GCD 大中枢派发 简单应用实例

@interface ViewController ()

{
    UIImageView* iv;
    UIButton* btn;
    UILabel* lbl;
}

@end

@implementation ViewController

- (void)viewDidLoad {

    [super viewDidLoad];
    iv=[[UIImageView alloc]initWithFrame:CGRectMake(, , , )];
    lbl=[[UILabel alloc]initWithFrame:CGRectMake(, , , )];
    btn=[[UIButton alloc]initWithFrame:CGRectMake(, , , )];
    [btn addTarget:self action:@selector(loadData) forControlEvents:UIControlEventTouchUpInside];
    btn.backgroundColor=[UIColor orangeColor];

    [self.view addSubview:iv];
    [self.view addSubview:lbl];
    [self.view addSubview:btn];
}

-(void)loadData
{
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, ), ^{
        NSString* url=@"http://f4.topitme.com/4/e4/5e/1132253263b8b5ee44o.jpg";
        NSData* data=[[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:url]];
        UIImage* image=[[UIImage alloc]initWithData:data];
        if (image) {
            dispatch_async(dispatch_get_main_queue(), ^{
                self->iv.image=image;
            });
        }
        else
        {
            NSLog(@"下载错误");
        }
    });
 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, ), ^{
        NSString* str=@"";
        if (str) {
            dispatch_async(dispatch_get_main_queue(), ^{
                self->lbl.text=str;
            });
        }
        else
        {
            NSLog(@"下载错误");
        }
    });
}