为什么播放音乐时控制按钮在锁定屏幕和命令中心禁用？

Question

为什么播放音乐时控制按钮在锁定屏幕和命令中心禁用？我使用MPRemoteCommandCenter来控制通过远程访问。请帮助我，非常好。

顺便说一下

1. 应用程序没有被杀死。
2. 我添加了admob SDK作为我的应用程序非页内广告。

control button disable on command center

control button disable on lock screen

    if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7.1) {
    MPRemoteCommandCenter *commandCenter = [MPRemoteCommandCenter sharedCommandCenter];
    commandCenter.playCommand.enabled = YES;
    [commandCenter.playCommand addTarget:self action:@selector(playAction:)];

    [commandCenter.pauseCommand addTarget:self action:@selector(pauseAction:)];

    commandCenter.previousTrackCommand.enabled = YES;
    [commandCenter.previousTrackCommand addTarget:self action:@selector(previousTrackAction:)];

    commandCenter.nextTrackCommand.enabled = YES;
    [commandCenter.nextTrackCommand addTarget:self action:@selector(nextTrackAction:)];

    commandCenter.togglePlayPauseCommand.enabled = YES;
    [commandCenter.togglePlayPauseCommand addTarget:self action:@selector(playOrPauseAction:)];
}