MPNowPlayingInfoCenter投掷EXC_BAD_ACCESS

Question

我正在制作一个回放音频的应用程序，我已经设置了它，以便通过MPNowPlayingInfoCenter更新锁定屏幕，但我遇到了一个问题。

在看似随机的时间，我得到一个EXC_BAD_ACCESS错误时，试图更新现在播放的信息。

下面是这样做的代码：

- (void)updatePlayback
{
    if(!active)
        return;

    NowPlayingController* npc = [AudioController nowPlayingController];
    CMTime elapsed = player.currentTime;
    Float64 elInterval = CMTimeGetSeconds(elapsed);
    [npc setElapsed:elInterval];

    CMTime duration = player.currentItem.duration;
    Float64 durInterval = CMTimeGetSeconds(duration);
    [npc setRemaining:ceilf(durInterval - elInterval)];

    [npc setPlayPauseValue:isPlaying];
    if(durInterval > 0)
    {
        [npc setProgressValue:elInterval/durInterval];
        [npc setAudioDuration:durInterval];
    }

    _activeMetadata[MPMediaItemPropertyPlaybackDuration] = @(durInterval);
    _activeMetadata[MPNowPlayingInfoPropertyPlaybackRate] = @(isPlaying);
    _activeMetadata[MPNowPlayingInfoPropertyElapsedPlaybackTime] = @(elInterval);

    MPNowPlayingInfoCenter* npInfoCenter = [MPNowPlayingInfoCenter defaultCenter];
    if(npInfoCenter && _activeMetadata)
    {
        if([npInfoCenter respondsToSelector:@selector(setNowPlayingInfo:)])
        {

//////////THE FOLLOWING LINE TRIGGERS EXC_BAD_ACCESS SOMETIMES////////////
            [npInfoCenter setNowPlayingInfo:_activeMetadata];
        }

    }
}

99.9%的情况下，这是可行的，但有时当应用程序退出后台或更改音频文件时，或者只是随机地，

[npInfoCenter setNowPlayingInfo:_activeMetadata];

抛出EXC_BAD_ACCESS。

此外，_activeMetadata声明为：

@property (atomic, strong, retain) NSMutableDictionary* activeMetadata;

在创建AVPlayer时实例化它：

    AVAsset* asset = [AVAsset assetWithURL:[NSURL fileURLWithPath:path]];
    AVPlayerItem* playerItem = [AVPlayerItem playerItemWithAsset:asset];
    player = [AVPlayer playerWithPlayerItem:playerItem];

    CMTime duration = player.currentItem.duration;
    NSTimeInterval durInterval = CMTimeGetSeconds(duration);
    NSLog(@"%f", durInterval);

    MPMediaItemArtwork* albumArtwork = [[MPMediaItemArtwork alloc] initWithImage:[downloader useCachedImage:CacheKeySeriesBanners withName:nil withURL:info[@"image"]]];
    NSDictionary* nowPlayingInfo = @{MPMediaItemPropertyTitle:ptString,
                                     MPMediaItemPropertyArtist:spString,
                                     MPMediaItemPropertyArtwork:albumArtwork,
                                     MPMediaItemPropertyAlbumTitle:info[@"title"],
                                     MPMediaItemPropertyPlaybackDuration:@(durInterval),
                                     MPNowPlayingInfoPropertyPlaybackRate:@(1),
                                     MPNowPlayingInfoPropertyElapsedPlaybackTime:@(0)};
    [[MPNowPlayingInfoCenter defaultCenter] setNowPlayingInfo:nowPlayingInfo];

    _activeMetadata = [nowPlayingInfo mutableCopy];

updatePlayback是通过每个帧上的CADisplayLink调用的。

有什么可以导致例外的原因吗？

Answer 1

我想你给setNowPlayingInfo打电话太频繁了。当然，它不应该崩溃，但是没有必要使用CADisplayLink每秒调用它60次。

那你为什么经常打电话给它？如果这是因为你想要进度条顺利跟踪，仍然没有必要。来自MPNowPlayingInfoPropertyElapsedPlaybackTime声明：

// The elapsed time of the now playing item, in seconds.
// Note the elapsed time will be automatically extrapolated from the previously 
// provided elapsed time and playback rate, so updating this property frequently
// is not required (or recommended.)

附注：我用m4a文件尝试了代码，发现durInterval是NotANumber。使用正确的持续时间并且只调用setNowPlayingInfo一次，进度条跟踪良好&没有崩溃。

Answer 2

苹果公司在iOS 10.3及以上版本中修复了这一故障。因此，如果您希望支持iOS 10.2.1及更低版本，请务必控制设置[MPNowPlayingInfoCenter defaultCenter].nowPlayingInfo属性的频率。可能限制每秒钟只设置一次属性。