performance - 设置 UIImageView 的图像属性会导致严重滞后

Question

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 加载 subview 作为一个从左到右滚动。每个 subview 有 10-20 个图像，每个图像大约 400x200。当我从一个 View 滚动到另一个 View 时，我遇到了相当多的滞后。

经过调查，我发现在卸载所有 View 并再次尝试后，延迟消失了。我认为图像的同步缓存是导致延迟的原因。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;
        });
    });
}

但是，在将所有 UIImageViews 更改为这个子类后，我仍然遇到滞后(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多的延迟(但仅在第一次加载时)？

请帮我。 =(

Answer 1

编辑 3:iOS 15 现在提供 UIImage.prepareForDisplay(completionHandler:) .

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

或者

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

编辑:实际上，我相信所有必要的是以下内容。 (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:事实证明它不是 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 View 刷新显示时执行此操作。所以解决方案是降低到 Core Graphics 的级别，并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;
        });
    });
}