iOS多线程应用开发中自定义NSOperation类的实例解析

-(NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return self.apps.count;
}
-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *ID=@"ID";
    UITableViewCell *cell=[tableView dequeueReusableCellWithIdentifier:ID];
    if (cell==nil) {
        cell=[[UITableViewCell alloc]initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:ID];
    }
    YYappModel *app=self.apps[indexPath.row];
    cell.textLabel.text=app.name;
    cell.detailTextLabel.text=app.download;
    
    //保证一个url对应一个image对象
    UIImage *image=self.images[app.icon];
    if (image) {//缓存中有图片
        cell.imageView.image=image;
    }else       //  缓存中没有图片，得下载
    {
        //先设置一张占位图片
        cell.imageView.image=[UIImage imageNamed:@"57437179_42489b0"];
        YYdownLoadOperation *operation=self.operations[app.icon];
        if (operation) {//正在下载
            //什么都不做
        }else  //当前没有下载，那就创建操作
        {
            operation=[[YYdownLoadOperation alloc]init];
            operation.url=app.icon;
            operation.indexPath=indexPath;
            operation.delegate=self;
            [self.queue addOperation:operation];//异步下载
            self.operations[app.icon]=operation;
        }
    }
   

    return cell;
}
-(void)downLoadOperation:(YYdownLoadOperation *)operation didFishedDownLoad:(UIImage *)image
{
    //1.移除执行完毕的操作
    [self.operations removeObjectForKey:operation.url];