ---> false  (crrent isPlaybackBufferEmpty ---> true)
2023-05-25 13:26:31.528310+0900 Lab_AVPlayer_livestream[1757:465625] Metal API Validation Enabled
[KVO] isPlaybackBufferFull ---> true
[KVO] isPlaybackBufferFull ---> false
[KVO] isPlaybackLikelyToKeepUp ---> true  (crrent isPlaybackBufferEmpty ---> false)

I conducted this test on iPadOS 16.5 using a real device. You can also refer to the provided demo AVPlayer_livestream for further clarification.

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