在 iPhone 上播放背景音频

Question

如何在我的应用程序运行时播放背景音频？

谢谢。

Answer 1

好的。这是 iOS4 和 iOS5 上的背景声音解决方案（绝对适用于 iOS 5.0.1），我仅使用 AVPlayer 对其进行了测试。它应该也适用于 MPMusicPlayerController。

所需的框架：

• AVFoundation.framework
• AudioToolbox.framework

在您的Info.plist 中，为密钥UIBackgroundModes 添加audio。

在MyAppDelegate.h:

• 参考<AVFoundation/AVFoundation.h> & <AudioToolbox/AudioToolbox.h>

• 实现协议AVAudioSessionDelegate:

  @interface MyAppDelegate : NSObject <UIApplicationDelegate, AVAudioSessionDelegate>
  

• 定义一个方法ensureAudio:

  // Ensures the audio routes are setup correctly
  - (BOOL) ensureAudio;
  

在MyAppDelegate.m:

• 实现ensureAudio 方法：

  - (BOOL) ensureAudio
  {
      // Registers this class as the delegate of the audio session (to get background sound)
      [[AVAudioSession sharedInstance] setDelegate: self];  
  
      // Set category
      NSError *categoryError = nil;
      if (![[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error:&categoryError]) {
          NSLog(@"Audio session category could not be set"); 
          return NO;
      }
  
      // Activate session
      NSError *activationError = nil;
      if (![[AVAudioSession sharedInstance] setActive: YES error: &activationError]) {
          NSLog(@"Audio session could not be activated");
          return NO;
      }
  
      // Allow the audio to mix with other apps (necessary for background sound)
      UInt32 doChangeDefaultRoute = 1;
      AudioSessionSetProperty(kAudioSessionProperty_OverrideCategoryMixWithOthers, sizeof(doChangeDefaultRoute), &doChangeDefaultRoute);
  
      return YES;
  }
  

• 在application:didFinishLaunchingWithOptions: 方法中，在分配根视图控制器之前，运行[self ensureAudio]：

  - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
  {
      // Configure audio session
      [self ensureAudio];
  
      // Add the navigation controller's view to the window and display.
      self.window.rootViewController = self.navigationController;
      [self.window makeKeyAndVisible];
  
      return YES;
  }
  

• 像这样实现AVAudioSessionDelegate 方法：

  #pragma mark - AVAudioSessionDelegate
  
  - (void) beginInterruption
  {
  
  }
  
  - (void) endInterruption
  {
      // Sometimes the audio session will be reset/stopped by an interruption
      [self ensureAudio];
  }
  
  - (void) inputIsAvailableChanged:(BOOL)isInputAvailable
  {
  
  }
  

• 确保您的应用程序继续在后台运行。如果您愿意，可以使用 ol'[[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler]，但我认为还有更好的方法。

• 播放实际音频（注意我使用的是 ARC，这就是没有 release 调用的原因）：

  NSURL * file = [[NSBundle mainBundle] URLForResource:@"beep" withExtension:@"aif"];    
  AVURLAsset * asset = [[AVURLAsset alloc] initWithURL:file options:nil];
  AVPlayerItem * item = [[AVPlayerItem alloc] initWithAsset:asset];
  __block AVPlayer * player = [[AVPlayer alloc]initWithPlayerItem:item];
  __block id finishObserver = [[NSNotificationCenter defaultCenter] addObserverForName:AVPlayerItemDidPlayToEndTimeNotification 
                                                                        object:player.currentItem 
                                                                         queue:[NSOperationQueue mainQueue] 
                                                                    usingBlock:^(NSNotification *note) {
      [[NSNotificationCenter defaultCenter] removeObserver:finishObserver];
  
      // Reference the 'player' variable so ARC doesn't release it until it's
      // finished playing.
      player = nil;
  }];
  
  // Trigger asynchronous load
  [asset loadValuesAsynchronouslyForKeys:[NSArray arrayWithObject:@"tracks"] completionHandler:^{
      // Start playing the beep (watch out - we're not on the main thread here)!
      [player play];
   }];
  

• 它太棒了！

Answer 2

如果您还使用您的应用进行录音 - 请不要忘记将 setCategory 更改为 AVAudioSessionCategoryPlayAndRecord。在其他情况下，您将无法录制

[[AVAudioSession sharedInstance] setCategory: AVAudioSessionCategoryPlayAndRecord error:&setCategoryErr];