Grand Central Dispatch 笔记

https://www.evernote.com/shard/s20/sh/54fbd735-53b6-4436-9bd4-88d46b628527/22bc221c73b55dbf1ddfb5515649c239

 

 

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(放进去的task只会等前一个执行完了才会执行下一个) 并行(放进去的task不用等前一个执行完了，但他们开始执行的次序还是FIFO的) 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. 普通的外部变量的值是task（block）被创建时的值（闭包）

    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，　group可以等好几个queue执行完

    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

10. 当你使用的是obj-c时，　block会自动retain 被它包住的任何对象，当block执行完释放，所以你不必担心你在block内使用的obj-c对象会消失

当使用的是c时（ CoreFoundation types in a C function ），　你必须手动retain release,

void SaveArrayAsync(CFArrayRef array) {
    CFRetain(array);

    dispatch_queue_t queue = dispatch_get_global_queue(
                                 DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_async(queue, ^{
        SaveToDisk(array);
        CFRelease(array);
    }
} 

11.  block的自动retain特性有时会造成互相引用，造成彼此都不能被释放，　可以用__block来解决这个问题，因为使用__block时不会retain

This copying/retaining semantics of blocks is very useful as it means your objects won’t go away while your block is being executed on some other thread, but you can also get into trouble if you happen to be using them only on one thread and the context in a block contains a reference to some parent structure. You’ll get into a retain-loop.

// inside your controller's tableView:cellForRowAtIndexPath:
cell = [[MyCell alloc] initWithBlock:^(MyCell* cell) {
           [self adjustCell:cell];
       }]; 

So here we have a block that calls a method on the controller. Simple enough. But when that cell is created, it’s init method might copy the block to ensure the block is valid at all times. The problem is, it references  self . But that  self  is the view controller. So you have a view controller retained by one of its descendants. This creates a retained object loop that ultimately means this controller will never get released.

  __block  variables are actually  not  retained when the lock is copied. So you can do this

// inside your controller's tableView:cellForRowAtIndexPath:
__block me = self;
cell = [[MyCell alloc] initWithBlock:^(MyCell* cell) {
           [me adjustCell:cell];
       }];

Since the self doesn’t get retained this time, we avoid the retain cycle. While it strikes me as iffy, it does work, and it’s the only way I’ve found to deal with this situation. You can’t simply avoid copying the block if you want to call it later, because the scope will be gone by the time the function that declared the block exits. That will lead to nothing but sadness.