设置 UIImageView 的图像属性会导致严重滞后 [英] Setting image property of UIImageView causes major lag
问题描述
让我告诉您我遇到的问题以及我如何尝试解决它.我有一个 UIScrollView 加载子视图作为一个从左到右滚动.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，我遇到了相当多的延迟.
Let me tell you about the problem I am having and how I tried to solve it. I have a UIScrollView which loads subviews as one scrolls from left to right. Each subview has 10-20 images around 400x200 each. When I scroll from view to view, I experience quite a bit of lag.
经过调查，我发现在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).
After investigating, I discovered that after unloading all the views and trying it again, the lag was gone. I figured that the synchronous caching of the images was the cause of the lag. So I created a subclass of UIImageView which loaded the images asynchronously. The loading code looks like the following (self.dispatchQueue returns a serial dispatch queue).
- (void)loadImageNamed:(NSString *)name {
dispatch_async(self.dispatchQueue, ^{
UIImage *image = [UIImage imageNamed:name];
dispatch_sync(dispatch_get_main_queue(), ^{
self.image = image;
});
});
}
然而，在将我所有的 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)?
However, after changing all of my UIImageViews to this subclass, I still experienced lag (I'm not sure if it was lessened or not). I boiled down the cause of the problem to self.image = image;. Why is this causing so much lag (but only on the first load)?
请帮帮我.=(
推荐答案
EDIT 3:iOS 15 现在提供 UIImage.prepareForDisplay(completionHandler:).
EDIT 3: iOS 15 now offers UIImage.prepareForDisplay(completionHandler:).
image.prepareForDisplay { decodedImage in
imageView.image = decodedImage
}
或
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
实际上，我相信所有必要的是以下内容.(未经测试.)
Actually, I believe all that is necessary is the following. (Untested.)
- (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;
});
});
}
ORIGINAL ANSWER:原来不是 self.image = image; 直接.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.
ORIGINAL ANSWER: It turns out that it wasn't self.image = image; directly. The UIImage image loading methods don't decompress and process the image data right away; they do it when the view refreshes its display. So the solution was to go a level lower to Core Graphics and decompress and process the image data myself. The new code looks like the following.
- (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;
});
});
}
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