dispatch_async 与 dispatch_get_global_queue
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#define kBgQueue dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)- (void)viewDidLoad{ [super viewDidLoad]; dispatch_async(kBgQueue, ^{ NSData* data = [NSData dataWithContentsOfURL: kLatestKivaLoansURL]; [self performSelectorOnMainThread:@selector(fetchedData:) withObject:datawaitUntilDone:YES]; });}
dispatch_async会向kBgQueue队列中添加新的任务去执行,这里kBgQueue队列使用dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)获得。
Dispatch Queues
Dispatch Queues从使用的角度将更象另一种形式的Operation Queues只是 Operation Queuse是用ObjectC的Dispatch Queues是C的
dispatch Queues有serial Queues 也被称为私有dispatch Queues,一个时间只能运行一个task,顺序运行
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");
这里使用了同步dispatch和异步dispatch,推荐使用dispatch_async这样才能真正体现其中的优势同步相当于WaitUntil = YES
还有一种就是Concurrent Queues每个程序系统自动提供了3个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个优先级别的concurrent queues
最后一个特殊的Dispatch Queue就是main dispatch Queue 也是程序启动自动生成
dispatch_queue_t mainQueue = dispatch_get_main_queue();
concurrent queues和main queue 都是由系统生成而且 dispatch_suspend, dispatch_resume, dispatch_set_context,这些函数对他们无效
但是我们的应用不是简单的同步也异步的运行,应用经常是混合的
比如我们要task1 task2 task3 都运行完成后才能异步运行task4 task5 task6我们该怎么做呢?这里我们可以引入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);
有时候我们也可以将设定一个数据在queue中 也可以定义一个结束函数
dispatch_set_finalizer_f 是在dispatch_release时候被调用
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);
转自:http://blog.csdn.net/ericsuper/article/details/6998856
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