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];