iOS源码解析—SDWebImage(SDWebImageManager)

概述

SDWebImageManager是SDWebImage框架的核心调度类，负责图片的缓存和下载逻辑，本文分析一下该类的相关方法。

初始化方法

首先SDWebImageManager维护类几个属性，类的定义注释如下：

@interface SDWebImageManager ()
@property (strong, nonatomic, readwrite) SDImageCache *imageCache; //缓存类
@property (strong, nonatomic, readwrite) SDWebImageDownloader *imageDownloader; //下载类
@property (strong, nonatomic) NSMutableSet *failedURLs; //下载失败的url
@property (strong, nonatomic) NSMutableArray *runningOperations; //当前执行的operation
@end

imageCache用于缓存图片数据的类，具体可以参考第一篇文章，imageDownloader用于下载图片数据，具体可以参考上一篇文章，failedURLs是一个数组，里面存储了获取图片失败对应的url，runningOperations存储了当前正在执行的operation，operation是SDWebImageCombinedOperation类型的对象，定义注释如下：

@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>
@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;
@property (copy, nonatomic) SDWebImageNoParamsBlock cancelBlock; //cancel block
@property (strong, nonatomic) NSOperation *cacheOperation; //执行缓存操作的operaton
@end

SDWebImageOperation是protocol，SDWebImageCombinedOperation实现该protocol，实现了cancel方法。注释如下；

- (void)cancel {
self.cancelled = YES;
if (self.cacheOperation) {
[self.cacheOperation cancel]; //取消缓存operation的操作
self.cacheOperation = nil;
}
if (self.cancelBlock) {
self.cancelBlock(); //执行cancelBlock
_cancelBlock = nil;
}
}

然后SDWebImageManager通过-(void)initWithCache:downloader:方法初始化，注释如下：

- (instancetype)initWithCache:(SDImageCache *)cache downloader:(SDWebImageDownloader *)downloader {
if ((self = [super init])) {
_imageCache = cache; //赋值cache
_imageDownloader = downloader; //赋值downloader
_failedURLs = [NSMutableSet new];
_runningOperations = [NSMutableArray new];
}
return self;
}

下载图片方法

SDWebImageManager中的核心方法是-(void)downloadImageWithURL:options:progress:completed:方法，主要负责从缓存中取图片，然后根据策略下载图片。改方法的步骤主要分为以下几步：

判断前置条件：

if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
//url判空
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}
//创建operation对象
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;

BOOL isFailedUrl = NO;
@synchronized (self.failedURLs) {
isFailedUrl = [self.failedURLs containsObject:url];
}
//当前url是否是FailedUrl，如果是直接返回上层。
if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
dispatch_main_sync_safe(^{
NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil];
completedBlock(nil, error, SDImageCacheTypeNone, YES, url);
});
return operation;
}

首先对url判空，如果不为空，判断当前url是否属于failedURLs中，因为failedURLs中的url属于之前下载失败的图片url，不能被下载。除非options中包含SDWebImageRetryFailed选项，表示之前下载失败过，但是本次需要重新下载。如果不包含SDWebImageRetryFailed选项，调用completedBlock回调。

将operation加入runningOperations中，从缓存中去图片数据：

operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) {
if (operation.isCancelled) {
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
return;
}
//不存在image，或者需要刷新image，重新下载
if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
...
}
else if (image) { //存在image，直接返回
dispatch_main_sync_safe(^{
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(image, nil, cacheType, YES, url);
}
});
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
else {
dispatch_main_sync_safe(^{
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation && !weakOperation.isCancelled) {
completedBlock(nil, nil, SDImageCacheTypeNone, YES, url);
}
});
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
}]

该方法首先从缓存中取image，如果没有取到，说明是第一次下载图片，或者存在image，但是需要刷新图片，如果delegate实现shouldDownloadImageForURL方法，返回YES，说明需要下载，这些情况下开始下载图片。当不属于这些情况且image存在时，说明之前已经下载过url对应的图片了，则直接将缓存中的图片通过completedBlock返回，并且将operation从runningOperations中删除。如果image不存在，则返回nil给外部。

当需要下载图片时，设置标记位，将SDWebImageOptions转化成SDWebImageDownloaderOptions：

SDWebImageDownloaderOptions downloaderOptions = 0;
if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache; //使用NSURLCache
if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
if (image && options & SDWebImageRefreshCached) {
//禁用ProgressiveDownload
downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
//开启忽略CachedResponse
downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
}

然后调用imageDownloader下载url对应图片数据，并处理不同情况：

id <SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (!strongOperation || strongOperation.isCancelled) {
}
else if (error) { //下载失败
dispatch_main_sync_safe(^{
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(nil, error, SDImageCacheTypeNone, finished, url);
}
});
if (error.code != NSURLErrorNotConnectedToInternet
&& error.code != NSURLErrorCancelled
&& error.code != NSURLErrorTimedOut
&& error.code != NSURLErrorInternationalRoamingOff
&& error.code != NSURLErrorDataNotAllowed
&& error.code != NSURLErrorCannotFindHost
&& error.code != NSURLErrorCannotConnectToHost) {
@synchronized (self.failedURLs) {
[self.failedURLs addObject:url]; //添加url进failedURLs
}
}
}
else {
if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url]; //下载成功，将url从failedURLs中删除
}
}
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
if (options & SDWebImageRefreshCached && image && !downloadedImage) {
}
else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)])
{ //将下载后的图片变换后写入缓存                  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];
[self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:(imageWasTransformed ? nil : data) forKey:key toDisk:cacheOnDisk];
}
dispatch_main_sync_safe(^{
//回调给外层
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(transformedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
});
}
else { //将下载后的图片数据写入缓存
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
}
dispatch_main_sync_safe(^{
//回调给外层
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
}
}
if (finished) {
@synchronized (self.runningOperations) {
if (strongOperation) {
//从runningOperations中删除
[self.runningOperations removeObject:strongOperation];
}
}
}
}];
operation.cancelBlock = ^{
[subOperation cancel]; //取消下载
@synchronized (self.runningOperations) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation) { //从runningOperations中删除
[self.runningOperations removeObject:strongOperation];
}
}
};

如果下载失败，则返回nil给外层，同时判断errorCode不是超时、取消、找不到主机地址等错误时，将url加入failedURLs。如果下载成功，且option标记为SDWebImageRetryFailed，则键url从failedURLs数组中删除。然后判断是否需要将下载的图片进行变换，如果需要transform，则进行transform后将图片写入缓存，否则直接将原图片写入缓存。当操作执行完后，将operation从runningOperations中删除。最后指定了operation的cancelBlock，该block主要讲负责下载的subOperation取消，然后将当前operation从runningOperations数组中删除。

其他方法

SDWebImageManager还提供了一些辅助方法，如下：

-(void)saveImageToCache: forURL:方法，该方法负责将图片存入缓存，url作为key。

- (void)saveImageToCache:(UIImage *)image forURL:(NSURL *)url {
if (image && url) {
NSString *key = [self cacheKeyForURL:url];
[self.imageCache storeImage:image forKey:key toDisk:YES];
}
}

-(void)cancelAll方法，该方法将runningOperations中的所有operation取消。

- (void)cancelAll {
@synchronized (self.runningOperations) {
NSArray *copiedOperations = [self.runningOperations copy];
[copiedOperations makeObjectsPerformSelector:@selector(cancel)]; //取消
[self.runningOperations removeObjectsInArray:copiedOperations];
}
}

调用makeObjectsPerformSelector方法遍历operation，调用cancel方法，SDWebImageCombinedOperation的cancel方法在上文有注释，取消cacheOperation，执行cancelBlock取消下载的operation。

-(BOOL)cachedImageExistsForURL:方法和-(BOOL)diskImageExistsForURL:方法

这两个同步方法返回图片是否在缓存中存在，前一个方法先判断图片是否在内存中存在，如果没有再去文件中查找，返回查找的结果。后一个方法直接去文件中查找，返回查找的结果。

-(void)cachedImageExistsForURL:completion:方法和-(void)diskImageExistsForURL:completion:方法

这两个方法的功能和上面两个是相同的，前一个方法同时从内存和文件缓存中查找图片是否存在，后一个方法只从文件缓存中查找，区别之处在于，这两个方法是通过completionBlock异步返回结果的。