dispatch_queue_t初学(转载)

1. GCD 使用后不用程序去管理线程的开闭，GCD会在系统层面上去动态检测系统状态，开闭线程
2. Dispatch Queues   单行 并行 2种  FIFO  把task依次放入单行queue可以实现顺序执行
3. Operation Queues 可以指定任务之间的优先级  task之间的先后依赖关系
4. __block变量是可以改变的　共享的

    dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

    for (__block int i = 0; i<10000; i++) {
        dispatch_async(aQueue, ^{
            NSLog(@"%d",i);
        });
    }

2011-07-05 17:11:38.346 ttt[41418:1803] 61
2011-07-05 17:11:38.346 ttt[41418:5f03] 1292
2011-07-05 17:11:38.348 ttt[41418:1803] 4096
2011-07-05 17:11:38.348 ttt[41418:5f03] 4954
2011-07-05 17:11:38.349 ttt[41418:1803] 5823
2011-07-05 17:11:38.349 ttt[41418:5f03] 6159
2011-07-05 17:11:38.349 ttt[41418:1803] 6575
2011-07-05 17:11:38.349 ttt[41418:5f03] 6634
2011-07-05 17:11:38.350 ttt[41418:1803] 7936
2011-07-05 17:11:38.350 ttt[41418:5f03] 8428
2011-07-05 17:11:38.351 ttt[41418:1803] 8895
2011-07-05 17:11:38.351 ttt[41418:5f03] 9364
2011-07-05 17:11:38.351 ttt[41418:1803] 9836
2011-07-05 17:11:38.351 ttt[41418:5f03] 10000
2011-07-05 17:11:38.354 ttt[41418:1803] 10000

5. 一般外部变量是copy正在运行时的外部状态

    dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

    for (int i = 0; i<1000; i++) {
        dispatch_async(aQueue, ^{
            NSLog(@"%d",i);
        });
    }

2011-07-05 17:15:37.525 ttt[41697:1803] 0
2011-07-05 17:15:37.526 ttt[41697:1803] 2
2011-07-05 17:15:37.527 ttt[41697:1803] 3
2011-07-05 17:15:37.527 ttt[41697:1803] 4
2011-07-05 17:15:37.527 ttt[41697:1803] 5
2011-07-05 17:15:37.527 ttt[41697:1803] 6
2011-07-05 17:15:37.526 ttt[41697:5f03] 1
2011-07-05 17:15:37.530 ttt[41697:5f03] 8
2011-07-05 17:15:37.530 ttt[41697:5f03] 9
2011-07-05 17:15:37.530 ttt[41697:5f03] 10
2011-07-05 17:15:37.530 ttt[41697:5f03] 11
2011-07-05 17:15:37.532 ttt[41697:6203] 13


6. queue可以有结束时执行的方法

void myFinalizerFunction(){
    NSLog(@"xxx");
}

- (void)viewDidLoad {
    [superviewDidLoad];
   
   
    dispatch_queue_t queue = dispatch_queue_create("com.example.MyQueue", NULL);
    dispatch_set_context(queue, @"xxx");
    dispatch_set_finalizer_f(queue, &myFinalizerFunction);
    for (int i = 0; i<1000; i++) {
        dispatch_async(queue, ^{
            NSLog(@"%d",i);
        });
    }
    dispatch_release(queue);


7. dispatch_sync(queue,task)  会阻塞当前线程 直到queue完成了你给的task， 但queue要完成你给的task，因为queue是FIFO的，意味着要完成之前的任务，才有机会执行你刚才给的task, 相当于当前线程等待queue里面所有任务执行完毕，　　所以这句话不能在当前queue的任务代码里面调用，会造成死锁

    dispatch_queue_t queue = dispatch_queue_create("com.example.MyQueue", NULL);
    dispatch_set_context(queue, @"xxx");
    dispatch_set_finalizer_f(queue, &myFinalizerFunction);
    for (int i = 0; i<10; i++) {
        dispatch_async(queue, ^{
            NSLog(@"%d",i);
        });
    }
    NSLog(@"waiting");
    dispatch_sync(queue, ^{
        NSLog(@"wait done");
    });
    dispatch_release(queue);

2011-07-05 17:54:35.479 ttt[44203:207] waiting
2011-07-05 17:54:35.479 ttt[44203:1803] 0
2011-07-05 17:54:35.481 ttt[44203:1803] 1
2011-07-05 17:54:35.482 ttt[44203:1803] 2
2011-07-05 17:54:35.482 ttt[44203:1803] 3
2011-07-05 17:54:35.483 ttt[44203:1803] 4
2011-07-05 17:54:35.483 ttt[44203:1803] 5
2011-07-05 17:54:35.484 ttt[44203:1803] 6
2011-07-05 17:54:35.484 ttt[44203:1803] 7
2011-07-05 17:54:35.485 ttt[44203:1803] 8
2011-07-05 17:54:35.485 ttt[44203:1803] 9
2011-07-05 17:54:35.486 ttt[44203:207] wait done
2011-07-05 17:54:35.487 ttt[44203:1803] xxx


8. Waiting on Groups of Queued Tasks　　　　//dispatch_sync只能对一个queue等待

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t group = dispatch_group_create();
   
        // Add a task to the group
    dispatch_group_async(group, queue, ^{
        NSLog(@"first task");
    });
   
    dispatch_queue_t otherqueue = dispatch_queue_create("com.example.MyQueue", NULL);
    for (int i = 0; i<100; i++) {
        dispatch_group_async(group, otherqueue, ^{
            NSLog(@"otherqueue task");
        });
    }

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

    NSLog(@"end waiting");

    dispatch_release(group);

2011-07-05 19:32:31.919 ttt[50138:5f03] otherqueue task
2011-07-05 19:32:31.919 ttt[50138:1803] first task
2011-07-05 19:32:31.922 ttt[50138:5f03] otherqueue task
2011-07-05 19:32:31.923 ttt[50138:5f03] otherqueue task
...
2011-07-05 19:32:32.078 ttt[50138:5f03] otherqueue task
2011-07-05 19:32:32.079 ttt[50138:5f03] otherqueue task
2011-07-05 19:32:32.080 ttt[50138:207] end waiting


9. Although you can obtain information about the underlying thread running a task, it is better to avoid doing so