打算用几篇文章整理一下 SDWebImage 的源码
源码有点小多, 决定把每个模块分开来整理
这其中包括 : 调度模块、下载模块、缓存模块、解码模块和一些代码整理
调度模块看这里
缓存模块看这里
下载模块看这里
解码模块看这里
整理模块看这里
本文简介每个模块的调度类 SDWebImageManager
SDWebImage 有几个核心类, 他们分别负责对应的核心模块:
| 核心类 | 负责模块 |
|---|---|
| SDWebImageManager | 调度各个类 |
| SDWebImageDownloader | 下载 |
| SDImageCache | 缓存, 包括内存缓存和磁盘缓存 |
| SDWebImageCodersManager | 图片解码 |
他们之间的关系是这样 :
SDWebImageManager
SDWebImageManager 负责调度其他核心类配合工作
也是与 SD 的调用者接交互的一个类
例如UIImageView+WebCache 中的 - (nullable NSURL *)sd_setImageWithURL 方法
会直接来到 SDWebImageManager 的这个方法 :
[manager loadImageWithURL:imageURL
options:0
progress:nil
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
if (image) {
// do something with image
}
}];
就是在这个方法中进行了上面流程图中的一顿猛虎操作
看一下这个方法的大概流程
关键部分都写了注释
/** 加载图片 */
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
// Invoking this method without a completedBlock is pointless
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
/** 容错 */
if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
// 防止应用程序崩溃在参数类型错误，如发送NSNull而不是NSURL
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}
/** 创建操作组合 */
SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
operation.manager = self;
BOOL isFailedUrl = NO;
if (url) {
LOCK(self.failedURLsLock);
/** 查看当前链接是否失败过 */
isFailedUrl = [self.failedURLs containsObject:url];
UNLOCK(self.failedURLsLock);
}
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;
}
/**
添加本次任务到 runningOperations
runningOperations 是存放正在 load 的所有操作的集合
*/
LOCK(self.runningOperationsLock);
[self.runningOperations addObject:operation];
UNLOCK(self.runningOperationsLock);
NSString *key = [self cacheKeyForURL:url];
SDImageCacheOptions cacheOptions = 0;
if (options & SDWebImageQueryDataWhenInMemory) cacheOptions |= SDImageCacheQueryDataWhenInMemory;
if (options & SDWebImageQueryDiskSync) cacheOptions |= SDImageCacheQueryDiskSync;
if (options & SDWebImageScaleDownLargeImages) cacheOptions |= SDImageCacheScaleDownLargeImages;
__weak SDWebImageCombinedOperation *weakOperation = operation;
/** load 时, 先让 ImageCache 先去缓存中取, 如果取不到, 再去下载 */
operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key options:cacheOptions done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
/** 内存和磁盘查找结束后, 来到这里, 主线程回调 */
NSLog(@"-*/-*-*/-/*-*/-*/-*/-*/-*/-*/-*/-*/-*/ %@",[NSThread currentThread]);
/** 如果检测到操作被取消，则直接 return */
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (!strongOperation || strongOperation.isCancelled) {
[self safelyRemoveOperationFromRunning:strongOperation];
return;
}
BOOL shouldDownload = (!(options & SDWebImageFromCacheOnly))
&& (!cachedImage || options & SDWebImageRefreshCached)
&& (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url]);
// 如果缓存没找到图片, 并且允许下载
if (shouldDownload) {
// 如果满足条件, 开始下载
if (cachedImage && options & SDWebImageRefreshCached) {
//如果在缓存中找到了图像但提供了SDWebImageRefreshCached，则通知缓存图像
//并尝试重新下载它，以便让 NSURLCache 有机会从服务器刷新它。
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
}
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;
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;
}
// 再次避免循环引用
// `SDWebImageCombinedOperation` -> `SDWebImageDownloadToken` -> `downloadOperationCancelToken`, which is a `SDCallbacksDictionary` and retain the completed block below, so we need weak-strong again to avoid retain cycle
__weak typeof(strongOperation) weakSubOperation = strongOperation;
/** 正式开始下载图片 */
strongOperation.downloadToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
/** 下载结束后, 来到这里, 图片已经是解码过了 */
__strong typeof(weakSubOperation) strongSubOperation = weakSubOperation;
if (!strongSubOperation || strongSubOperation.isCancelled) {
// 不进行任何操作, 如果操作被取消
// 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) {
/** 处理下载 error */
[self callCompletionBlockForOperation:strongSubOperation completion:completedBlock error:error url:url];
BOOL shouldBlockFailedURL;
// Check whether we should block failed url
if ([self.delegate respondsToSelector:@selector(imageManager:shouldBlockFailedURL:withError:)]) {
shouldBlockFailedURL = [self.delegate imageManager:self shouldBlockFailedURL:url withError:error];
} else {
shouldBlockFailedURL = ( error.code != NSURLErrorNotConnectedToInternet
&& error.code != NSURLErrorCancelled
&& error.code != NSURLErrorTimedOut
&& error.code != NSURLErrorInternationalRoamingOff
&& error.code != NSURLErrorDataNotAllowed
&& error.code != NSURLErrorCannotFindHost
&& error.code != NSURLErrorCannotConnectToHost
&& error.code != NSURLErrorNetworkConnectionLost);
}
if (shouldBlockFailedURL) {
LOCK(self.failedURLsLock);
[self.failedURLs addObject:url];
UNLOCK(self.failedURLsLock);
}
} else {
/**
如果下载没出错
*/
/** 默认情况下，当URL无法下载时，URL会被列入黑名单，因此库不会继续尝试。 SDWebImageRetryFailed 标志禁用此黑名单。 */
if ((options & SDWebImageRetryFailed)) {
LOCK(self.failedURLsLock);
[self.failedURLs removeObject:url];
UNLOCK(self.failedURLsLock);
}
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
/** 如果不是单例 SDWebImageManager, 那么要缩放一下, 因为单例 SDWebImageManager 已经帮我们缩放过了 */
if (self != [SDWebImageManager sharedManager] && self.cacheKeyFilter && downloadedImage) {
downloadedImage = [self scaledImageForKey:key image:downloadedImage];
}
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:)]) {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
@autoreleasepool {
UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
if (transformedImage && finished) {
BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
NSData *cacheData;
// pass nil if the image was transformed, so we can recalculate the data from the image
if (self.cacheSerializer) {
cacheData = self.cacheSerializer(transformedImage, (imageWasTransformed ? nil : downloadedData), url);
} else {
cacheData = (imageWasTransformed ? nil : downloadedData);
}
[self.imageCache storeImage:transformedImage imageData:cacheData forKey:key toDisk:cacheOnDisk completion:nil];
}
[self callCompletionBlockForOperation:strongSubOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
}
});
} else {
/** 如果正常下载, 没出什么幺蛾子, 是来到这个分支的 */
if (downloadedImage && finished) {
if (self.cacheSerializer) {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
@autoreleasepool {
NSData *cacheData = self.cacheSerializer(downloadedImage, downloadedData, url);
[self.imageCache storeImage:downloadedImage imageData:cacheData forKey:key toDisk:cacheOnDisk completion:nil];
}
});
} else {
/** 缓存图片 */
[self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
}
}
/** 回调 block */
[self callCompletionBlockForOperation:strongSubOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
}
}
if (finished) {
[self safelyRemoveOperationFromRunning:strongSubOperation];
}
}];
} else if (cachedImage) {
// 磁盘找到了图 回调
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
[self safelyRemoveOperationFromRunning:strongOperation];
} else {
// 图片没找到 还不允许下载
// Image not in cache and download disallowed by delegate
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
[self safelyRemoveOperationFromRunning:strongOperation];
}
}];
return operation;
}
SDWebImageManager 类中有一个 SDWebImageCombinedOperation
只在内部自己使用
看看他的属性 :
@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>
/** 是否取消当前所有操作 */
@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;
/** 下载操作, 可以用来取消下载 */
@property (strong, nonatomic, nullable) SDWebImageDownloadToken *downloadToken;
/** 缓存查找操作, 可以用来取消查找 */
@property (strong, nonatomic, nullable) NSOperation *cacheOperation;
/** SDWebImageManager */
@property (weak, nonatomic, nullable) SDWebImageManager *manager;
@end
他的作用就是把一个加载图片任务的所有子任务集中在一起
加载一次图片任务的子任务包括缓存查找任务和下载任务
这样可以很方便的取消所有子任务
SDWebImageManager 另外还有一些实用的方法
/** 缓存图片根据指定的 url 和 image */
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url;
/** 取消所有 load */
- (void)cancelAll;
/** 检查是否有图片正在下载 */
- (BOOL)isRunning;
/** 检查缓存中是否缓存了当前url对应的图片-先判断内存缓存、再判断磁盘缓存 */
- (void)cachedImageExistsForURL:(nullable NSURL *)url
completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
/** 检查图片是否缓存 在磁盘中 */
- (void)diskImageExistsForURL:(nullable NSURL *)url
completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
/** 根据下载链接生成 cacheKey */
- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url;
