SDWebImage 源代码剖析 - 缓存策略
SDWebImage 源代码剖析 - 缓存策略
今天我们将对另外一个在 iOS 开发中广泛使用的库的源代码进行分析，这个库就是鼎鼎大名的 SDWebImage。
使用方法
SDWebImage 的使用非常简洁，往往可以用一行代码来完成图片设置工作。下面列出一些常用设置方法。
- (void) sd_setImageWithURL:(nullable NSURL *) url;
- (void) sd_setImageWithURL:(nullable NSURL *) url placeholderImage:(nullable UIImage *) placeholder;
- (void) sd_setImageWithURL:(nullable NSURL *) url placeholderImage:(nullable UIImage *) placeholder options:(SDWebImageOptions) options;
- (void）sd_setImageWithURL:(nullable NSURL *) url completed:(nullable SDExternalCompletionBlock) completedBlock;
- (void) sd_setImageWithURL:(nullable NSURL *) url placeholderImage:(nullable UIImage *) placeholder options:(SDWebImageOptions) options completed:(nullable SDExternalCompletionBlock) completedBlock;url 是远程图片的 URL 地址，placeholderImage 是远程图片尚未下载完成时显示的占位图片，completedBlock 是远程图片下载完成后将要执行的 block，options 是一组 NS_OPTIONS 枚举值：
typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
// 当按照给出的 URL 下载失败后，这个 URL 会被加入黑名单，
// 如果下次这个 URL 再次出现，就不会尝试去下载
SDWebImageRetryFailed = 1 << 0,
// 通常来说，图像下载是在 UI 交互过程中进行的，
// 如果使用这个 flag 的话就会延迟图片的下载
SDWebImageLowPriority = 1 << 1,
// 禁止磁盘缓存，只允许内存缓存
SDWebImageCacheMemoryOnly = 1 << 2,
// 允许渐进式加载。默认的是加载完成才显示
SDWebImageProgressiveDownload = 1 << 3,
// 磁盘缓存将会由 NSURLCache 而不是 SDWebImage 来处理，因此可能带来轻微的性能下降。
// 使用于使用固定的图片 URL 但是图片内容可能变化的场景
SDWebImageRefreshCached = 1 << 4,
// 如果应用进入后台状态，继续图片下载，应用因此将会额外活跃一段时间，
// 如果这段时间用完但是下载任务尚未完成，那么下载就会被取消
SDWebImageContinueInBackground = 1 << 5,
// 处理存储在 NSHTTPCookieStore 中的 cookie
SDWebImageHandleCookie = 1 << 6,
// 允许不受信任的 SSL 认证。通常用于测试环境，很少用于生产环境
SDWebImageAllowInvalidSSLCertificates = 1 << 7,
// 提高该图片下载的优先级
SDWebImageHighPriority = 1 << 8,
// 通常在图片加载时都会显示 placeholder。但是这个 flag 会将 placeholder 的显示延迟到
// 图片加载之后（不是很懂这个选项的意思）
/**
* By default, placeholder images are loaded while the image is loading. This flag will delay the loading
* of the placeholder image until after the image has finished loading.
*/
SDWebImageDelayPlaceholder = 1 << 9,
// 并不常用的方法。用于对下载的图片进行变换。
// 这个变换工作由实现了 transformDownloadedImage 的协议的类完成
SDWebImageTransformAnimatedImage = 1 << 10,
// 在下载完成之后将图片设置成 imageView.image 之前，
// 允许你对下载的图片进行额外的处理
SDWebImageAvoidAutoSetImage = 1 << 11,
// 图片默认会被解码成它们的原始尺寸。这个 flag 会将图片按照设备的内存来进行缩放。
// 如果 SDWebImageProgressiveDownload 被设置了，那么这个选项就不起作用
SDWebImageScaleDownLargeImages = 1 << 12
};内部实现
UIView+WebCache
上面列出的方法其实是一个核心方法接受不同参数时的不同版本。
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock;这个核心方法内部是这样实现的：
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
[self sd_internalSetImageWithURL:url
placeholderImage:placeholder
options:options
operationKey:nil
setImageBlock:nil
progress:progressBlock
completed:completedBlock];
}进入sd_internalSetImageWithURL: 这个方法的内部：
// UIView+WebCache.m
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
operationKey:(nullable NSString *)operationKey
setImageBlock:(nullable SDSetImageBlock)setImageBlock
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
// 1
NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
// 2
[self sd_cancelImageLoadOperationWithKey:validOperationKey];
// 3
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
// 4
if (!(options & SDWebImageDelayPlaceholder)) {
dispatch_main_async_safe(^{
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
});
}
if (url) {
// check if activityView is enabled or not
if ([self sd_showActivityIndicatorView]) {
[self sd_addActivityIndicator];
}
__weak __typeof(self)wself = self;
// 5
id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
__strong __typeof (wself) sself = wself;
[sself sd_removeActivityIndicator];
if (!sself) {
return;
}
dispatch_main_async_safe(^{
if (!sself) {
return;
}
// 6
if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
completedBlock(image, error, cacheType, url);
return;
} else if (image) {
// 7
[sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
} else {
// 8
if ((options & SDWebImageDelayPlaceholder)) {
[sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
}
}
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
// 9
[self sd_setImageLoadOperation:operation forKey:validOperationKey];
} else {
dispatch_main_async_safe(^{
[self sd_removeActivityIndicator];
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
completedBlock(nil, error, SDImageCacheTypeNone, url);
}
});
}
}- 用当前视图的类名来作为一个key。
SDWebImage不仅能用来设置UIImageView，也可以用来设置UIButton。 - 在当前视图的operationDictionary 中进行查找，如果已经有key为operationKey 的operation，则取消这个operation。
- 将该远程图片的url 与当前视图的imageURLKey 用关联对象设置在一起。关于关联对象，网上也已经有很多不错的分析文章。
- 如果没有设置
SDWebImageDelayPlaceholder这个选项，那么就先将当前视图设置成placeholder。 - 使用
SDWebImageManager的loadImageWithURL:创建一个operation 对象。 - 如果 image 下载完成了，并且设置了
SDWebImageAvoidAutoSetImage选项，而且传入了对下载的图片进行处理的 block，那么就进行对应处理。 - 如果 image 下载完成了，没有额外处理要求，那么将当前视图设置为 image。
- 如果 image 下载失败了，那么还是将当前视图设置为 placeholder。
- 将 5 中创建的 operation 的 key 设置为 operationKey，然后加入当前视图的 operationDictionary 中。
SDWebImageManager
由代码可知，operation 这个对象是设置过程的核心与关键。既然它是 SDWebImageManager 创建的，我们自然要去探究下 SDWebImageManager 的内部秘密。
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
// 1
// Invoking this method without a completedBlock is pointless
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
...
// 2
@synchronized (self.runningOperations) {
[self.runningOperations addObject:operation];
}
// 3
NSString *key = [self cacheKeyForURL:url];
// 4
...
}1
// Invoking this method without a completedBlock is pointless
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
// Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, Xcode won't
// throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.
if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
// Prevents app crashing on argument type error like sending NSNull instead of NSURL
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;
BOOL isFailedUrl = NO;
if (url) {
@synchronized (self.failedURLs) {
isFailedUrl = [self.failedURLs containsObject:url];
}
}
if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
[self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
return operation;
}首先进行异常处理。这是编程时一个常见的习惯，将可能遇到的各种问题和对应的解决方案放在方法的开头，可以使得逻辑变得清晰，同时也避免了无谓的函数调用开销。SDWebImage 团队贴心地为我们处理了常见的误将 NSString 类型的对象传入 NSURL 类型的参数的错误。这启示我们，在编写自己的库时，应尽可能考虑到各种常见错误，并对它们进行处理，这样可以使得你的库对于别的开发者更加友好。
2
将 operation 加入 SDWebImageManager 的 runningOperations 数组中。
3
获得远程图片 URL 所对应的 key。
- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url {
if (!url) {
return @"";
}
if (self.cacheKeyFilter) {
// 如果定义了用来对 URL 进行过滤的 filter，那么就用 filter 来处理
return self.cacheKeyFilter(url);
} else {
// 否则就返回 URL 的 string 表示
return url.absoluteString;
}
}4
先查询operation 的缓存操作。
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
if (!key) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}
// First check the in-memory cache...
// 首先检查该图片在内存中是否有缓存
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
NSData *diskData = nil;
if ([image isGIF]) {
diskData = [self diskImageDataBySearchingAllPathsForKey:key];
}
if (doneBlock) {
doneBlock(image, diskData, SDImageCacheTypeMemory);
}
// 如果在内存中有缓存，这里就会直接返回了
return nil;
}
// 当该图片在内存中没有缓存的时候才会执行下面的代码
NSOperation *operation = [NSOperation new];
dispatch_async(self.ioQueue, ^{
if (operation.isCancelled) {
// do not call the completion if cancelled
return;
}
@autoreleasepool {
// 获得该图片在磁盘中的缓存
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage = [self diskImageForKey:key];
if (diskImage && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = SDCacheCostForImage(diskImage);
// 将磁盘缓存保存在内存中
[self.memCache setObject:diskImage forKey:key cost:cost];
}
if (doneBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
});
}
}
});
return operation;
}在查询到operation 的缓存操作后，设置doneBlock:
operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
if (operation.isCancelled) {
[self safelyRemoveOperationFromRunning:operation];
return;
}
// 1
if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
// 2
if (cachedImage && options & SDWebImageRefreshCached) {
// If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
// AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
[self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
}
// download if no image or requested to refresh anyway, and download allowed by delegate
// 3
SDWebImageDownloaderOptions downloaderOptions = 0;
if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;
// 4
if (cachedImage && options & SDWebImageRefreshCached) {
// force progressive off if image already cached but forced refreshing
downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
// ignore image read from NSURLCache if image if cached but force refreshing
downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
}
// 5
SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (!strongOperation || strongOperation.isCancelled) {
// Do nothing if the operation was cancelled
// See #699 for more details
// if we would call the completedBlock, there could be a race condition between this block and another completedBlock for the same object, so if this one is called second, we will overwrite the new data
} else if (error) {
[self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];
// 6
if ( error.code != NSURLErrorNotConnectedToInternet
&& error.code != NSURLErrorCancelled
&& error.code != NSURLErrorTimedOut
&& error.code != NSURLErrorInternationalRoamingOff
&& error.code != NSURLErrorDataNotAllowed
&& error.code != NSURLErrorCannotFindHost
&& error.code != NSURLErrorCannotConnectToHost
&& error.code != NSURLErrorNetworkConnectionLost) {
@synchronized (self.failedURLs) {
[self.failedURLs addObject:url];
}
}
}
else {
if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
// 7
if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
// Image refresh hit the NSURLCache cache, do not call the completion block
} else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
// 8
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
if (transformedImage && finished) {
BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
// pass nil if the image was transformed, so we can recalculate the data from the image
[self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
}
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
});
} else {
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
}
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
}
}
if (finished) {
[self safelyRemoveOperationFromRunning:strongOperation];
}
}];
operation.cancelBlock = ^{
[self.imageDownloader cancel:subOperationToken];
__strong __typeof(weakOperation) strongOperation = weakOperation;
[self safelyRemoveOperationFromRunning:strongOperation];
};
} else if (cachedImage) {
// 9
__strong __typeof(weakOperation) strongOperation = weakOperation;
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
[self safelyRemoveOperationFromRunning:operation];
} else {
// Image not in cache and download disallowed by delegate
// 10
__strong __typeof(weakOperation) strongOperation = weakOperation;
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
[self safelyRemoveOperationFromRunning:operation];
}
}];- 由 if 条件，假设后一条件成立，如果缓存存在，那么只有在设置了
SDWebImageRefreshCache才会进入 if 语句体中；如果缓存不存在，那么肯定会进入 if 语句体中。 - 如果说缓存存在，而且设置了
SDWebImageRefreshCache，那么就应该从 server 上重新下载图片以更新本地缓存。 - 设置下载时的选项。
- 如果设置了
SDWebImageRefreshCache，那么必须取消渐进式下载，而且还要忽略从 NSURLCache 中获得的缓存响应。 - 调用 imageDownloader 进行图片下载。
- 如果发生了错误，且错误原因不是列出来的这些原因中的一个，那么就把这个 URL 加入黑名单中。
- 如果缓存存在，设置了
SDWebImageRefreshCache，而且 downloadedImage 为 nil，那么说明命中了 NSURLCache 缓存，什么事也不做。 - 如果 downloadedImage 非 nil，并且设置了
SDWebImageTransformAnimatedImage，那么就在主线程中对图片进行变换，然后将变换后的图片存储在内存中或内存和磁盘上。如果不需要变换，那么直接将 downloadedImage 存储在内存中或内存和磁盘上。 - 如果缓存存在，且其他条件都不成立，那么直接取出缓存中的图片，然后在主线程中更新视图。
- 如果缓存不存在，并且也不允许下载，那么直接调用 completedBlock。
小结
我们来小结一下 SDWebImage 的缓存策略。在给一个 UIImageView 或者 UIButton 设置了远端图片的 URL 后，SDWebImage 首先以 URL 为 key 在内存中寻找图片缓存，如果在内存中没找到就会去磁盘中寻找，如果找到了，则将磁盘中的缓存拷贝一份到内存中，然后使用缓存来设置视图。如果在磁盘和内存中都没有找到，那么才会下载远程图片，然后将远程图片缓存在内存中或者是内存和磁盘上。