ios - 对 UITableViewCell 使用 NSInvocationOperation 会导致某些 UIImageView 显示错误的图像？可能的重入问题？

Question

我试图让我的 UITableView 在浏览大约 700 张从 Internet 下载、缓存(到内部存储)并显示在表格的每个单元格上的图片时保持平滑。就滚动性能而言，我的代码到目前为止似乎还不错。但是，我注意到有时，如果连接很糟糕或者如果我滚动得非常快，一个单元格将显示错误的图片(另一个单元格的图片)大约每秒 1/2 秒，然后更新到它应该的图像显示。

到目前为止我怀疑两件事:

一个-从我的 NSInvocationOperation 使用 [self performSelectorOnMainThread:] 回调到主线程到执行主线程中的选择器的点，我可能会遇到重入问题.虽然我没有真正发现任何共享变量。

B-主线程和 NSInvocationOperation 之间的某种竞争？喜欢:

1个主线程调用cacheImageFromURL

2 在此调用中，UIImage 跨越工作线程

3 个工作线程几乎完成并开始调用 performSelectorOnMainThread

4 有问题的单元格在此时出列以便重新使用，因此主线程再次调用 cahceImageFromURL 以获得新图像。

5 在此调用中，UIImage 停止 NSOPerationQueue，这会导致先前的 NSInvocationOperation 线程终止。

6 但是，线程已经调用了 performSelectorOnMainThread

7 所以选择器会很兴奋导致加载旧图像。

8 在此之后，最近生成的线程完成获取新图像并再次调用 performSelectorOnMainThread，导致更新到正确的图像。

如果是这种情况，我想我需要在进入 cacheImageFromURL 方法时设置一个标志，这样工作线程代码就不会调用 performSelectorOnMainThread cacheImageFromURL 中是否已经有另一个线程(主线程)？

这是我的 UIImageView 子类的代码，表格中的每个单元格都使用它:

@implementation UIImageSmartView

//----------------------------------------------------------------------------------------------------------------------
@synthesize defaultNotFoundImagePath;
//----------------------------------------------------------------------------------------------------------------------
#pragma mark - init
//----------------------------------------------------------------------------------------------------------------------
- (void)dealloc
{ 
  if(!opQueue)
  {
    [opQueue cancelAllOperations];
    [opQueue release];
  }
  [super dealloc];
}
//----------------------------------------------------------------------------------------------------------------------
#pragma mark - functionality
//----------------------------------------------------------------------------------------------------------------------
- (bool)cacheImageFromURL:(NSString*)imageURL
{
  /*  If using for the first time, create the thread queue and keep it 
      around until the object goes out of scope*/
  if(!opQueue)
    opQueue = [[NSOperationQueue alloc] init];
  else
    [opQueue cancelAllOperations];

  NSString *imageName = [[imageURL pathComponents] lastObject];
  NSString* cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
  NSString *cachedImagePath = [cachePath stringByAppendingPathComponent:imageName];

  /*  If the image is already cached, load it from the local cache dir.
      Else span a thread and go get it from the internets.*/
  if([[NSFileManager defaultManager] fileExistsAtPath:cachedImagePath])
    [self setImage:[UIImage imageWithContentsOfFile:cachedImagePath]];
  else
  {
    [self setImage:[UIImage imageWithContentsOfFile:self.defaultNotFoundImagePath]];
    NSMutableArray *payload = [NSMutableArray arrayWithObjects:imageURL, cachedImagePath, nil];

    /*  Dispatch thread*/
    concurrentOp = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(loadURI:) object:payload];
    [opQueue addOperation: concurrentOp];
    [concurrentOp release];
  }

  return YES;
}
//----------------------------------------------------------------------------------------------------------------------
/*  Thread code*/
-(void)loadURI:(id)package
{
  NSArray *payload = (NSArray*)package;
  NSString *imageURL = [payload objectAtIndex:0];  
  NSString *cachedImagePath = [payload objectAtIndex:2];

  /*  Try fetching the image from the internets.
      If we got it, write it to disk. If fail, set the path to the not found again.*/
  UIImage *newThumbnail = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:imageURL]]];  
  if(!newThumbnail)
    cachedImagePath = defaultNotFoundImagePath;
  else  
    [UIImagePNGRepresentation(newThumbnail) writeToFile:cachedImagePath atomically:YES];

  /*  Call to the main thread - load the image from the cache directory
      at this point it'll be the recently downloaded one or the NOT FOUND one.*/
  [self performSelectorOnMainThread:@selector(updateImage:) withObject:cachedImagePath waitUntilDone:NO];
}
//----------------------------------------------------------------------------------------------------------------------
- (void)updateImage:(NSString*)cachedImagePath
{
  [self setImage:[UIImage imageWithContentsOfFile:cachedImagePath]];
}
//----------------------------------------------------------------------------------------------------------------------
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
  // Return YES for supported orientations
  return (interfaceOrientation == UIInterfaceOrientationPortrait);
}

@end

并且这个 UIImage 的使用方式是在 cellForRowAtIndexPath 的上下文中，像这样:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
  UIImageSmartView *cachedImage;
  // and some other stuff...

  static NSString *CellIdentifier = @"Cell";
  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
  if (cell == nil) 
  {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
                                   reuseIdentifier:CellIdentifier] autorelease];

    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
    cell.selectionStyle = UITableViewCellSelectionStyleGray;

    // some labels and tags stuff..

    cachedImage = [[UIImageSmartView alloc] initWithFrame:CGRectMake(5, 5, 57, 80)];
    cachedImage.contentMode = UIViewContentModeCenter;
    cachedImage.defaultNotFoundImagePath = [[NSBundle mainBundle] pathForResource:@"DefaultNotFound" ofType:@"png"];
    cachedImage.tag = PHOTO_TAG;

    [cell.contentView addSubview:cachedImage];
    [cell.contentView addSubview:mainLabel];
    [cell.contentView addSubview:secondLabel];

  }
  else
  {
    cachedImage = (UIImageSmartView*)[cell.contentView viewWithTag:PHOTO_TAG];
    mainLabel = (UILabel*)[cell.contentView viewWithTag:MAINLABEL_TAG]; 
  }

  // Configure the cell...

  NSString *ImageName = [[[self.dbData objectAtIndex:indexPath.row] objectAtIndex:2] 
                         stringByReplacingOccurrencesOfString:@".jpg" 
                         withString:@"@57X80.png"];

  NSString *imageURL = [NSString stringWithFormat:@"www.aServerAddress.com/%@/thumbnail5780/%@", 
                        self.referencingTable, 
                        ImageName];

  [cachedImage cacheImageFromURL:imageURL];

  mainLabel.text = [[self.dbData objectAtIndex:indexPath.row] objectAtIndex:0]; 
  return cell;
}

Answer 1

问题是单元格的重用，一个单元格同时请求各种图像，并在下载每个图像时显示，我知道你正在取消操作队列但是由于处理调用者是同步的，操作继续执行。我建议尝试保存请求的 indexPath，并在设置 UIImage 之前将其与单元格的索引路径匹配。