iOS 获取视频当前缓冲的时间 - 即可以播放的时间

- (NSTimeInterval)availableDuration {
	///获取缓冲的时间数组 ，数组中的对象是 CMTimeRangeValue 
    NSArray *timeRangeArray = _playerItem.loadedTimeRanges;
    CMTime currentTime = [_player currentTime];
    BOOL foundRange = NO;
    CMTimeRange aTimeRange = {0};
    if (timeRangeArray.count) {
        aTimeRange = [[timeRangeArray objectAtIndex:0] CMTimeRangeValue];
        if (CMTimeRangeContainsTime(aTimeRange, currentTime)) {
        /// 判断缓存的时间是否大于播放进度
            foundRange = YES;
        }
    }
    
    if (foundRange) {
        CMTime maxTime = CMTimeRangeGetEnd(aTimeRange);
        NSTimeInterval playableDuration = CMTimeGetSeconds(maxTime);
        if (playableDuration > 0) {
            return playableDuration;
        }
    }
    return 0;
}