Question

我正在尝试从iPad照片库中创建所选照片的缩略图（288x288）。我在UITableView中显示了一组ALAsset对象，当我选择一行时，会显示该图像的更大预览（288x288）。为了防止主线程阻塞，我正在尝试在后台线程上创建缩略图，并将缩略图的副本缓存到文件系统。

在视图控制器中选择tableview行时，我在后台调用loadPreviewImage：

- (void)tableView:(UITableView *)tableView
        didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    // get the upload object from an array that contains a ALAsset object
    upload = [uploads objectAtIndex:[indexPath row]];

    [self performSelectorInBackground:@selector(loadPreviewImage:)
                           withObject:upload];

}

我传递了一个包含asseturl属性的自定义上传对象：

- (void)loadPreviewImage:(MyUploadClass*)upload
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; 

    UIImage *preview = [upload previewImage];       
    [self performSelectorOnMainThread:@selector(setPreviewImage:)
                           withObject:preview
                        waitUntilDone:YES];

    [pool release];
}

在主线程上调用它以在加载后显示缩略图：

- (void)setPreviewImage:(UIImage*)image
{
    self.imageViewPreview.image = image;
    [self layoutSubviews];
}

这是MyUploadClass的一种方法：

- (UIImage *)previewImage
{
    __block UIImage *previewImage = [[UIImage imageWithContentsOfFile:
            [self uploadPreviewFilePath]] retain];

    if (previewImage == nil && asseturl)
    {
        ALAssetsLibrary* library = [[ALAssetsLibrary alloc] init];
        [library assetForURL:self.asseturl resultBlock:^(ALAsset *asset)
        {                         
            ALAssetRepresentation *rep = [asset defaultRepresentation]; 

            previewImage = [UIImage imageWithCGImage: [rep fullScreenImage]];
            previewImage = [[previewImage resizedImageWithContentMode:UIViewContentModeScaleAspectFit
                                             bounds:CGSizeMake(288, 288)
                               interpolationQuality:kCGInterpolationHigh] retain];

            NSData *previewData = UIImageJPEGRepresentation(previewImage, 1.0); 
            [previewData writeToFile:[self uploadPreviewFilePath] atomically:YES];   
         }
         failureBlock:^(NSError *error){ }];
         [library release];
    } 
    return [previewImage autorelease];
}

问题是我第一次得到nil previewImage，只有在缓存缩放后我得到一个图像对象。我究竟做错了什么？有没有更好的方法解决这个问题？

Answer 1

我没有清楚地理解ALAssetsLibrary的resultBlock如何运作，我的错误是认为执行是线性的。事实证明，在我的情况下，resultBlock在主线程上执行，而previewImage中的其余代码在后台线程上执行。因为在resultBlock有机会结束执行之前返回了previewImage，所以我得到了nil。我通过使用以下方法替换previewImage来解决问题：

- (void) loadPreviewImage:(CGSize)size withTarget:(id)target andCallback:(SEL)callback
{
    NSString *path = [self uploadPreviewFilePath];    
    UIImage *previewImage = [UIImage imageWithContentsOfFile:path];  

    if (previewImage == nil && asseturl)
    {
        ALAssetsLibrary* library = [[ALAssetsLibrary alloc] init];
        [library assetForURL:self.asseturl resultBlock:^(ALAsset *asset)
        {
            if (asset) {                                  
              ALAssetRepresentation *rep = [asset defaultRepresentation];   

              UIImage *img = [UIImage imageWithCGImage: [rep fullScreenImage]];

              img = [img resizedImageWithContentMode:UIViewContentModeScaleAspectFit
                    bounds:size interpolationQuality:kCGInterpolationHigh];

              NSData *previewData = UIImageJPEGRepresentation(img, 1.0);                            
              [previewData writeToFile:path atomically:YES];     
              [target performSelectorOnMainThread:callback 
                                       withObject:img
                                    waitUntilDone:YES];
          }
       }
      failureBlock:^(NSError *error){ }];                        
      [library release];
  }
  else {
      [target performSelectorOnMainThread:callback withObject:img waitUntilDone:YES];
    }
}

在后台线程上保存缩略图

1 个答案: