GCD实现倒计时且后台运行不影响计数

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 or streams audio/video using AirPlay

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！

附带GCD实现倒计时方法：

__block NSInteger timeout = totalTimeInterval;

    // 拿到一个队列

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    // 创建一个_timer放到队列里

    _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);

    // 设置_timer的首次执行时间、执行时间间隔、精确度

    dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 1.0 * NSEC_PER_SEC, 0);

    // 设置_timer执行的事件

    typeof(self) __weak wself = self;

    dispatch_source_set_event_handler(_timer, ^{

        if (timeout <= 0) {

            // 倒计时结束，取消_timer

            dispatch_source_cancel(_timer);

            dispatch_async(dispatch_get_main_queue(), ^{

                // 重置ItemContainer

                if (_infiniteLoop) {

                    [wself setInfiniteCycleFromCurrentTime];

                }

                else {

                    [wself reloadItemsWithTimes:nil];

                }

                // 调用代理事件

                if ([wself.delegate respondsToSelector:@selector(RBCountDownViewDidStopCounting:)]) {

                    [wself.delegate RBCountDownViewDidStopCounting:wself];

                }

            });

        }

        else {

            // 计算显示时间数组

            NSArray *times = [wself calculateTimesWithTotalTimeInterval:timeout];

            dispatch_async(dispatch_get_main_queue(), ^{

                [wself reloadItemsWithTimes:times];

            });

            timeout--;

        }

    });

    // 激活_timer

    dispatch_resume(_timer);

参考文章：

http://blog.youkuaiyun.com/u013009873/article/details/50979415

http://jingyan.baidu.com/article/d8072ac47d3c00ec94cefd5b.html