歌曲不可用时的警报视图

Question

我有一个很好的小查询，当用户单击"播放按钮"时，会调用一个标题，如果该标题的歌曲存在于他们的iTunes中，则歌曲开始播放。我现在被困住了，因为我不确定当歌曲不存在时如何显示警报视图。

下面是查询：

- (void) queryMusic
{
MPMediaQuery *q = [[MPMediaQuery alloc] init];
[q addFilterPredicate:[MPMediaPropertyPredicate predicateWithValue:@"Beyoncé"  forProperty:MPMediaItemPropertyArtist comparisonType:MPMediaPredicateComparisonEqualTo]];

[q addFilterPredicate:[MPMediaPropertyPredicate predicateWithValue:_DetailModal[0]
                                                         forProperty:MPMediaItemPropertyTitle comparisonType:MPMediaPredicateComparisonEqualTo]];

[q setGroupingType:MPMediaGroupingTitle];
self.player = [MPMusicPlayerController applicationMusicPlayer];
[self.player setRepeatMode:MPMusicRepeatModeAll];
[self.player setShuffleMode:MPMusicShuffleModeSongs];
[self.player setQueueWithQuery: q];

[[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(handleStateChanged:)
                                             name:MPMusicPlayerControllerPlaybackStateDidChangeNotification
                                           object:nil];
[[NSNotificationCenter defaultCenter]  addObserver: self
                                          selector: @selector (handle_PlaybackStateChanged:)
                                              name:    MPMusicPlayerControllerPlaybackStateDidChangeNotification
                                            object: self.player];
[self.player beginGeneratingPlaybackNotifications];
 }

handle_PlaybackStateChanged是：

- (void) handle_PlaybackStateChanged: (id) notification {
MPMusicPlaybackState playbackState = [self.player playbackState];

if (playbackState == MPMusicPlaybackStatePaused) {
    self.textLabel.text = @"play";
    [PlayButton setTitle:@"STOP" forState:UIControlStateNormal]; // To set the title
    NSLog (@"This is paused");
    self.playBarButton.title = @"Play";
    self.PlayButton = PlayButton;

    [self.PlayButton setTitleColor:[UIColor redColor] forState:UIControlStateNormal];
    [playPauseButton setImage:[UIImage imageNamed:@"playbss.png"] forState:UIControlStateNormal];

} else if (playbackState == MPMusicPlaybackStatePlaying) {
    self.textLabel.text = @"pause";
    [PlayButton setTitle:@"Pause" forState:UIControlStateNormal];
    self.PlayButton = PauseButton;
    [playPauseButton setImage:[UIImage imageNamed:@"[e-mail]"] forState:UIControlStateNormal];
    NSLog (@"This is playing");


} else if (playbackState == MPMusicPlaybackStateStopped) {
    self.textLabel.text = @"Play";
    self.PlayButton = PlayButton;
    [playPauseButton setImage:[UIImage imageNamed:@"playbss.png"] forState:UIControlStateNormal];

    [self.player stop];
}
}

我刚刚尝试过这个：

MPMediaItem *nowPlayingItem = self.player.nowPlayingItem;

NSString *title  = [nowPlayingItem valueForProperty:MPMediaItemPropertyTitle];
if (title == (id)[NSNull null] || title.length == 0) {
    UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"Hello World!"
                                                     message:@"This is your first UIAlertview message."
                                                    delegate:nil
                                           cancelButtonTitle:@"OK"
                                           otherButtonTitles:nil];
    [message show];
}

但不起作用。帮助。

Answer 1

尝试

if( q.items.count < 1 )  {
    [self showAlert];
    return;
}

以前

[self.player setQueueWithQuery: q]