为什么 AVPlayer 在“playbackLikelyToKeepUp”之后立即以“playbackBufferEmpty”状态失败？

Question

我有一个要播放的音频流，然后 KVO 观察了两次playbackLikelyToKeepUp，然后立即停止播放，观察了两次playbackBufferEmpty。 首先，我不明白为什么每条消息都应该调用两次 observeValueForKeyPath 。其次，playbackLikelyToKeepUp 的存在应该表明playbackBufferEmpty 不应该发生。

我应该注意到这发生在模拟器和设备中。

- (void)setRetryConnectionTimer:(NSTimer *)newTimer
{
    if (_retryConnectionTimer) { // Take care of releasing old Timers here to make things easy.
        NSLog(@"[Timer Setter] Timer Exists. Invalidating and releasing.");
        [_retryConnectionTimer invalidate];
        [_retryConnectionTimer release];
        _retryConnectionTimer = nil;
    }
    _retryConnectionTimer = [newTimer retain];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
                        change:(NSDictionary *)change context:(void *)context {
    if (object == self.audioStream && [keyPath isEqualToString:@"status"]) 
    {
        if(self.audioStream.status == AVPlayerStatusReadyToPlay)
        {
            //AVPlayer ready. Kill retry timer.
            if(self.retryConnectionTimer){
                if([self.retryConnectionTimer isValid]){
                    NSLog(@"Timer is valid. invalidating and setting to nil.");
                    [self.retryConnectionTimer invalidate];
                    self.retryConnectionTimer = nil;
                }
            }
            NSLog(@"Buffering..."); // Allow the buffer to fill some by delaying play message.
            [self.audioPlayer performSelector:@selector(play) withObject:nil afterDelay:kBufferSize];
        }
    }
    if ([keyPath isEqualToString:@"playbackBufferEmpty"]){
        NSLog(@"BUFFER EMPTY!!!"); // Buffer empty.... why when playbackShouldKeep up was just sent milliseconds ago.
        _retries = 0;
        [self.audioPlayer pause];
        self.retryConnectionTimer = [NSTimer scheduledTimerWithTimeInterval:20.0f
                                                         target:self
                                                       selector:@selector(tryReconnect:)
                                                       userInfo:nil
                                                        repeats:YES];
    }
    if ([keyPath isEqualToString:@"playbackLikelyToKeepUp"]){
        NSLog(@"playbackLikelyToKeepUp");
    }
}
- (void)tryReconnect:(NSTimer *)sender {
    NSLog(@"tryReconnect Called. Retry: %i", _retries);
    if (_retries <= kReconnectRetries) {
        [self restartStream];
        _retries ++;
    } else {
        NSLog(@"Connection Dropped: invalidating Timer.");
        [self.retryConnectionTimer invalidate];
        self.retryConnectionTimer = nil;
        _retries = 0;
    }
}

- (void)restartStream
{
    [self.audioStream removeObserver:self forKeyPath:@"status"];
    [self.audioStream removeObserver:self forKeyPath:@"playbackBufferEmpty"];
    [self.audioStream removeObserver:self forKeyPath:@"playbackLikelyToKeepUp"];
    self.audioStream = [AVPlayerItem playerItemWithURL:[NSURL URLWithString:@"http://myaudiostream.com:9094"]];
    [self.audioStream addObserver:self forKeyPath:@"status" options:0 context:nil];
    [self.audioStream addObserver:self forKeyPath:@"playbackBufferEmpty" options:NSKeyValueObservingOptionNew context:nil];
    [self.audioStream addObserver:self forKeyPath:@"playbackLikelyToKeepUp" options:NSKeyValueObservingOptionNew context:nil];
    NSLog(@"Playing audioPlayer with new AVPlayerItem");
    self.audioPlayer = [AVPlayer playerWithPlayerItem:self.audioStream];
}

来自日志。

2014-01-04 13:32:48.121 Audio Test [833:70b] Playing audioPlayer with new AVPlayerItem
2014-01-04 13:32:55.749 Audio Test [833:70b] Buffering... 
2014-01-04 13:32:55.752 Audio Test [833:70b] playbackLikelyToKeepUp
2014-01-04 13:36:00.266 Audio Test [833:70b] playbackLikelyToKeepUp
2014-01-04 13:36:00.267 Audio Test [833:70b] BUFFER EMPTY!!!
2014-01-04 13:36:00.267 Audio Test [833:70b] Creating new timer and assigning to self.retryConnectionTimer
2014-01-04 13:36:03.701 Audio Test [833:70b] BUFFER EMPTY!!!
2014-01-04 13:36:03.702 Audio Test [833:70b] Creating new timer and assigning to self.retryConnectionTimer
2014-01-04 13:36:03.702 Audio Test [833:70b] [Timer Setter] Timer Exists. Invalidating and releasing.
2014-01-04 13:36:07.171 Audio Test [833:70b] playbackLikelyToKeepUp
2014-01-04 13:36:23.697 Audio Test [833:70b] tryReconnect Called. Retry: 0
2014-01-04 13:36:23.698 Audio Test [833:70b] Playing audioPlayer with new AVPlayerItem
2014-01-04 13:36:34.002 Audio Test [833:70b] Buffering... 
2014-01-04 13:36:34.002 Audio Test [833:70b] Timer is valid. invalidating and setting to nil.
2014-01-04 13:36:34.002 Audio Test [833:70b] [Timer Setter] Timer Exists. Invalidating and releasing.

Answer 1

你真的在检查结果吗？ playbackLikelyToKeepUp 是 BOOL，所以它很可能是 NO。