iOS GCD

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//耗时的操作
    NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"];
    NSData * data = [[NSData alloc]initWithContentsOfURL:url];
    UIImage *image = [[UIImage alloc]initWithData:data];
    if (data != nil) {
        dispatch_async(dispatch_get_main_queue(), ^{
 //刷新数据
            self.imageView.image = image;
         });
    }
});
系统给每一个应用程序提供了三个concurrent dispatch queues。这三个并发调度队列是全局的，它们只有优先级的不同。因为是全局的，我们不需要去创建。我们只需要通过使用函数dispath_get_global_queue去得到队列，如下：

1.dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);    

 这里也用到了系统默认就有一个串行队列main_queue

2.dispatch_queue_t mainQ = dispatch_get_main_queue();    

虽然dispatch queue是引用计数的对象，但是以上两个都是全局的队列，不用retain或release。

2.使用Dispatch Group异步下载两张图片

@interface ViewController ()

@property (nonatomic, strong) UIImage *imageOne;

@property (nonatomic, strong) UIImage *imageTwo;

@property (nonatomic, weak) UILabel *textLabel;

@end

@implementation ViewController

- (void)viewDidLoad {

    [super viewDidLoad];

    

    [self operation1];

}

- (void)operation1

{

    UILabel *textLabel = [[UILabel alloc] initWithFrame:CGRectMake(50, 450, 0, 0)];

    textLabel.text = @"正在下载图片";

    [textLabel sizeToFit];

    [self.view addSubview:textLabel];

    self.textLabel = textLabel;

    [self group];

    NSLog(@"在下载图片的时候，主线程貌似还可以干点什么");

}

- (void)group

{

    UIImageView *imageView = [[UIImageView alloc] init];

    [self.view addSubview:imageView];

dispatch_group_t group = dispatch_group_create();

dispatch_queue_t queue = dispatch_queue_create("com.wuniu.www", DISPATCH_QUEUE_CONCURRENT);

dispatch_group_async(group, queue, ^{

    NSLog(@"正在下载第一张图片");

    NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://img3.imgtn.bdimg.com/it/u=3484903974,2907035455&fm=206&gp=0.jpg"]];

    NSLog(@"第一张图片下载完毕");

    self.imageOne = [UIImage imageWithData:data];

});

dispatch_group_async(group, queue, ^{

    NSLog(@"正在下载第二张图片");

    NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://img4.imgtn.bdimg.com/it/u=2718132976,3419769724&fm=206&gp=0.jpg"]];

    NSLog(@"第二张图片下载完毕");

    self.imageTwo = [UIImage imageWithData:data];

});

dispatch_group_notify(group, queue, ^{

    UIGraphicsBeginImageContext(CGSizeMake(400, 200));

    

    [self.imageOne drawInRect:CGRectMake(0, 0, 150, 200)];

    [self.imageTwo drawInRect:CGRectMake(150, 0, 150, 200)];

    

    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    

    dispatch_async(dispatch_get_main_queue(), ^{

        UIImageView *imageView = [[UIImageView alloc] initWithImage:newImage];

        [self.view addSubview:imageView];

        self.textLabel.text = @"图片合并完毕";

    });

});

}

//打印输出

2016-08-18 15:26:27.750 LX[66966:7575374] 正在下载第一张图片

2016-08-18 15:26:27.750 LX[66966:7575372] 正在下载第二张图片

2016-08-18 15:26:27.750 LX[66966:7575341] 在下载图片的时候，主线程貌似还可以干点什么

2016-08-18 15:26:28.076 LX[66966:7575372] 第二张图片下载完毕

2016-08-18 15:26:28.080 LX[66966:7575374] 第一张图片下载完毕