iOS：在某些设备上多次调用MPMusicPlayerControllerPlaybackStateDidChangeNotification [英] iOS: MPMusicPlayerControllerPlaybackStateDidChangeNotification called multiple times on certain devices

问题描述

我有一个播放音乐的应用程序。

I have an application that plays back music.

我正在使用以下代码从MPMusicPlayerController监听播放状态更改以更新UI。更确切地说，我在播放和暂停之间切换播放按钮的外观。

I'm using the following code to listen to playback state changes from the MPMusicPlayerController to update the UI. More precisely I toggle the look of the play button between play and pause.

NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];

[notificationCenter addObserver: self
                       selector: @selector (handle_NowPlayingItemChanged:)
                           name: MPMusicPlayerControllerNowPlayingItemDidChangeNotification
                         object: self.musicPlayer];

[notificationCenter addObserver: self
                       selector: @selector (handle_PlaybackStateChanged:)
                           name: MPMusicPlayerControllerPlaybackStateDidChangeNotification
                         object: self.musicPlayer];

[self.musicPlayer beginGeneratingPlaybackNotifications];

这适用于 iPod Touch（iOS 5）和 iPhone 3GS（iOS 5）。每次回放状态改变时，我得到以下回调：

This works great on an iPod Touch (iOS 5) and iPhone 3GS (iOS 5). Every time the playback state changes I get the following callback:

[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 1

其中1表示 MPMusicPlaybackStatePlaying 。

但是，如果我在 iPad 1（iOS 5）， iPad 2（iOS 5）或 iPad 3（ iOS 6）我得到以下序列而不是只有一个回调：

However if I run the same on a iPad 1 (iOS 5), iPad 2 (iOS 5) or iPad 3 (iOS 6) I get the following sequence instead of just one single callback:

-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 1
-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 2
-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 1
-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 2

其中2表示 MPMusicPlaybackStatePaused 并导致我的申请在UI中显示错误的状态，因为该歌曲实际上正在播放。

where 2 means MPMusicPlaybackStatePaused and causes my application to display the wrong state in the UI, because the song is actually being played back.

有趣的hing是，偶尔序列是

The funny thing is, that once in a while the sequence is

-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 1
-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 2
-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 1
-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 2
-[JBMediaPlayer handle_PlaybackStateChanged:] :: playbackState: 1

以1 MPMusicPlaybackStatePlaying正确结束，但仍然没有意义，回调被调用5次，交替值。

which ends up correctly with 1 MPMusicPlaybackStatePlaying, however still doesn't make sense that the callback gets called 5 times, with alternating values.

关于如何解决这个或建议什么的想法否则我可以测试缩小问题范围？

Any ideas on how to solve this or suggestion what else I can test to narrow down the problem?

由于到目前为止我还没有收到答案，我也是将问题交叉发布到 Apple开发人员论坛： https://devforums.apple .COM / thread / 158426

Since I haven't received an answer here so far, I also cross-posted the question to the Apple Developer Forum: https://devforums.apple.com/thread/158426

推荐答案

您可以使用currentPlaybackRate属性检查实际播放状态。 MPMusicPlaybackStatePaused必须与速率0.0匹配。它如何实现的示例如下所示...

You can check real playback state using currentPlaybackRate property. MPMusicPlaybackStatePaused must match rate 0.0. An example how can it implemented is shown below...

- (void)musicPlayerControllerPlaybackStateDidChangeNotification:(NSNotification *)notification {
    float playbackRate = [((MPMusicPlayerController *)notification.object) currentPlaybackRate];
    MPMusicPlaybackState playbackState = (MPMusicPlaybackState)[notification.userInfo[@"MPMusicPlayerControllerPlaybackStateKey"] integerValue];
    switch (playbackState) {
        case MPMusicPlaybackStatePaused:
            if (playbackRate <= .0f) {
                self.playbackState = playbackState;
            }
            break;
        default:
            self.playbackState = playbackState;
            break;
    }
}

因此可以切断误暂停通知。

Thus it is possible to cut off false pause notification.

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