Timer 计时器进入后台仍能继续运行

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利用苹果给出的三种类型的程序可以保持在后台运行：音频播放类AVFoundation 在Build Phases添加依赖库

1、步骤一：在Info.plist中，添加"Required background modes"键，value为：App plays audio

2、步骤二：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];    // Override point for customization after application launch.    NSError *setCategoryErr = nil;    NSError *activationErr  = nil;    [[AVAudioSession sharedInstance]     setCategory: AVAudioSessionCategoryPlayback     error: &setCategoryErr];    [[AVAudioSession sharedInstance]     setActive: YES     error: &activationErr];    self.window.backgroundColor = [UIColor whiteColor];    [self.window makeKeyAndVisible];    return YES;}

3、步骤三：将以下代码添加到appDelegate文件中的- (void)applicationDidEnterBackground:(UIApplication *)application函数，也可添加到在具体类中注册的应用进入后台后的通知方法

- (void)applicationDidEnterBackground:(UIApplication *)application{    UIApplication*   app = [UIApplication sharedApplication];    __block    UIBackgroundTaskIdentifier bgTask;    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{        dispatch_async(dispatch_get_main_queue(), ^{            if (bgTask != UIBackgroundTaskInvalid)            {                bgTask = UIBackgroundTaskInvalid;            }        });    }];    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{        dispatch_async(dispatch_get_main_queue(), ^{            if (bgTask != UIBackgroundTaskInvalid)            {                bgTask = UIBackgroundTaskInvalid;            }        });    });}

完成以上步骤你会发现，程序进入后台后仍可运行定时器！

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