NSOperation使用系统提供子类的方法--处理复杂任务

 //创建一个队列

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

    //把任务放在NSBlockOperation里面

    NSBlockOperation *myOperation=[NSBlockOperation blockOperationWithBlock:^{

        NSData *firstData=[NSData dataWithContentsOfURL:[NSURL URLWithString:KURL1]];

        dispatch_async(dispatch_get_main_queue(), ^{

            _firstImageView.image=[UIImage imageWithData:firstData];

       });

    }];

    //把第二任务添加到操作队列

    [myOperation addExecutionBlock:^{

        NSData *secondData=[NSData dataWithContentsOfURL:[NSURL URLWithString:KURL1]];

        dispatch_async(dispatch_get_main_queue(), ^{

            _secondImageView.image=[UIImage imageWithData:secondData];

        });

    }];

    [operation addOperation:myOperation];