iOS开发多线程-下载合并图片

方法一

先下载第1张图片，然后下载第2张图片，最后合并图片

弊病：由于图片比较耗时，一张一张的下载图片更耗时。

// 异步下载
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 1.下载第1张
        NSURL *url1 = [NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/f2deb48f8c5494ee460de6182ff5e0fe99257e80.jpg"];
        NSData *data1 = [NSData dataWithContentsOfURL:url1];
        UIImage *image1 = [UIImage imageWithData:data1];
        
        // 2.下载第2张
        NSURL *url2 = [NSURL URLWithString:@"http://su.bdimg.com/static/superplus/img/logo_white_ee663702.png"];
        NSData *data2 = [NSData dataWithContentsOfURL:url2];
        UIImage *image2 = [UIImage imageWithData:data2];
        
        // 3.合并图片
        // 开启一个位图上下文
        UIGraphicsBeginImageContextWithOptions(image1.size, NO, 0.0);
        
        // 绘制第1张图片
        CGFloat image1W = image1.size.width;
        CGFloat image1H = image1.size.height;
        [image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];
        
        // 绘制第2张图片
        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)];
        
        // 得到上下文中的图片
        UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();
        
        // 结束上下文
        UIGraphicsEndImageContext();
        
        // 4.回到主线程显示图片
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = fullImage;
        });
    });

方法二

同时开启两个线程分别下载图片

弊病：增加额外的属性

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    // 异步下载
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 1.下载第1张
        NSURL *url1 = [NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/f2deb48f8c5494ee460de6182ff5e0fe99257e80.jpg"];
        NSData *data1 = [NSData dataWithContentsOfURL:url1];
        self.image1 = [UIImage imageWithData:data1];
        
        [self bindImages];
    });
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 2.下载第2张
        NSURL *url2 = [NSURL URLWithString:@"http://su.bdimg.com/static/superplus/img/logo_white_ee663702.png"];
        NSData *data2 = [NSData dataWithContentsOfURL:url2];
        self.image2 = [UIImage imageWithData:data2];
        
        [self bindImages];
    });
    
}
- (void)bindImages
{
    if (self.image1 == nil || self.image2 == nil) return;
    
    // 3.合并图片
    // 开启一个位图上下文  （相当于创建了一个空的图片）
    // UIGraphicsBeginImageContextWithOptions(<#CGSize size#>（新的图片有多大）, <#BOOL opaque#>（NO代表透明，YES代表不透明）, <#CGFloat scale#> （缩放比例）)
    UIGraphicsBeginImageContextWithOptions(self.image1.size, NO, 0.0);
    
    // 绘制第1张图片 （绘制到上面创建的那个位图上下文上面 也就是那张空的图片上面）
    CGFloat image1W = self.image1.size.width;
    CGFloat image1H = self.image1.size.height;
    [self.image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];
    
    // 绘制第2张图片
    CGFloat image2W = self.image2.size.width * 0.5;
    CGFloat image2H = self.image2.size.height * 0.5;
    CGFloat image2Y = image1H - image2H;
    //把image2 绘制到后面的那个区域
    [self.image2 drawInRect:CGRectMake(0, image2Y, image2W, image2H)];
    
    // 得到上下文中的图片
    UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();
    
    // 结束上下文
    UIGraphicsEndImageContext();
    
    // 4.回到主线程显示图片
    dispatch_async(dispatch_get_main_queue(), ^{
        self.imageView.image = fullImage;
    });
}

方法三

队列组

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event  
{  
      
    // 1.队列组  
    dispatch_group_t group = dispatch_group_create();  
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
      
    // 2.下载图片1  
    __block UIImage *image1 = nil;  //要加一个 __block因为 block代码默认不能改外面的东西（记住语法即可）  
    dispatch_group_async(group, queue, ^{  
        NSURL *url1 = [NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/f2deb48f8c5494ee460de6182ff5e0fe99257e80.jpg"];  
        NSData *data1 = [NSData dataWithContentsOfURL:url1];  
        image1 = [UIImage imageWithData:data1];  
    });  
      
    // 3.下载图片2  
    __block UIImage *image2 = nil;  
    dispatch_group_async(group, queue, ^{  
        NSURL *url2 = [NSURL URLWithString:@"http://su.bdimg.com/static/superplus/img/logo_white_ee663702.png"];  
        NSData *data2 = [NSData dataWithContentsOfURL:url2];  
        image2 = [UIImage imageWithData:data2];  
    });  
      
    // 4.合并图片 用Quartz2D的知识，则要先要搞一个空的大图片，然后再把小图片画上去(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)  
    //队列组：要把队列组里面的所有任务都执行完后调用dispatch_group_notify(group, queue, ^{ }  
    dispatch_group_notify(group, queue, ^{  
        // 开启一个位图上下文  
        UIGraphicsBeginImageContextWithOptions(image1.size, NO, 0.0);  
          
        // 绘制第1张图片  
        CGFloat image1W = image1.size.width;  
        CGFloat image1H = image1.size.height;  
        [image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];  
          
        // 绘制第2张图片  
        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)];  
          
        // 得到上下文中的图片  
        UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();  
          
        // 结束上下文  
        UIGraphicsEndImageContext();  
          
        // 5.回到主线程显示图片  
        dispatch_async(dispatch_get_main_queue(), ^{  
            self.imageView.image = fullImage;  
        });  
    });  
      
}  

参考 ** iOS开发多线程之队列组——下载合并图片**

方法四

线程依赖

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    
    /**
     假设有A、B、C三个操作，要求：
     1. 3个操作都异步执行
     2. 操作C依赖于操作A
     3. 操作C依赖于操作B
     */
    //创建一个队列
    NSOperationQueue *queue = [[NSOperationQueue alloc]init];
    //可开辟线程的最大数量
    queue.maxConcurrentOperationCount = 3;
    
    //创建三个任务
    NSBlockOperation *operationA = [NSBlockOperation blockOperationWithBlock:^{
        NSURL *url1 = [NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/f2deb48f8c5494ee460de6182ff5e0fe99257e80.jpg"];
        NSData *data1 = [NSData dataWithContentsOfURL:url1];
        self.image1 = [UIImage imageWithData:data1];
    }];
    NSBlockOperation *operationB = [NSBlockOperation blockOperationWithBlock:^{
        NSURL *url2 = [NSURL URLWithString:@"https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1504108008401&di=6297b96e3a13728c58d806b71e4590bb&imgtype=0&src=http%3A%2F%2Fwww.logoing.net%2Fwp-content%2Fuploads%2F2012%2F08%2Fbaidulogo1.jpg"];
        NSData *data2 = [NSData dataWithContentsOfURL:url2];
        self.image2 = [UIImage imageWithData:data2];
    }];
    NSBlockOperation *operationC = [NSBlockOperation blockOperationWithBlock:^{
        // 开启一个位图上下文
        UIGraphicsBeginImageContextWithOptions(self.image1.size, NO, 0.0);

        // 绘制第1张图片
        CGFloat image1W = self.image1.size.width;
        CGFloat image1H = self.image1.size.height;
        [self.image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];

        // 绘制第2张图片
        CGFloat image2W = self.image2.size.width * 0.5;
        CGFloat image2H = self.image2.size.height * 0.5;
        CGFloat image2Y = image1H - image2H;
        [self.image2 drawInRect:CGRectMake(0, image2Y, image2W, image2H)];

        // 得到上下文中的图片
        UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();

        // 结束上下文
        UIGraphicsEndImageContext();

        // 5.回到主线程显示图片
        dispatch_async(dispatch_get_main_queue(), ^{
            self.ww_imgView.image = fullImage;
        });
    }];
    //设置任务之间的依赖
    [operationC addDependency:operationA];
    [operationC addDependency:operationB];
    
    //添加操作到队列中-自动异步执行，并发
    [queue addOperation:operationA];
    [queue addOperation:operationB];
    [queue addOperation:operationC];
}

参考iOS-网络图片通过NSOperation线程依赖异步加载并保存到沙盒中