iOS-GCD 隊列組的使用

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    // 1.隊列組    dispatch_group_t group = dispatch_group_create();    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);        // 2.下載圖片1    __block UIImage *image1 = nil;    dispatch_group_async(group, queue, ^{        NSURL *url1 = [NSURL URLWithString:@"http://111.jpg"];        NSData *data1 = [NSData dataWithContentsOfURL:url1];        image1 = [UIImage imageWithData:data1];    });        // 3.下載圖片2    __block UIImage *image2 = nil;    dispatch_group_async(group, queue, ^{        NSURL *url2 = [NSURL URLWithString:@"http://222.png"];        NSData *data2 = [NSData dataWithContentsOfURL:url2];        image2 = [UIImage imageWithData:data2];    });        // 4.合并圖片 (保證執行完組裡面的所有任務之後，再執行notify函數裡面的block)    dispatch_group_notify(group, queue, ^{        // 開啟一個位元影像上下文        UIGraphicsBeginImageContextWithOptions(image1.size, NO, 0.0);                // 繪製第1張圖片        CGFloat image1W = image1.size.width;        CGFloat image1H = image1.size.height;        [image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];                // 繪製第2張圖片        CGFloat image2W = image2.size.width * 0.5;        CGFloat image2H = image2.size.height * 0.5;        CGFloat image2Y = image1H - image2H;        [image2 drawInRect:CGRectMake(0, image2Y, image2W, image2H)];                // 得到上下文中的圖片        UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();                // 結束上下文        UIGraphicsEndImageContext();                // 5.回到主線程顯示圖片        dispatch_async(dispatch_get_main_queue(), ^{            self.imageView.image = fullImage;        });    });

 

iOS-GCD 隊列組的使用