如何强制一个方法在另一个方法之前执行？

Question

我有2个方法。 一种方法开始播放音频文件 (.mp3)，另一种方法更新 UIToolBar 以显示按钮（播放或暂停）。这两个方法按以下顺序调用：

//Adds some UIBarButtonItems to a UIToolBar
[self togglePlayer];
//Uses AVAudioPlayer
[audioPlayer play];

TogglePlayer 执行以下操作：

-(void)togglePlayer
{

    NSLog(@"Toggling Player");

    NSArray *barButtonItems;
    UIBarButtonItem *barButtonSpaceL;
    UIBarButtonItem *barButtonSpaceR;
    UIBarButtonItem *barButtonItemPlayer;
    UIBarButtonItem *barButtonItemCancel;
    UIBarButtonItem *barButtonItemLyrics;
    UIBarButtonItem *barButtonItemTweet;

    if([myToolbar.items count] > 0){

        NSEnumerator *enumerator = [myToolbar.items objectEnumerator];
        id object;
        while ((object = [enumerator nextObject])) {

            [object release];
            object = nil;
        }   
    }

    if(!downloadInProgress){

    barButtonSpaceL = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:self action:nil];
    barButtonSpaceR = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:self action:nil];

    if(thePlayerState == PLAYER_PLAYING){
    barButtonItemPlayer = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"pauseIcon.png"] style:UIBarButtonItemStylePlain target:self action:@selector(pauseButtonPressed:)];
    barButtonItemLyrics = [[UIBarButtonItem alloc] initWithTitle:@"Lyrics" style:UIBarButtonItemStyleBordered target:self action:@selector(switchPageLyrics:)];
    barButtonItemTweet = [[UIBarButtonItem alloc] initWithTitle:@"Tweet This" style:UIBarButtonItemStyleBordered target:self action:@selector(tweetSong:)];

        if(canTweet){
        barButtonItems = [NSArray arrayWithObjects:barButtonItemTweet, barButtonSpaceR, barButtonItemPlayer, barButtonSpaceL, barButtonItemLyrics, nil];
        }else{
        barButtonItems = [NSArray arrayWithObjects:barButtonItemPlayer, barButtonSpaceR, barButtonItemLyrics, nil];         
        }

    }else if(thePlayerState == PLAYER_PAUSED){

    barButtonItemPlayer = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"playIcon.png"] style:UIBarButtonItemStylePlain target:self action:@selector(playButtonPressed:)];
    barButtonItemLyrics = [[UIBarButtonItem alloc] initWithTitle:@"Lyrics" style:UIBarButtonItemStyleBordered target:self action:@selector(switchPageLyrics:)];
    barButtonItemTweet = [[UIBarButtonItem alloc] initWithTitle:@"Tweet This" style:UIBarButtonItemStyleBordered target:self action:@selector(tweetSong:)];


        if(canTweet){
            barButtonItems = [NSArray arrayWithObjects:barButtonItemTweet, barButtonSpaceR, barButtonItemPlayer, barButtonSpaceL, barButtonItemLyrics, nil];
        }else{
            barButtonItems = [NSArray arrayWithObjects:barButtonItemPlayer, barButtonSpaceR, barButtonItemLyrics, nil];         
        }   

    }else{

    //PLAYER OFF    
    barButtonItems = [NSArray arrayWithObjects:barButtonSpaceL, nil];

    }
        [myToolbar setItems:barButtonItems];    

    }else{


        barButtonSpaceL = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:self action:nil];
        barButtonSpaceR = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:self action:nil];
        barButtonItemCancel = [[UIBarButtonItem alloc] initWithTitle:NSLocalizedString(@"VC1_DownloadCancel", @"") style:UIBarButtonItemStyleBordered target:self action:@selector(downloadCancelled:)];        

        if(thePlayerState == PLAYER_PLAYING){
            barButtonItemPlayer = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"pauseIcon.png"] style:UIBarButtonItemStylePlain target:self action:@selector(pauseButtonPressed:)];
            barButtonItems = [NSArray arrayWithObjects:barButtonItemPlayer, barButtonSpaceR, barButtonItemCancel, nil]; 

        }else if(thePlayerState == PLAYER_PAUSED){
            barButtonItemPlayer = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"playIcon.png"] style:UIBarButtonItemStylePlain target:self action:@selector(playButtonPressed:)];
            barButtonItems = [NSArray arrayWithObjects:barButtonItemPlayer, barButtonSpaceR, barButtonItemCancel, nil]; 

        }else{

        //PLAYER OFF
            barButtonItems = [NSArray arrayWithObjects:barButtonSpaceL, barButtonItemCancel, barButtonSpaceR, nil]; 
        }

        [myToolbar setItems:barButtonItems];


    }

}

我按上述顺序调用方法，以便在歌曲开始播放时显示（暂停）按钮。但是，问题是歌曲首先开始播放，并且 UIToolBar 在相当长一段时间内（从 2 到 5 秒）保持不变，直到添加并显示按钮。

我想要的是在歌曲开始播放的同时显示按钮（即无延迟）。有什么办法可以做到这一点吗？

Answer 1

为了更新 GUI，您需要给它一个实际更新 GUI 的机会。由于音频播放发生在不同的线程中，因此它将立即开始播放。但是，由于您可能处于 gui 的事件中（例如触摸事件）并且尚未从该函数返回，因此负责更新 gui 的代码尚未运行。

请参阅 http://developer.apple.com/iphone/library/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/ApplicationEnvironment/ApplicationEnvironment.html#//apple_ref/doc/uid/TP40007072-CH7-SW2有关 Cocoa 框架的信息

使用此函数来调用 play 应该可以按照您想要的方式工作。这将在处理当前事件后将事件置于调用播放位置。

- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay

你的例子：

//Adds some UIBarButtonItems to a UIToolBar
[self togglePlayer];
//Uses AVAudioPlayer
[audioPlayer performSelector:@SEL(play) withObject:nil afterDelay:0];

Answer 2

来准备系统播放音频文件。

[audioPlayer prepareToPlay];

另一种方法可能是使用初始化代码中的某个位置（例如 viewDidLoad） 这样，一旦您想要播放音频，所有内容都会被设置，并且 play 方法不会导致明显的延迟。 （不在您的用户界面代码中，也不在实际播放中）