在 UITableView 中使用 SDWebImage 调整图像大小

Question

我在 UITableView 中使用 SDWebImage。 我需要根据图像框架创建单元格大小，我正在使用这种方法：

-(UIImage *)imageManager:(SDWebImageManager *)imageManager transformDownloadedImage:(UIImage *)image withURL:(NSURL *)imageURL{



    CGSize newSize = CGSizeMake(self.cmImage.frame.size.width, self.cmImage.frame.size.width * image.size.height / image.size.width);
    // Create a graphics image context
    UIGraphicsBeginImageContext(newSize);

    // draw in new context, with the new size
    [image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];

    // Get the new image from the context
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();

    // End the context
    UIGraphicsEndImageContext();

    return newImage;

}

我打电话给这个

- (void)awakeFromNib {
    SDWebImageManager.sharedManager.delegate = self;
}

在 cellForRowAtIndexPath 我这样做了：

 [SDWebImageManager.sharedManager downloadImageWithURL:[NSURL URLWithString:_model.data] options:0 progress:nil completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            self.cmImage.image = image;
        }];

在单元格中调整图像大小并更改cmImage的框架

但我认为此方法对于创建 cmImage 框架的响应为时已晚，并且在我滚动 tableView 后某些图像正在调整大小，并且对于某些单元格我加载了不同的图像

我还收到警告 [UIView frame] must be used only from main thread 从这一行：

CGSize newSize = CGSizeMake(self.cmImage.frame.size.width, self.cmImage.frame.size.width * image.size.height / image.size.width);

我到底做错了什么？

Answer 1

SDWebImage 在后台线程中下载图像。所以：

[SDWebImageManager.sharedManager downloadImageWithURL:[NSURL URLWithString:_model.data] options:0 progress:nil completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {  
 dispatch_async(dispatch_get_main_queue(), ^{
        self.cmImage.image = image;
    });
}];

另外 cellForRowAtIndexPath 调用很频繁。所以你应该在 iamgeview 上制作一个占位符大小。图片下载完成后调整 imageview 的大小。

-

---------- 再次编辑-------- -----

-

看SDWebImage原注，这个方法是从全局队列中调用的。所以你应该改变主队列的图像大小。

/**
 * Allows to transform the image immediately after it has been downloaded and just before to cache it on disk and memory.
 * NOTE: This method is called from a global queue in order to not to block the main thread.
 *
 * @param imageManager The current `SDWebImageManager`
 * @param image        The image to transform
 * @param imageURL     The url of the image to transform
 *
 * @return The transformed image object.
 */
- (nullable UIImage *)imageManager:(nonnull SDWebImageManager *)imageManager transformDownloadedImage:(nullable UIImage *)image withURL:(nullable NSURL *)imageURL;

所以用下面的代码补充这个方法

-(UIImage *)imageManager:(SDWebImageManager *)imageManager transformDownloadedImage:(UIImage *)image withURL:(NSURL *)imageURL{

    __block UIImage *newImage = nil ;
    dispatch_async(dispatch_get_main_queue(), ^{
        CGSize newSize = CGSizeMake(self.cmImage.frame.size.width, self.cmImage.frame.size.width * image.size.height / image.size.width);
        // Create a graphics image context
        UIGraphicsBeginImageContext(newSize);

        // draw in new context, with the new size
        [image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];

        // Get the new image from the context
         newImage = UIGraphicsGetImageFromCurrentImageContext();

        // End the context
        UIGraphicsEndImageContext();

    });
        return newImage;
}