iphone:Grand Central Dispatch

queue有三种 

• Main: tasks execute serially on your application’s main thread

• Concurrent: tasks start executing in FIFO order, but can run concurrently.

• Serial: tasks execute one at a time in FIFO order

• (1)serial queues（串行队列）又称私有调度队列（private），一般用再对特定资源的同步访问上。我们可以根据需要创建任意数量的串行队列，每一个串行队列之间是并发的。
• (2)并行队列，又称global dispatch queue。并行队列虽然可以并发的执行多个任务，但是任务开始执行的顺序和其加入队列的顺序相同。我们自己不能去创建并行调度队列。只有三个可用的global concurrent queues。
• (3)main dispatch queue 是一个全局可用的串行队列，其在行用程序的主线程上执行任务。此队列的任务和应用程序的主循环（run loop）要执行的事件源交替执行。因为其运行在应用程序的主线程，main queue经常用来作为应用程序的一个同步点

 

Important functions in this C API
Creating and releasing queues
dispatch_queue_t dispatch_queue_create(const char *label, NULL); // serial queue void dispatch_release(dispatch_queue_t);

Putting blocks in the queue

typedef void (^dispatch_block_t)(void);

void dispatch_async(dispatch_queue_t queue, dispatch_block_t block);

Getting the current or main queue

dispatch_queue_t dispatch_get_current_queue();
void dispatch_queue_retain(dispatch_queue_t); // keep it in the heap until dispatch_release

Main Queue:         dispatch_queue_t  dispatch_get_main_queue();

Global Queue：      dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

Serial：                 dispatch_queue_create("identifier",NULL);

 

 

Example ... assume we fetched an image from the network (this would be slow).

- (void)viewWillAppear:(BOOL)animated

{
    dispatch_queue_t   downloadQueue = dispatch_queue_create(“image downloader”, NULL);

    dispatch_async(downloadQueue, ^{
         NSData  *imageData = [NSData dataWithContentsOfURL:networkURL];
        //UIKit只能在main_queue中
         dispatch_async(dispatch_get_main_queue(), ^{

             UIImage *image = [UIImage imageWithData:imageData];

             self.imageView.image = image;

             self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);

             self.scrollView.contentSize = image.size;
         });

    });
    dispatch_release(downloadQueue);

}