NSOperationQueue的簡單使用過程：

1.  建立一個NSOperationQueue的對象

2.  建立一個NSOperation的對象

3.  將operation加入到NSOperationQueue中

     MRC 需要手動釋放

4.  release掉operation

 NSOperationQueue *queue = [NSOperationQueue new];

  NSInvocationOperation *operation = [[NSInvocationOperation alloc]

                                        initWithTarget:self

                                        selector:nil)

                                        object:nil];

[queue addOperation:operation];

/***********************************GCD**************************************************/

//一次性執行

    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{

        //code be executed once

    });

//延遲調用

    double delayInSeconds = 2.0;

    dispatch_time_t timer = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds *NSEC_PER_SEC);

    dispatch_after(timer, dispatch_get_main_queue(), ^{

        //code be executed once after delay

    });

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{

    　//code be executed once after delay

    });

//自定義queque

    dispatch_queue_t fun = dispatch_queue_create("funfun", NULL);

    dispatch_async(fun, ^{

        // your code

    });

MRC 需要釋放

//dispatch_release(fun);

//GCD組 同時進行多個任務，任務完成之后再主線程執行其他任務

    dispatch_group_t group = dispatch_group_create();

    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{

        //your code

    });

    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{

        //your code

    });

       dispatch_group_notify(group, dispatch_get_main_queue(), ^{

        //your code

    });

//多線程的線程安全