dispatch_async 與 dispatch_get_global_queue

#define kBgQueue dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

- (void)viewDidLoad

{

   [superviewDidLoad];

dispatch_async(kBgQueue, ^{

NSData* data = [NSData dataWithContentsOfURL: kLatestKivaLoansURL];

       [self performSelectorOnMainThread:@selector(fetchedData:) withObject:datawaitUntilDone:YES];

   });

}

dispatch_async會向kBgQueue隊列中添加新的任務去執行，這裏kBgQueue隊列使用dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)得到。

Dispatch Queues

Dispatch Queues從使用的角度將更象另外一種形式的Operation Queues只是 Operation Queuse是用ObjectC的Dispatch Queues是C的

dispatch Queues有serial Queues 也被稱爲私有dispatch Queues，一個時間只能運行一個task，順序運行

dispatch_queue_t queue;

queue = dispatch_queue_create("myQueue", NULL);

dispatch_async(queue, ^{

   printf("Do some work here.\n");

});

printf("The first block may or may not have run.\n");

dispatch_sync(queue, ^{

   printf("Do some more work here.\n");

});

printf("Both blocks have completed.\n");

這裏使用了同步dispatch和異步dispatch，推薦使用dispatch_async這樣才能真正體現其中的優點同步至關於WaitUntil ＝YES

還有一種就是Concurrent Queues每一個程序系統自動提供了3個Concurrent Queues

dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_queue_t aHQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

dispatch_queue_t aLQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);

啥意思一看就明白，3個優先級別的concurrent queues

最後一個特殊的Dispatch Queue就是main dispatch Queue 也是程序啓動自動生成

dispatch_queue_t mainQueue = dispatch_get_main_queue();

concurrent queues和main queue 都是由系統生成並且 dispatch_suspend, dispatch_resume, dispatch_set_context,這些函數對他們無效

可是咱們的應用不是簡單的同步也異步的運行，應用常常是混合的

好比咱們要task1 task2 task3 都運行完成後才能異步運行task4 task5 task6咱們該怎麼作呢？這裏咱們能夠引入group的概念

dispatch_queue_t aDQueue = 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, aDQueue, ^{

   printf("task 1 \n");

});

dispatch_group_async(group, aDQueue, ^{

   printf("task 2 \n");

});

dispatch_group_async(group, aDQueue, ^{

   printf("task 3 \n");

});

printf("wait 1 2 3 \n");

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

printf("task 1 2 3 finished \n");

dispatch_release(group);

group = dispatch_group_create();

// Add a task to the group

dispatch_group_async(group, aDQueue, ^{

   printf("task 4 \n");

});

dispatch_group_async(group, aDQueue, ^{

   printf("task 5 \n");

});

dispatch_group_async(group, aDQueue, ^{

   printf("task 6 \n");

});

printf("wait 4 5 6 \n");

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

printf("task 4 5 6 finished \n");

dispatch_release(group);

有時候咱們也能夠將設定一個數據在queue中也能夠定義一個結束函數

dispatch_set_finalizer_f 是在dispatch_release時候被調用

dispatch_queue_t serialQueue = dispatch_queue_create("com.example.CriticalTaskQueue", NULL);

if (serialQueue)

{

   dispatch_set_context(serialQueue, self);

   dispatch_set_finalizer_f(serialQueue, &myFinalizerFunction);

}

dispatch_group_t group = dispatch_group_create();

// Add a task to the group

dispatch_group_async(group, serialQueue, ^{

   printf("task 1 \n");

});

dispatch_group_async(group, serialQueue, ^{

   printf("task 2 \n");

});

dispatch_group_async(group, serialQueue, ^{

   printf("task 3 \n");

});

printf("wait 1 2 3 \n");

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

dispatch_release(group);

dispatch_release(serialQueue);