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1.系统提供的dispatch方法

   为了方便的使用GCD，苹果提供了一些方法方便我们将BLOCK放在主线程或者后台程序执行，或者延后执行。

    //后台执行：
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //something
    });
    //主线程执行
    dispatch_async(dispatch_get_main_queue(), ^{
        //something
    });
    //一次执行完
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        //something
    });
    //延迟2秒执行
    double delayInSeconds = 2.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
    dispatch_after(popTime, dispatch_get_main_queue(), ^{
        //something
    });

    dispatch_queue_t my_queue = dispatch_queue_create("myQueue", NULL);
    dispatch_async(my_queue, ^{
        //something
    });
    dispatch_release(my_queue);

    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //并行执行的线程1
    });
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //并行执行的线程2
    });
    dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //汇总结果
    });

2.后台运行

//AppDelegate.h文件
@property (nonatomic, assign) UIBackgroundTaskIdentifier backgroundUpdateTask;

//APPDelegate.m
- (void)applicationDidEnterBackground:(UIApplication *)application {
    
    [self beginBackgrooundUpdateTask];
    //在这里加上你需要长久运行的代码
    [self  endBackgrooundUpdateTask];
}

-(void)beginBackgrooundUpdateTask
{
    self.backBackgroundUpdateTask = [[UIApplication sharedApplication]beginBackgroundTaskWithExpirationHandler:^{
        [self  endBackgrooundUpdateTask];
    }];
}

-(void)endBackgrooundUpdateTask
{
    [[UIApplication sharedApplication] endBackgroundTask:self.backBackgroundUpdateTask];
    self.backBackgroundUpdateTask = UIBackgroundTaskInvalid;
}

小胖说事20--------GCD笔记

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原文地址：http://blog.csdn.net/haogaoming123/article/details/46646637