设置 UIImageView 的图像属性会导致严重滞后

让我告诉你我遇到的问题以及我如何尝试解决它。我有一个 UIScrollView ，它在从左到右滚动时加载子视图。每个子视图有 10-20 个图像，每个图像大约 400x200。当我从一个视图滚动到另一个视图时，我遇到了相当大的延迟。

经过调查，我发现卸载所有视图并再次尝试后，延迟消失了。我认为图像的同步缓存是造成延迟的原因。所以我创建了一个 UIImageView 的子类，它异步加载图像。加载代码如下所示（self.dispatchQueue返回一个串行调度队列）。

- (void)loadImageNamed:(NSString *)name {
    dispatch_async(self.dispatchQueue, ^{
        UIImage *image = [UIImage imageNamed:name];

        dispatch_sync(dispatch_get_main_queue(), ^{
            self.image = image;
        });
    });
}

然而，在将我的所有 UIImageViews 更改为这个子类之后，我仍然遇到延迟（我不确定它是否减少了）。我把问题的原因归结为self.image = image;。为什么这会导致如此大的延迟（但仅在第一次加载时）？

请帮我。 =(

编辑 3：iOS 15 现在提供UIImage.prepareForDisplay(completionHandler:) https://developer.apple.com/documentation/uikit/uiimage/3750844-preparefordisplay.

image.prepareForDisplay { decodedImage in
    imageView.image = decodedImage
}

or

imageView.image = await image.byPreparingForDisplay()

编辑 2：这是一个包含一些改进的 Swift 版本。 （未经测试。）https://gist.github.com/fumoboy007/d869e66ad0466a9c246d https://gist.github.com/fumoboy007/d869e66ad0466a9c246d

编辑：实际上，我相信所有必要的就是以下内容。 （未经测试。）

- (void)loadImageNamed:(NSString *)name {
    dispatch_async(self.dispatchQueue, ^{
        // Determine path to image depending on scale of device's screen,
        // fallback to 1x if 2x is not available
        NSString *pathTo1xImage = [[NSBundle mainBundle] pathForResource:name ofType:@"png"];
        NSString *pathTo2xImage = [[NSBundle mainBundle] pathForResource:[name stringByAppendingString:@"@2x"] ofType:@"png"];

        NSString *pathToImage = ([UIScreen mainScreen].scale == 1 || !pathTo2xImage) ? pathTo1xImage : pathTo2xImage;


        UIImage *image = [[UIImage alloc] initWithContentsOfFile:pathToImage];

        // Decompress image
        if (image) {
            UIGraphicsBeginImageContextWithOptions(image.size, NO, image.scale);

            [image drawAtPoint:CGPointZero];

            image = UIGraphicsGetImageFromCurrentImageContext();

            UIGraphicsEndImageContext();
        }


        // Configure the UI with pre-decompressed UIImage
        dispatch_async(dispatch_get_main_queue(), ^{
            self.image = image;
        });
    });
}

原始答案：事实证明并非如此self.image = image;直接地。 UIImage图像加载方法不会立即解压缩和处理图像数据；他们在视图刷新其显示时执行此操作。所以解决方案是降低到 Core Graphics 级别，自己解压和处理图像数据。新代码如下所示。

- (void)loadImageNamed:(NSString *)name {
    dispatch_async(self.dispatchQueue, ^{
        // Determine path to image depending on scale of device's screen,
        // fallback to 1x if 2x is not available
        NSString *pathTo1xImage = [[NSBundle mainBundle] pathForResource:name ofType:@"png"];
        NSString *pathTo2xImage = [[NSBundle mainBundle] pathForResource:[name stringByAppendingString:@"@2x"] ofType:@"png"];
        
        NSString *pathToImage = ([UIScreen mainScreen].scale == 1 || !pathTo2xImage) ? pathTo1xImage : pathTo2xImage;
        
        
        UIImage *uiImage = nil;
        
        if (pathToImage) {
            // Load the image
            CGDataProviderRef imageDataProvider = CGDataProviderCreateWithFilename([pathToImage fileSystemRepresentation]);
            CGImageRef image = CGImageCreateWithPNGDataProvider(imageDataProvider, NULL, NO, kCGRenderingIntentDefault);
            
            
            // Create a bitmap context from the image's specifications
            // (Note: We need to specify kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little
            // because PNGs are optimized by Xcode this way.)
            CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
            CGContextRef bitmapContext = CGBitmapContextCreate(NULL, CGImageGetWidth(image), CGImageGetHeight(image), CGImageGetBitsPerComponent(image), CGImageGetWidth(image) * 4, colorSpace, kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
            
            
            // Draw the image into the bitmap context
            CGContextDrawImage(bitmapContext, CGRectMake(0, 0, CGImageGetWidth(image), CGImageGetHeight(image)), image);
            
            //  Extract the decompressed image
            CGImageRef decompressedImage = CGBitmapContextCreateImage(bitmapContext);
            
            
            // Create a UIImage
            uiImage = [[UIImage alloc] initWithCGImage:decompressedImage];
            
            
            // Release everything
            CGImageRelease(decompressedImage);
            CGContextRelease(bitmapContext);
            CGColorSpaceRelease(colorSpace);
            CGImageRelease(image);
            CGDataProviderRelease(imageDataProvider);
        }
        
        
        // Configure the UI with pre-decompressed UIImage
        dispatch_async(dispatch_get_main_queue(), ^{
            self.image = uiImage;
        });
    });
}