iOS：如何阻止后台线程更新主线程中的UI？

我实现了一个UITmageView UITableView单元,每个单元通过NSTimer每隔5秒定期刷新一次.每个图像都是从后台线程中的服务器加载的,从后台线程我也通过调用performSELEctorOnMainThread更新UI,显示新图像.到现在为止还挺好.

目前,当我不等到当前单元格中加载图像并快速向下滚动到新单元格时,我遇到了问题.然而,该新单元显示先前单元的图像而不是新单元的图像.虽然下一轮NSTimer会正确显示图像,但这可能会让用户感到困惑.

如果我不重用UITableView的单元格,问题就会消失,但考虑到我的应用程序中显示的单元格数量,这不是一个选项.

因此,我能想到的唯一解决方案是,如果我知道用户执行滚动操作,则取消(或终止)将显示旧图像的后台线程.

我想知道这可能不是最好的做法,因此,寻求你的意见.

(我也不能使用SDWebImage,因为我的要求是在从服务器加载的循环中显示一组图像)

// In MyViewController.m
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowATindexPath:(NSIndexPath *)indexPath
{
    ...
    NSTimer* timer=[NSTimer scheduledTimerWithTimeInterval:ANIMATION_scheDULED_AT_TIME_INTERVAL 
                                                target:self 
                                              SELEctor:@SELEctor(updateImageInBACkground:) 
                                              userInfo:cell.imageView 
                                               repeats:YES];
    ...
}

- (void) updateImageInBACkground:(NSTimer*)aTimer
{  
    [self performSELEctorInBACkground:@SELEctor(updateImage:)
                       withObject:[aTimer userInfo]];
}  

- (void) updateImage:(AnimatedImageView*)animatedImageView 
{      
    @autoreleasepool { 
        [animatedImageView refresh];
    }
}  

// In AnimatedImageView.m
-(void)refresh
{
    if(self.currenTindex>=self.urls.count)
        self.currenTindex=0;

    ASIhttprequest *request=[[ASIhttprequest alloc] initWithURL:[self.urls objectATindex:self.currenTindex]];
    [request startSynchronous];

    UIImage *image = [UIImage imageWithData:[request responseData]];

    // How do I cancel this operation if I kNow that a user performs a scrolling action,therefore deparTing from this cell.
    [self performSELEctorOnMainThread:@SELEctor(performTransition:)
                       withObject:image
                    waitUntilDone:YES];
}

-(void)performTransition:(UIImage*)anImage
{
    [UIView transitionWithView:self duration:1.0 options:(UIViewAnimationOptionTransitionCrossDissolve | UIViewAnimationOptionAllowUserInteraction) animations:^{ 
        self.image=anImage;
        currenTindex++;
    } completion:^(BOOL finished) {
    }];
}

// your controller
    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowATindexPath:(NSIndexPath *)indexPath
    {
    ...
    [cell.imageView setDistantImage:imageURL];
    ...    
    }

//your ImageView class

@property (...) ASIhttprequest *distantImagerequest;

- (void) setDistantImageUrl:(NSUrl *)imageUrl
{
    //clear prevIoUs request
    [distantImagerequest clearDelegatesAndCancel];
    //set a new request,with the callBACk embedded directly in this ImageView class
    ... 
}

//animation and callBACks methods in this ImageView class too...