您可以为这些键添加观察者:

you can add observer for those keys:

[playerItem addObserver:self forKeyPath:@"playbackBufferEmpty" options:NSKeyValueObservingOptionNew context:nil];
[playerItem addObserver:self forKeyPath:@"playbackLikelyToKeepUp" options:NSKeyValueObservingOptionNew context:nil];

第一个会在您的缓冲区为空时警告您，第二个会在您的缓冲区可以再次使用时发出警告.

The first one will warn you when your buffer is empty and the second when your buffer is good to go again.

然后要处理密钥更改，您可以使用以下代码:

Then to handle the key change you can use this code:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
                        change:(NSDictionary *)change context:(void *)context {
    if (!player)
    {
        return;
    }

    else if (object == playerItem && [keyPath isEqualToString:@"playbackBufferEmpty"])
    {
        if (playerItem.playbackBufferEmpty) {
            //Your code here
        }
    }

    else if (object == playerItem && [keyPath isEqualToString:@"playbackLikelyToKeepUp"])
    {
        if (playerItem.playbackLikelyToKeepUp)
        {
            //Your code here
        }
    }
}