NSOperationQueue 设置依赖

 NSOperationQueue*queue=[[NSOperationQueue alloc]init];

    

    NSBlockOperation*blockOperation=[NSBlockOperation blockOperationWithBlock:^{

        NSURL *url = [NSURL URLWithString:@"http://img.pconline.com.cn/images/photoblog/9/9/8/1/9981681/200910/11/1255259355826.jpg"];

        NSData*data=[NSData dataWithContentsOfURL:url];

        [[NSOperationQueue mainQueue] addOperationWithBlock:^{

            self.singleImageView.image=[UIImage imageWithData:data];

        }];

    }];

    

    NSBlockOperation*blockOperation1=[NSBlockOperation blockOperationWithBlock:^{

        NSURL *url = [NSURL URLWithString:@"http://img.pconline.com.cn/images/photoblog/9/9/8/1/9981681/200910/11/1255259355826.jpg"];

        NSData*data=[NSData dataWithContentsOfURL:url];

        [[NSOperationQueue mainQueue] addOperationWithBlock:^{

            self.secondImageView.image=[UIImage imageWithData:data];

        }];

    }];

    __weak __typeof__(self) weakSelf = self;

    NSBlockOperation*blockOperation3=[NSBlockOperation blockOperationWithBlock:^{

      

        UIGraphicsBeginImageContext(self.bottomImageView.frame.size);

        CGSize  size= self.bottomImageView.frame.size;

        [weakSelf.singleImageView.image drawInRect:CGRectMake(0, 0, size.width/2, size.height)];

        [weakSelf.secondImageView.image drawInRect:CGRectMake(size.width/2, 0, size.width/2, size.height)];

        UIImage *finalImage=UIGraphicsGetImageFromCurrentImageContext();

        [[NSOperationQueue mainQueue] addOperationWithBlock:^{

            weakSelf.bottomImageView.image=finalImage;

        }];

        

        UIGraphicsEndImageContext();

        

    }];

    

    

    [blockOperation3 addDependency:blockOperation1];

    [blockOperation3 addDependency:blockOperation];

    

    [queue addOperation:blockOperation3];

    [queue addOperation:blockOperation1];

    [queue addOperation:blockOperation];