GCD队列组2

// 下载 - > 合并 - > 显示
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    
    
    
    
    // 组    // 一个并行队列队列中 有两个任务  然后放入一个组中
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    __block UIImage *image1 = nil;
    dispatch_group_async(group,queue, ^{
       
        
        NSString *urlString1 = [NSString stringWithFormat:@"http://img04.tooopen.com/images/20130712/tooopen_17270713.jpg"];
        NSURL *url1 = [NSURL URLWithString:urlString1];
        image1 = [UIImage imageWithData:[NSData dataWithContentsOfURL:url1]];
    });
    
        __block UIImage *image2 = nil;
    dispatch_group_async(group,queue, ^{
        
        NSString *urlString2 = [NSString stringWithFormat:@"https://www.baidu.com/img/bd_logo1.png"];
        NSURL *url2 = [NSURL URLWithString:urlString2];
        image2 = [UIImage imageWithData:[NSData dataWithContentsOfURL:url2]];
    });
    
    // 保证执行完组里面的所有任务之后，再执行notify函数里面的block
    dispatch_group_notify(group, queue, ^{
        
        UIGraphicsBeginImageContextWithOptions(image1.size, NO, 0.0f);
        [image1 drawInRect:CGRectMake(0, 0, image1.size.width, image1.size.height)];
        
        [image2 drawInRect:CGRectMake(0, 0, image2.size.width, image2.size.height)];
        
        UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();
        
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = fullImage;
        });
        
        UIGraphicsEndImageContext();

    });
    
    
    
    
    
    
    
    
    
#if 0
    //        @"http://img04.tooopen.com/images/20130712/tooopen_17270713.jpg"
    //        @"https://www.baidu.com/img/bd_logo1.png"
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 下载第一张
        NSString *urlString1 = [NSString stringWithFormat:@"http://img04.tooopen.com/images/20130712/tooopen_17270713.jpg"];
        NSURL *url1 = [NSURL URLWithString:urlString1];
        self.image1 = [UIImage imageWithData:[NSData dataWithContentsOfURL:url1]];
        
        NSLog(@"---%@---",[NSThread currentThread]);
        [self bindImages];
        
    });
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        
        // 下载第二张
        NSString *urlString2 = [NSString stringWithFormat:@"https://www.baidu.com/img/bd_logo1.png"];
        NSURL *url2 = [NSURL URLWithString:urlString2];
         self.image2 = [UIImage imageWithData:[NSData dataWithContentsOfURL:url2]];
                NSLog(@"---%@---",[NSThread currentThread]);
        
        [self bindImages];
    });
    
#endif
    
}

- (void)bindImages {
    
    
    if (self.image1 == nil || self.image2 == nil) {
        
        UIGraphicsBeginImageContextWithOptions(self.image1.size, NO, 0.0f);
        [self.image1 drawInRect:CGRectMake(0, 0, self.image1.size.width, self.image1.size.height)];
        
        [self.image2 drawInRect:CGRectMake(0, 0, self.image2.size.width, self.image2.size.height)];
        
        UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();
        
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = fullImage;
        });
        
        UIGraphicsEndImageContext();
        
    } else return;
}