GCD调度组、自定义队列来实现多个下载任务

#define KURL1 @"http://img0.imgtn.bdimg.com/it/u=79920747,2237302485&fm=90&gp=0.jpg"
#define KURL2 @"http://img5.imgtn.bdimg.com/it/u=2983485216,1361143125&fm=90&gp=0.jpg"
#import "ViewController.h"

@interface ViewController ()
{
    dispatch_queue_t myQueue;
}
@property (weak, nonatomic) IBOutlet UILabel *remindLabel;
@property (weak, nonatomic) IBOutlet UIImageView *firstImageView;
@property (weak, nonatomic) IBOutlet UIImageView *secondImageView;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    //自定义队列
    myQueue=dispatch_queue_create("myQueue", DISPATCH_QUEUE_CONCURRENT);
}


- (IBAction)sendBtnClick:(UIButton *)sender
{
    //创建调度组
    dispatch_group_t myGroup=dispatch_group_create();
    
    //延时执行
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(NSEC_PER_SEC*2)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_group_async(myGroup, myQueue, ^{
            
            dispatch_async(myQueue, ^{
                
                NSData *firstData=[NSData dataWithContentsOfURL:[NSURL URLWithString:KURL1]];
                //主队列刷新UI
                dispatch_async(dispatch_get_main_queue(), ^{
                    _firstImageView.image=[UIImage imageWithData:firstData];
                    
                });
                
            });
        });
        
        dispatch_group_async(myGroup, myQueue, ^{
            
            dispatch_async(myQueue, ^{
                
                NSData *secondData=[NSData dataWithContentsOfURL:[NSURL URLWithString:KURL2]];
                //主队列刷新UI
                dispatch_async(dispatch_get_main_queue(), ^{
                    _secondImageView.image=[UIImage imageWithData:secondData];
                    
                });
                
            });
            
        });
        
    });
    
    //异步函数监听调度组里面的任务完成后刷新UI
    dispatch_group_notify(myGroup, myQueue, ^{
        _remindLabel.text=@"下载完成";
        
    });
    /*
    //同步函数 主线程阻塞 主要用于数据库操作
    dispatch_group_wait(myGroup, dispatch_time(DISPATCH_TIME_NOW, (int64_t)(NSEC_PER_SEC)));
     _remindLabel.text=@"下载完成";
     */
}