AFNetWorking 和 SDWebImage 图片缓存对比
文章 篇幅 较长，其中有很多对比源码的地方。如果没有耐心看分析，可以直接看 最后的结论。
首先 提下： NSURLCache 和 NSCache。
NSURLCache: 可以在memory 和 disk 上缓存。但是在memory上缓存时，没有自动清理机制。单纯性的拷贝到memory上。（ BTW: 网上很多对比人说AFNetwork的图片缓存使用了NSURLCache,但是 经过笔者研究最新版本的AFNetwork 并没有使用NSURLCache。反而在SDWebImage 中的网络请求中使用了。但SDWebImage 并不是用NSURLCache来实现磁盘缓存的。）
NSCache: 在memory上缓存，类似于NSMutableDictionary ,以 哈希算法 管理。有自动清理机制，当缓存到memory时，如果memory空间不够，则会自动删除memory中当前界面不使用的空间。SDWebImage 中的图片缓存 在memory上使用的是 NSCache,在disk 是则用到的是 NSFileManager。AFNetWorking 的图片缓存，在在memory上使用的是 NSCache，没有磁盘缓存。
接下来 通过源码 来 对比 AFNetWorking 和 SDWebImage 图片缓存
AFNetWorking 最新版：（2015-07-28 ）
UIImageView+AFNetworking.m
- (void)setImageWithURL:(NSURL *)url {
[self setImageWithURL:url placeholderImage:nil];
}
- (void)setImageWithURL:(NSURL *)url
placeholderImage:(UIImage *)placeholderImage
{
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
[request addValue:@"image/*" forHTTPHeaderField:@"Accept"];
[self setImageWithURLRequest:request placeholderImage:placeholderImage success:nil failure:nil];
}
- (void)setImageWithURLRequest:(NSURLRequest *)urlRequest
placeholderImage:(UIImage *)placeholderImage
success:(void (^)(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image))success
failure:(void (^)(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error))failure
{
[self cancelImageRequestOperation];
<span style="color:#ff0000;">UIImage *cachedImage = [[[self class] sharedImageCache] cachedImageForRequest:urlRequest];</span>
if (cachedImage) {
if (success) {
success(nil, nil, cachedImage);
} else {
self.image = cachedImage;
}
self.af_imageRequestOperation = nil;
} else {
if (placeholderImage) {
self.image = placeholderImage;
}
__weak __typeof(self)weakSelf = self;
self.af_imageRequestOperation = [[AFHTTPRequestOperation alloc] initWithRequest:urlRequest];
self.af_imageRequestOperation.responseSerializer = self.imageResponseSerializer;
[self.af_imageRequestOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
__strong __typeof(weakSelf)strongSelf = weakSelf;
if ([[urlRequest URL] isEqual:[strongSelf.af_imageRequestOperation.request URL]]) {
if (success) {
success(urlRequest, operation.response, responseObject);
} else if (responseObject) {
strongSelf.image = responseObject;
}
if (operation == strongSelf.af_imageRequestOperation){
strongSelf.af_imageRequestOperation = nil;
}
}
[[[strongSelf class] sharedImageCache] cacheImage:responseObject forRequest:urlRequest];
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
__strong __typeof(weakSelf)strongSelf = weakSelf;
if ([[urlRequest URL] isEqual:[strongSelf.af_imageRequestOperation.request URL]]) {
if (failure) {
failure(urlRequest, operation.response, error);
}
if (operation == strongSelf.af_imageRequestOperation){
strongSelf.af_imageRequestOperation = nil;
}
}
}];
[[[self class] af_sharedImageRequestOperationQueue] addOperation:self.af_imageRequestOperation];
}
}
UIImage *cachedImage = [[[selfclass] sharedImageCache]cachedImageForRequest:urlRequest];
这句即是查找缓存，那么我们到 AFImageCache 中来看看：
@interface AFImageCache : NSCache <AFImageCache> @end
可见 AFImageCache 继承自NSCache. 所以 AFNetWorking 在memory中的缓存使用了（cocoa kit提供的）NSCache。
如果 没有找到memory中的图片 就开始使用：
self.af_imageRequestOperation = [[AFHTTPRequestOperation alloc] initWithRequest:urlRequest];
self.af_imageRequestOperation.responseSerializer = self.imageResponseSerializer;
开始网络请求。再来看看 这个 AFHTTPRequestOperation
@interface AFHTTPRequestOperation : AFURLConnectionOperation而 AFURLConnectionOperation
@interface AFURLConnectionOperation : NSOperation <NSURLConnectionDelegate, NSURLConnectionDataDelegate, NSSecureCoding, NSCopying>
@interface AFURLConnectionOperation () @property (readwrite, nonatomic, assign) AFOperationState state; @property (readwrite, nonatomic, strong) NSRecursiveLock *lock; <span style="color:#ff0000;">@property (readwrite, nonatomic, strong) NSURLConnection *connection; @property (readwrite, nonatomic, strong) NSURLRequest *request; @property (readwrite, nonatomic, strong) NSURLResponse *response;</span> @property (readwrite, nonatomic, strong) NSError *error; @property (readwrite, nonatomic, strong) NSData *responseData;
来看看 initWithRequest 这个方法，到底 AFNetWorking 是使没使用 NSURLCache 缓存。
- (instancetype)initWithRequest:(NSURLRequest *)urlRequest {
NSParameterAssert(urlRequest);
self = [super init];
if (!self) {
return nil;
}
_state = AFOperationReadyState;
self.lock = [[NSRecursiveLock alloc] init];
self.lock.name = kAFNetworkingLockName;
self.runLoopModes = [NSSet setWithObject:NSRunLoopCommonModes];
self.request = urlRequest;
self.shouldUseCredentialStorage = YES;
self.securityPolicy = [AFSecurityPolicy defaultPolicy];
return self;
}
没有。根本就没有使用 NSURLCache 。再看看 最后获取图片data 完成后进行了什么操作：
[self.af_imageRequestOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
__strong __typeof(weakSelf)strongSelf = weakSelf;
if ([[urlRequest URL] isEqual:[strongSelf.af_imageRequestOperation.request URL]]) {
if (success) {
success(urlRequest, operation.response, responseObject);
} else if (responseObject) {
strongSelf.image = responseObject;
}
if (operation == strongSelf.af_imageRequestOperation){
strongSelf.af_imageRequestOperation = nil;
}
}
<span style="color:#ff0000;"> [[[strongSelf class] sharedImageCache] cacheImage:responseObject forRequest:urlRequest];</span>
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
__strong __typeof(weakSelf)strongSelf = weakSelf;
if ([[urlRequest URL] isEqual:[strongSelf.af_imageRequestOperation.request URL]]) {
if (failure) {
failure(urlRequest, operation.response, error);
}
if (operation == strongSelf.af_imageRequestOperation){
strongSelf.af_imageRequestOperation = nil;
}
}
}];
使用
[[[strongSelf class]sharedImageCache] cacheImage:responseObjectforRequest:urlRequest];
本质上 就是使用NSCache 在memory上进行缓存。
看看 SDWebImage 的图片缓存：
在 UIImageView+WebCache.m
- (void)sd_setImageWithURL:(NSURL *)url {
[self sd_setImageWithURL:url placeholderImage:nil options:0 progress:nil completed:nil];
}
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder {
[self sd_setImageWithURL:url placeholderImage:placeholder options:0 progress:nil completed:nil];
}
/**
* Andrew Zhang add
*/
- (void)sd_setVedioImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder{
[self sd_setVedioImageWithURL:url placeholderImage:placeholder options:0 progress:nil completed:nil];
}
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options {
[self sd_setImageWithURL:url placeholderImage:placeholder options:options progress:nil completed:nil];
}
- (void)sd_setImageWithURL:(NSURL *)url completed:(SDWebImageCompletionBlock)completedBlock {
[self sd_setImageWithURL:url placeholderImage:nil options:0 progress:nil completed:completedBlock];
}
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder completed:(SDWebImageCompletionBlock)completedBlock {
[self sd_setImageWithURL:url placeholderImage:placeholder options:0 progress:nil completed:completedBlock];
}
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options completed:(SDWebImageCompletionBlock)completedBlock {
[self sd_setImageWithURL:url placeholderImage:placeholder options:options progress:nil completed:completedBlock];
}
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
[self sd_cancelCurrentImageLoad];
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
if (!(options & SDWebImageDelayPlaceholder)) {
dispatch_main_async_safe(^{
self.image = placeholder;
});
}
if (url) {
__weak UIImageView *wself = self;
<span style="color:#ff0000;"> id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) </span>{
if (!wself) return;
dispatch_main_sync_safe(^{
if (!wself) return;
if (image) {
wself.image = image;
[wself setNeedsLayout];
} else {
if ((options & SDWebImageDelayPlaceholder)) {
wself.image = placeholder;
[wself setNeedsLayout];
}
}
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
[self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
} else {
dispatch_main_async_safe(^{
NSError *error = [NSError errorWithDomain:@"SDWebImageErrorDomain" code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
if (completedBlock) {
completedBlock(nil, error, SDImageCacheTypeNone, url);
}
});
}
}
使用了 id < SDWebImageOperation > operation = [ SDWebImageManager . sharedManager downloadImageWithURL :url options :options progress :progressBlock completed :^( UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) 来进行图片下载，接着我们去看看 这个方法怎么实现的。
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock {
// 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;
@synchronized (self.failedURLs) {
isFailedUrl = [self.failedURLs containsObject:url];
}
if (!url || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
dispatch_main_sync_safe(^{
NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil];
completedBlock(nil, error, SDImageCacheTypeNone, YES, url);
});
return operation;
}
@synchronized (self.runningOperations) {
[self.runningOperations addObject:operation];
}
NSString *key = [self cacheKeyForURL:url];
<span style="color:#ff0000;">operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) {</span>
if (operation.isCancelled) {
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
return;
}
if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
if (image && options & SDWebImageRefreshCached) {
dispatch_main_sync_safe(^{
// If image was found in the cache bug 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.
completedBlock(image, nil, cacheType, YES, url);
});
}
// download if no image or requested to refresh anyway, and download allowed by delegate
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 (image && 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;
}
id <SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
if (weakOperation.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) {
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(nil, error, SDImageCacheTypeNone, finished, url);
}
});
if (error.code != NSURLErrorNotConnectedToInternet && error.code != NSURLErrorCancelled && error.code != NSURLErrorTimedOut) {
@synchronized (self.failedURLs) {
if (![self.failedURLs containsObject:url]) {
[self.failedURLs addObject:url];
}
}
}
}
else {
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
if (options & SDWebImageRefreshCached && image && !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), ^{
UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
if (transformedImage && finished) {
BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
<span style="color:#ff0000;"> [self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:data forKey:key toDisk:cacheOnDisk];</span>
}
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(transformedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
});
}
else {
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
}
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
}
}
if (finished) {
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
}];
operation.cancelBlock = ^{
[subOperation cancel];
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:weakOperation];
}
};
}
<span style="color:#ff0000;"> else if (image) {
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(image, nil, cacheType, YES, url);
}
});
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}</span>
else {
// Image not in cache and download disallowed by delegate
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(nil, nil, SDImageCacheTypeNone, YES, url);
}
});
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
}];
return operation;
}
在 这个实现方法中，可以看到，首先使用：
operation.cacheOperation = [self.imageCachequeryDiskCacheForKey:key done:^(UIImage *image,SDImageCacheType cacheType) 去查询本地磁盘上和memory上是否存在图片。到内部去看看：
- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock {
if (!doneBlock) {
return nil;
}
if (!key) {
doneBlock(nil, SDImageCacheTypeNone);
return nil;
}
<span style="color:#ff0000;">// First check the in-memory cache...
UIImage *image = [self imageFromMemoryCacheForKey:key];</span>
if (image) {
doneBlock(image, SDImageCacheTypeMemory);
return nil;
}
NSOperation *operation = [NSOperation new];
dispatch_async(self.ioQueue, ^{
if (operation.isCancelled) {
return;
}
@autoreleasepool {
<span style="color:#ff0000;">UIImage *diskImage = [self diskImageForKey:key];
if (diskImage) {
CGFloat cost = diskImage.size.height * diskImage.size.width * diskImage.scale * diskImage.scale;
[self.memCache setObject:diskImage forKey:key cost:cost];
}
</span>
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, SDImageCacheTypeDisk);
});
}
});
return operation;
}
这个方法是先去 查询memory上是否命中，如果命中 直接返回图片，如果没有，则再去查找本地磁盘是否有缓存。
好了，再看看，如果memory 和 本地磁盘上都没有图片缓存 SDWebImage是怎么做的？
if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
if (image && options & SDWebImageRefreshCached) {
dispatch_main_sync_safe(^{
// If image was found in the cache bug 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.
completedBlock(image, nil, cacheType, YES, url);
});
}
// download if no image or requested to refresh anyway, and download allowed by delegate
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 (image && 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;
}
<span style="color:#ff0000;">id <SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished)</span> {
if (weakOperation.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) {
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(nil, error, SDImageCacheTypeNone, finished, url);
}
});
if (error.code != NSURLErrorNotConnectedToInternet && error.code != NSURLErrorCancelled && error.code != NSURLErrorTimedOut) {
@synchronized (self.failedURLs) {
if (![self.failedURLs containsObject:url]) {
[self.failedURLs addObject:url];
}
}
}
}
else {
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
if (options & SDWebImageRefreshCached && image && !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), ^{
UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
<span style="color:#ff0000;"> if (transformedImage && finished) {
BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
[self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:data forKey:key toDisk:cacheOnDisk];
}</span>
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(transformedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
});
}
else {
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
}
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
}
}
if (finished) {
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
}];
使用
id <SDWebImageOperation> subOperation = [self.imageDownloaderdownloadImageWithURL:url options:downloaderOptionsprogress:progressBlock completed:^(UIImage *downloadedImage,NSData *data, NSError *error,BOOL finished) 子操作，来下载图片，到 SDWebImageDownloader.h 中看看具体的源码：
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
__block SDWebImageDownloaderOperation *operation;
__weak SDWebImageDownloader *wself = self;
[self addProgressCallback:progressBlock andCompletedBlock:completedBlock forURL:url createCallback:^{
NSTimeInterval timeoutInterval = wself.downloadTimeout;
if (timeoutInterval == 0.0) {
timeoutInterval = 15.0;
}
// In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise
<span style="color:#ff0000;"> NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];</span>
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
request.HTTPShouldUsePipelining = YES;
if (wself.headersFilter) {
request.allHTTPHeaderFields = wself.headersFilter(url, [wself.HTTPHeaders copy]);
}
else {
request.allHTTPHeaderFields = wself.HTTPHeaders;
}
<span style="color:#ff0000;">operation = [[wself.operationClass alloc] initWithRequest:request
options:options
progress:^(NSInteger receivedSize, NSInteger expectedSize)</span> {
SDWebImageDownloader *sself = wself;
if (!sself) return;
NSArray *callbacksForURL = [sself callbacksForURL:url];
for (NSDictionary *callbacks in callbacksForURL) {
SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
if (callback) callback(receivedSize, expectedSize);
}
}
completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {
SDWebImageDownloader *sself = wself;
if (!sself) return;
NSArray *callbacksForURL = [sself callbacksForURL:url];
if (finished) {
[sself removeCallbacksForURL:url];
}
for (NSDictionary *callbacks in callbacksForURL) {
SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
if (callback) callback(image, data, error, finished);
}
}
cancelled:^{
SDWebImageDownloader *sself = wself;
if (!sself) return;
[sself removeCallbacksForURL:url];
}];
if (wself.username && wself.password) {
operation.credential = [NSURLCredential credentialWithUser:wself.username password:wself.password persistence:NSURLCredentialPersistenceForSession];
}
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
[wself.downloadQueue addOperation:operation];
if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
// Emulate LIFO execution order by systematically adding new operations as last operation's dependency
[wself.lastAddedOperation addDependency:operation];
wself.lastAddedOperation = operation;
}
}];
return operation;
}
注意：这句 NSMutableURLRequest *request = [[NSMutableURLRequestalloc] initWithURL:urlcachePolicy:(options & SDWebImageDownloaderUseNSURLCache ?NSURLRequestUseProtocolCachePolicy :NSURLRequestReloadIgnoringLocalCacheData)timeoutInterval:timeoutInterval];
可见默认使用了带缓存的网络请求。
然后 将 request使用
operation = [[wself.operationClassalloc] initWithRequest:request
options:options
progress:^(NSInteger receivedSize,NSInteger expectedSize)
进行请求。
SDWebImageDownloaderOperation.m 中 来看看 具体是怎么请求的？
- (id)initWithRequest:(NSURLRequest *)request
options:(SDWebImageDownloaderOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageDownloaderCompletedBlock)completedBlock
cancelled:(SDWebImageNoParamsBlock)cancelBlock {
if ((self = [super init])) {
_request = request;
_shouldUseCredentialStorage = YES;
_options = options;
_progressBlock = [progressBlock copy];
_completedBlock = [completedBlock copy];
_cancelBlock = [cancelBlock copy];
_executing = NO;
_finished = NO;
_expectedSize = 0;
responseFromCached = YES; // Initially wrong until `connection:willCacheResponse:` is called or not called
}
return self;
}
#pragma mark NSURLConnection (delegate)
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
//'304 Not Modified' is an exceptional one
if ((![response respondsToSelector:@selector(statusCode)] || [((NSHTTPURLResponse *)response) statusCode] < 400) && [((NSHTTPURLResponse *)response) statusCode] != 304) {
NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0;
self.expectedSize = expected;
if (self.progressBlock) {
self.progressBlock(0, expected);
}
self.imageData = [[NSMutableData alloc] initWithCapacity:expected];
}
else {
NSUInteger code = [((NSHTTPURLResponse *)response) statusCode];
//This is the case when server returns '304 Not Modified'. It means that remote image is not changed.
//In case of 304 we need just cancel the operation and return cached image from the cache.
if (code == 304) {
[self cancelInternal];
} else {
[self.connection cancel];
}
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:nil];
});
if (self.completedBlock) {
self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:[((NSHTTPURLResponse *)response) statusCode] userInfo:nil], YES);
}
CFRunLoopStop(CFRunLoopGetCurrent());
[self done];
}
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
[self.imageData appendData:data];
if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0 && self.completedBlock) {
// The following code is from http://www.cocoaintheshell.com/2011/05/progressive-images-download-imageio/
// Thanks to the author @Nyx0uf
// Get the total bytes downloaded
const NSInteger totalSize = self.imageData.length;
// Update the data source, we must pass ALL the data, not just the new bytes
CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)self.imageData, NULL);
if (width + height == 0) {
CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
if (properties) {
NSInteger orientationValue = -1;
CFTypeRef val = CFDictionaryGetValue(properties, kCGImagePropertyPixelHeight);
if (val) CFNumberGetValue(val, kCFNumberLongType, &height);
val = CFDictionaryGetValue(properties, kCGImagePropertyPixelWidth);
if (val) CFNumberGetValue(val, kCFNumberLongType, &width);
val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);
if (val) CFNumberGetValue(val, kCFNumberNSIntegerType, &orientationValue);
CFRelease(properties);
// When we draw to Core Graphics, we lose orientation information,
// which means the image below born of initWithCGIImage will be
// oriented incorrectly sometimes. (Unlike the image born of initWithData
// in connectionDidFinishLoading.) So save it here and pass it on later.
orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)];
}
}
if (width + height > 0 && totalSize < self.expectedSize) {
// Create the image
CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);
#ifdef TARGET_OS_IPHONE
// Workaround for iOS anamorphic image
if (partialImageRef) {
const size_t partialHeight = CGImageGetHeight(partialImageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef bmContext = CGBitmapContextCreate(NULL, width, height, 8, width * 4, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
CGColorSpaceRelease(colorSpace);
if (bmContext) {
CGContextDrawImage(bmContext, (CGRect){.origin.x = 0.0f, .origin.y = 0.0f, .size.width = width, .size.height = partialHeight}, partialImageRef);
CGImageRelease(partialImageRef);
partialImageRef = CGBitmapContextCreateImage(bmContext);
CGContextRelease(bmContext);
}
else {
CGImageRelease(partialImageRef);
partialImageRef = nil;
}
}
#endif
if (partialImageRef) {
UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
UIImage *scaledImage = [self scaledImageForKey:key image:image];
image = [UIImage decodedImageWithImage:scaledImage];
CGImageRelease(partialImageRef);
dispatch_main_sync_safe(^{
if (self.completedBlock) {
self.completedBlock(image, nil, nil, NO);
}
});
}
}
CFRelease(imageSource);
}
if (self.progressBlock) {
self.progressBlock(self.imageData.length, self.expectedSize);
}
}
+ (UIImageOrientation)orientationFromPropertyValue:(NSInteger)value {
switch (value) {
case 1:
return UIImageOrientationUp;
case 3:
return UIImageOrientationDown;
case 8:
return UIImageOrientationLeft;
case 6:
return UIImageOrientationRight;
case 2:
return UIImageOrientationUpMirrored;
case 4:
return UIImageOrientationDownMirrored;
case 5:
return UIImageOrientationLeftMirrored;
case 7:
return UIImageOrientationRightMirrored;
default:
return UIImageOrientationUp;
}
}
- (UIImage *)scaledImageForKey:(NSString *)key image:(UIImage *)image {
return SDScaledImageForKey(key, image);
}
- (void)connectionDidFinishLoading:(NSURLConnection *)aConnection {
SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
@synchronized(self) {
CFRunLoopStop(CFRunLoopGetCurrent());
self.thread = nil;
self.connection = nil;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:nil];
});
}
<span style="color:#ff0000;">if (![[NSURLCache sharedURLCache] cachedResponseForRequest:_request]) {
responseFromCached = NO;
}</span>
if (completionBlock) {
if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached) {
completionBlock(nil, nil, nil, YES);
}
else {
UIImage *image = [UIImage sd_imageWithData:self.imageData];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
image = [self scaledImageForKey:key image:image];
// Do not force decoding animated GIFs
if (!image.images) {
image = [UIImage decodedImageWithImage:image];
}
if (CGSizeEqualToSize(image.size, CGSizeZero)) {
completionBlock(nil, nil, [NSError errorWithDomain:@"SDWebImageErrorDomain" code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}], YES);
}
else {
completionBlock(image, self.imageData, nil, YES);
}
}
}
self.completionBlock = nil;
[self done];
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
@synchronized(self) {
CFRunLoopStop(CFRunLoopGetCurrent());
self.thread = nil;
self.connection = nil;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:nil];
});
}
if (self.completedBlock) {
self.completedBlock(nil, nil, error, YES);
}
self.completionBlock = nil;
[self done];
}
- (NSCachedURLResponse *)connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse {
responseFromCached = NO; // If this method is called, it means the response wasn't read from cache
if (self.request.cachePolicy == NSURLRequestReloadIgnoringLocalCacheData) {
// Prevents caching of responses
return nil;
}
else {
return cachedResponse;
}
}
依然使用最基本的 NSURLConnection 进行网络请求。但是注意 在
- (void)connectionDidFinishLoading:(NSURLConnection *)aConnection 方法中：
if (![[NSURLCachesharedURLCache] cachedResponseForRequest:_request]) {
responseFromCached =NO;
}
并且：
- (NSCachedURLResponse *)connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse {
responseFromCached = NO; // If this method is called, it means the response wasn't read from cache
if (self.request.cachePolicy == NSURLRequestReloadIgnoringLocalCacheData) {
// Prevents caching of responses
return nil;
}
else {
return cachedResponse;
}
}
可以看到 SDWebImage 使用了 NSURLCache 进行网络请求缓存。
回到最开始 ，假设图片下载完成了，SDWebImage 又做了什么事情呢？
if (transformedImage && finished) {
BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
[self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:data forKey:key toDisk:cacheOnDisk];
}
使用
[self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:data forKey:key toDisk:cacheOnDisk];
- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk {
if (!image || !key) {
return;
}
<span style="color:#ff0000;"> [self.memCache setObject:image forKey:key cost:image.size.height * image.size.width * image.scale * image.scale];</span>
if (toDisk) {
dispatch_async(self.ioQueue, ^{
NSData *data = imageData;
if (image && (recalculate || !data)) {
#if TARGET_OS_IPHONE
// We need to determine if the image is a PNG or a JPEG
// PNGs are easier to detect because they have a unique signature (http://www.w3.org/TR/PNG-Structure.html)
// The first eight bytes of a PNG file always contain the following (decimal) values:
// 137 80 78 71 13 10 26 10
// We assume the image is PNG, in case the imageData is nil (i.e. if trying to save a UIImage directly),
// we will consider it PNG to avoid loosing the transparency
BOOL imageIsPng = YES;
// But if we have an image data, we will look at the preffix
if ([imageData length] >= [kPNGSignatureData length]) {
imageIsPng = ImageDataHasPNGPreffix(imageData);
}
if (imageIsPng) {
data = UIImagePNGRepresentation(image);
}
else {
data = UIImageJPEGRepresentation(image, (CGFloat)1.0);
}
#else
data = [NSBitmapImageRep representationOfImageRepsInArray:image.representations usingType: NSJPEGFileType properties:nil];
#endif
}
if (data) {
<span style="color:#ff0000;"> if (![_fileManager fileExistsAtPath:_diskCachePath]) {
[_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
}
[_fileManager createFileAtPath:[self defaultCachePathForKey:key] contents:data attributes:nil];</span>
}
});
}
}
可见，先使用NSCache 缓存到memory上，然后 使用 NSFileManager 将图片数据（注意：依旧是 UIImage类型，而不是NSData类型）写入到本地磁盘路径。
综上所述：
得出如下结论：
UIImageView+AFNetworking 的图片缓存，并没有本地磁盘缓存，且网络请求图片中并未使用NSURLCache，来进行网络请求缓存。只是使用了NSCache 在memory上进行缓存。由于只是在memory上缓存了，所以当app从内存中退出后，重新打开app，每次都会重新对图片进行网络请求。
SDWebImage 的图片缓存，不仅使用了NSCache 在memory上进行缓存，而且还使用了NSFileManager在本地磁盘上进行缓存。同时在 使用NSURLConnection 时，使用了NSURLCache进行网络请求缓存（这样有什么好处？1: 当一次图片请求因某种原因中途断开，下次请求时，可以直接从NSURLCache获取之前请求的一部分数据，接着请求余下的数据即可。2:当缓存策略不同时，比如：每次都请求，不缓存。那么这时候，只要第一次进行了NSURLConnection，以后都可以直接从NSURLCache 获取数据。）
由此，可知：SDWebImage 在图片缓存上比UIImageView+AFNetworking 有着明显的优势，如果对于一款app在图片上的性能优化，推荐使用 SDWebImage 来替代其他框架。这样可以节省用户很多的流量。
－－－－－－－－－－－下面转载几篇关于 NSURLCache的文章－－－－－－－－－－－
1:NSURLCache 为您的应用的 URL 请求提供了内存中以及磁盘上的综合缓存机制。 作为基础类库 URL 加载系统 的一部分，任何通过 NSURLConnection 加载的请求都将被 NSURLCache 处理。
网络缓存减少了需要向服务器发送请求的次数，同时也提升了离线或在低速网络中使用应用的体验。
当一个请求完成下载来自服务器的回应，一个缓存的回应将在本地保存。下一次同一个请求再发起时，本地保存的回应就会马上返回，不需要连接服务器。NSURLCache 会 自动 且 透明 地返回回应。
为了好好利用 NSURLCache，你需要初始化并设置一个共享的 URL 缓存。在 iOS 中这项工作需要在 -application:didFinishLaunchingWithOptions: 完成，而 Mac OS X 中是在 –applicationDidFinishLaunching:：
- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
NSURLCache *URLCache = [[NSURLCache alloc] initWithMemoryCapacity:4 * 1024 * 1024
diskCapacity:20 * 1024 * 1024
diskPath:nil];
[NSURLCache setSharedURLCache:URLCache];
}
缓存策略由请求（客户端）和回应（服务端）分别指定。理解这些策略以及它们如何相互影响，是为您的应用程序找到最佳行为的关键。
NSURLRequestCachePolicy
NSURLRequest 有个 cachePolicy 属性，它根据以下常量指定了请求的缓存行为：
NSURLRequestUseProtocolCachePolicy： 对特定的 URL 请求使用网络协议中实现的缓存逻辑。这是默认的策略。NSURLRequestReloadIgnoringLocalCacheData：数据需要从原始地址加载。不使用现有缓存。NSURLRequestReloadIgnoringLocalAndRemoteCacheData：不仅忽略本地缓存，同时也忽略代理服务器或其他中间介质目前已有的、协议允许的缓存。NSURLRequestReturnCacheDataElseLoad：无论缓存是否过期，先使用本地缓存数据。如果缓存中没有请求所对应的数据，那么从原始地址加载数据。NSURLRequestReturnCacheDataDontLoad：无论缓存是否过期，先使用本地缓存数据。如果缓存中没有请求所对应的数据，那么放弃从原始地址加载数据，请求视为失败（即：“离线”模式）。NSURLRequestReloadRevalidatingCacheData：从原始地址确认缓存数据的合法性后，缓存数据就可以使用，否则从原始地址加载。
你并不会惊奇于这些值不被透彻理解且经常搞混淆。
NSURLRequestReloadIgnoringLocalAndRemoteCacheData 和 NSURLRequestReloadRevalidatingCacheData 根本没有实现（Link to Radar）更加加深了混乱程度！
关于NSURLRequestCachePolicy，以下才是你 实际 需要了解的东西：
| 常量 | 意义 |
|---|---|
| UseProtocolCachePolicy | 默认行为 |
| ReloadIgnoringLocalCacheData | 不使用缓存 |
| ReloadIgnoringLocalAndRemoteCacheData | 我是认真地，不使用任何缓存 |
| ReturnCacheDataElseLoad | 使用缓存（不管它是否过期），如果缓存中没有，那从网络加载吧 |
| ReturnCacheDataDontLoad | 离线模式：使用缓存（不管它是否过期），但是不从网络加载 |
| ReloadRevalidatingCacheData | 在使用前去服务器验证 |
HTTP 缓存语义
因为 NSURLConnection 被设计成支持多种协议——包括 FTP、HTTP、HTTPS——所以 URL 加载系统用一种协议无关的方式指定缓存。为了本文的目的，缓存用术语 HTTP 语义来解释。
HTTP 请求和回应用 headers 来交换元数据，如字符编码、MIME 类型和缓存指令等。
Request Cache Headers
在默认情况下，NSURLRequest 会用当前时间决定是否返回缓存的数据。为了更精确地控制，允许使用以下请求头：
If-Modified-Since- 这个请求头与Last-Modified回应头相对应。把这个值设为同一终端最后一次请求时返回的Last-Modified字段的值。If-None-Match- 这个请求头与与Etag回应头相对应。使用同一终端最后一次请求的Etag值。
Response Cache Headers
NSHTTPURLResponse 包含多个 HTTP 头，当然也包括以下指令来说明回应应当如何缓存：
Cache-Control- 这个头必须由服务器端指定以开启客户端的 HTTP 缓存功能。这个头的值可能包含max-age（缓存多久），是公共public还是私有private，或者不缓存no-cache等信息。详情请参阅Cache-Controlsection of RFC 2616。
除了 Cache-Control 以外，服务器也可能发送一些附加的头用于根据需要有条件地请求（如上一节所提到的）：
Last-Modified- 这个头的值表明所请求的资源上次修改的时间。例如，一个客户端请求最近照片的时间线，/photos/timeline，Last-Modified的值可以是最近一张照片的拍摄时间。Etag- 这是 “entity tag” 的缩写，它是一个表示所请求资源的内容的标识符。在实践中，Etag的值可以是类似于资源的MD5之类的东西。这对于那些动态生成的、可能没有明显的Last-Modified值的资源非常有用。
NSURLConnectionDelegate
一旦收到了服务器的回应，NSURLConnection 的代理就有机会在 -connection:willCacheResponse: 中指定缓存数据。
NSCachedURLResponse 是个包含 NSURLResponse 以及它对应的缓存中的 NSData 的类。
在 -connection:willCacheResponse: 中，cachedResponse 对象会根据 URL 连接返回的结果自动创建。因为 NSCachedURLResponse 没有可变部分，为了改变 cachedResponse 中的值必须构造一个新的对象，把改变过的值传入 –initWithResponse:data:userInfo:storagePolicy:，例如：
- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
willCacheResponse:(NSCachedURLResponse *)cachedResponse
{
NSMutableDictionary *mutableUserInfo = [[cachedResponse userInfo] mutableCopy];
NSMutableData *mutableData = [[cachedResponse data] mutableCopy];
NSURLCacheStoragePolicy storagePolicy = NSURLCacheStorageAllowedInMemoryOnly;
// ...
return [[NSCachedURLResponse alloc] initWithResponse:[cachedResponse response]
data:mutableData
userInfo:mutableUserInfo
storagePolicy:storagePolicy];
}
如果 -connection:willCacheResponse: 返回 nil，回应将不会缓存。
- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
willCacheResponse:(NSCachedURLResponse *)cachedResponse
{
return nil;
}
如果不实现此方法，NSURLConnection 就简单地使用本来要传入 -connection:willCacheResponse: 的那个缓存对象，所以除非你需要改变一些值或者阻止缓存，否则这个代理方法不必实现。
注意事项
正如它那个毫无关系但是名字相近的小伙伴 NSCache 一样，NSURLCache 也是有一些特别的。
在 iOS 5，磁盘缓存开始支持，但仅支持 HTTP，非 HTTPS（iOS 6 中增加了此支持）。Peter Steinberger 关于这个主题写了一篇优秀的文章，在深入研究内部细节后实现他自己的 NSURLCache 子类。
Daniel Pasco 在 Black Pixel 上的另一篇文章 描述了一些与服务器通信时不设置缓存头的意外的默认行为。
NSURLCache 提醒着我们熟悉我们正在操作的系统是多么地重要。开发 iOS 或 Mac OS X 程序时，这些系统中的重中之重，非 URL Loading System莫属。
无数开发者尝试自己做一个简陋而脆弱的系统来实现网络缓存的功能，殊不知 NSURLCache 只要两行代码就能搞定且好上100倍。甚至更多开发者根本不知道网络缓存的好处，也从未尝试过，导致他们的应用向服务器作了无数不必要的网络请求。
所以如果你想看到世界的变化，你想确保你有程序总以正确的方式开启，在 -application:didFinishLaunchingWithOptions: 设置一个共享的 NSURLCache 吧。
2:NSURLCache如何工作
默认是可以的，但最好还是手动配置一下
既然AFNetworking使用NSURLConnection，它利用了原生的缓存机制NSURLCache。NSURLCache缓存了从服务器返回的NSURLResponse对象。
NSURLCache的shareCache方法默认是可以使用的，缓存获取的内容。不幸的是，它的默认配置只是缓存在内存并没有写到硬盘。为了解决这个问题，你可以声明一个 sharedCache，像这样：
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NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:2 * 1024 * 1024
diskCapacity:100 * 1024 * 1024
diskPath:nil];
[NSURLCache setSharedURLCache:sharedCache];
|
这样，我们声明了一个2M内存，100M磁盘空间的NSURLCache。
对NSURLRequest对象设置缓存策略
NSURLCache对每个NSURLRequest对象都会遵守缓存策略（NSURLRequestCachePolicy）。策略定义如下：
● NSURLRequestUseProtocolCachePolicy:指定定义在协议实现里的缓存逻辑被用于URL请求。这是URL请求的默认策略
● NSURLRequestReloadIgnoringLocalCacheData:忽略本地缓存，从源加载
● NSURLRequestReloadIgnoringLocalAndRemoteCacheData:忽略本地&服务器缓存，从源加载
● NSURLRequestReturnCacheDataElseLoad:先从缓存加载，如果没有缓存，从源加载
● NSURLRequestReturnCacheDataDontLoad离线模式，加载缓存数据（无论是否过期），不从源加载
● NSURLRequestReloadRevalidatingCacheData存在的缓存数据先确认有效性，无效的话从源加载
用NSURLCache缓存到磁盘
Cache-Control HTTP Header
Cache-Control或者Expires header 必须在从服务器返回的 HTTP response header 中，用于客户端的缓存（Cache-Control header 优先权高于 Expires header）。这里边有很多需要注意的地方，Cache Control可以有被定义为 max-age的参数（在更新响应之前缓存多长时间），public/private 访问，或者 no-cache(不缓存响应数据)，这里有一个关于HTTP cache headers的文章。
Subclass NSURLCache for Ultimate Control
如果你想绕过 Cache-Control 需求，定义你自己的规则来读写一个带有 NSURLResponse对象的NSURLCache，你可以继承 NSURLCache。
这里有个例子，使用 CACHE_EXPIRES 来判断在获取源数据之前对缓存数据保留多长时间。
(感谢 Mattt Thompson的反馈)
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@interface CustomURLCache : NSURLCache
static
NSString *
const
CustomURLCacheExpirationKey = @
"CustomURLCacheExpiration"
;
static
NSTimeInterval
const
CustomURLCacheExpirationInterval = 600;
@implementation CustomURLCache
+ (instancetype)standardURLCache {
static
CustomURLCache *_standardURLCache = nil;
static
dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_standardURLCache = [[CustomURLCache alloc]
initWithMemoryCapacity:(2 * 1024 * 1024)
diskCapacity:(100 * 1024 * 1024)
diskPath:nil];
}
return
_standardURLCache;
}
#pragma mark - NSURLCache
- (NSCachedURLResponse *)cachedResponseForRequest:(NSURLRequest *)request {
NSCachedURLResponse *cachedResponse = [super cachedResponseForRequest:request];
if
(cachedResponse) {
NSDate* cacheDate = cachedResponse.userInfo[CustomURLCacheExpirationKey];
NSDate* cacheExpirationDate = [cacheDate dateByAddingTimeInterval:CustomURLCacheExpirationInterval];
if
([cacheExpirationDate compare:[NSDate date]] == NSOrderedAscending) {
[self removeCachedResponseForRequest:request];
return
nil;
}
}
}
return
cachedResponse;
}
- (
void
)storeCachedResponse:(NSCachedURLResponse *)cachedResponse
forRequest:(NSURLRequest *)request
{
NSMutableDictionary *userInfo = [NSMutableDictionary dictionaryWithDictionary:cachedResponse.userInfo];
userInfo[CustomURLCacheExpirationKey] = [NSDate date];
NSCachedURLResponse *modifiedCachedResponse = [[NSCachedURLResponse alloc] initWithResponse:cachedResponse.response data:cachedResponse.data userInfo:userInfo storagePolicy:cachedResponse.storagePolicy];
[super storeCachedResponse:modifiedCachedResponse forRequest:request];
}
@end
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既然你有了自己的 NSURLCache子类，不要忘了在AppDelegate里边初始化并使用它
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CustomURLCache *URLCache = [[CustomURLCache alloc] initWithMemoryCapacity:2 * 1024 * 1024
diskCapacity:100 * 1024 * 1024
diskPath:nil];
[NSURLCache setSharedURLCache:URLCache];
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Overriding the NSURLResponse before caching
-connection:willCacheResponse代理方法是在被缓存之前用于截断和编辑由NSURLConnection创建的NSURLCacheResponse的地方。为了编辑NSURLCacheResponse，返回一个可变的拷贝，如下（代码来自NSHipster blog）：
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- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
willCacheResponse:(NSCachedURLResponse *)cachedResponse {
NSMutableDictionary *mutableUserInfo = [[cachedResponse userInfo] mutableCopy];
NSMutableData *mutableData = [[cachedResponse data] mutableCopy];
NSURLCacheStoragePolicy storagePolicy = NSURLCacheStorageAllowedInMemoryOnly;
// ...
return
[[NSCachedURLResponse alloc] initWithResponse:[cachedResponse response]
data:mutableData
userInfo:mutableUserInfo
storagePolicy:storagePolicy];
}
// If you do not wish to cache the NSURLCachedResponse, just return nil from the delegate function:
- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
willCacheResponse:(NSCachedURLResponse *)cachedResponse {
return
nil;
}
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Disabling NSURLCache
不想使用 NSURLCache,可以，只需要将内存和磁盘空间容量设为零就可以了
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NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:0
diskCapacity:0
diskPath:nil];
[NSURLCache setSharedURLCache:sharedCache];
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总结
我写这篇博客是为了iOS社区贡献一份力，总结了一下我在处理关于 AFNetworking缓存相关的问题。我们有个内部App加载了好多图片，导致内存问题以及性能问题。我主要职责就是诊断这个App的缓存行为。在这个研究过程中，我在网上搜索了好多资料并且做了好多调试。然后我总结之后写到了这篇博客中。我希望这篇文章能够为其他人用AFNetworking的时候提供帮助，真心希望对你们有用处！
外文地址：http://blog.originate.com/blog/2014/02/20/afimagecache-vs-nsurlcache/
3: NSURLCache内存缓存
(2013-01-31 14:18:33) 标签： nsurlcache内存缓存it |
分类： iOS笔记 |
内存缓存我们可以使用sdk中的NSURLCache类。NSURLRequest需要一个缓存参数来说明它请求的url何如缓存数据的，我们先看下它的CachePolicy类型。
1、NSURLRequestUseProtocolCachePolicy NSURLRequest默认的cache policy，使用Protocol协议定义。
2、NSURLRequestReloadIgnoringCacheData 忽略缓存直接从原始地址下载。
3、NSURLRequestReturnCacheDataElseLoad 只有在cache中不存在data时才从原始地址下载。
4、NSURLRequestReturnCacheDataDontLoad 只使用cache数据，如果不存在cache，请求失败;用于没有建立网络连接离线模式;
5、NSURLRequestReloadIgnoringLocalAndRemoteCacheData：忽略本地和远程的缓存数据，直接从原始地址下载，与NSURLRequestReloadIgnoringCacheData类似。
6、NSURLRequestReloadRevalidatingCacheData:验证本地数据与远程数据是否相同，如果不同则下载远程数据，否则使用本地数据。
NSURLCache还提供了很多方法，来方便我们实现应用程序的缓存机制。下面我通过一个例子来说明，这个例子减少我们对同一个url多次请求。看下面代码：
#import
@interface ViewController : UIViewController
@property (strong, nonatomic) NSURLConnection *connection;
@property (strong, nonatomic) NSURLCache *urlCache;
@property (strong, nonatomic) NSURL *url;
@property (strong, nonatomic) NSMutableURLRequest *request;
- (IBAction)reloadWebView:(UIButton *)sender;
@end
#import "ViewController.h"
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad
{
[super viewDidLoad];
NSString *paramURLAsString= @"http://blog.sina.com.cn/u/2526279194";
self.urlCache = [NSURLCache sharedURLCache];
[self.urlCache setMemoryCapacity:1*1024*1024];
//创建一个nsurl
self.url = [NSURL URLWithString:paramURLAsString];
//创建一个请求
self.request=[NSMutableURLRequest requestWithURL:self.url
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:30.0f];
[self.myWebView loadRequest:self.request];
}
这个例子中，我们请求url为http://blog.sina.com.cn/u/2526279194的网站。如果这个url被缓存了，我们直接从缓存中获取数据，否则从http://blog.sina.com.cn/u/2526279194站点上重新获取数据。我们设置了缓存大小为1M。
- (IBAction)reloadWebView:(UIButton *)sender {
//从请求中获取缓存输出
NSCachedURLResponse *response =[self.urlCache cachedResponseForRequest:self.request];
//判断是否有缓存
if (response != nil){
NSLog(@"如果有缓存输出，从缓存中获取数据");
[self.request setCachePolicy:NSURLRequestReturnCacheDataDontLoad];
}
[self.myWebView loadRequest:self.request];
self.connection = nil;
NSURLConnection *newConnection = [[NSURLConnection alloc]initWithRequest:self.request
delegate:self
startImmediately:YES];
self.connection = newConnection;
}
使用下面代码，我将请求的过程打印出来
- (void) connection:(NSURLConnection *)connection
didReceiveResponse:(NSURLResponse *)response{
NSLog(@"将接收输出");
}
- (NSURLRequest *)connection:(NSURLConnection *)connection
willSendRequest:(NSURLRequest *)request
redirectResponse:(NSURLResponse *)redirectResponse{
NSLog(@"即将发送请求");
return(request);
}
- (void)connection:(NSURLConnection *)connection
didReceiveData:(NSData *)data{
NSLog(@"接受数据");
NSLog(@"数据长度为 = %lu", (unsigned long)[data length]);
}
- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
willCacheResponse:(NSCachedURLResponse *)cachedResponse{
NSLog(@"将缓存输出");
return(cachedResponse);
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection{
NSLog(@"请求完成");
}
- (void)connection:(NSURLConnection *)connection
didFailWithError:(NSError *)error{
NSLog(@"请求失败");
}
@end
第一次打印结果如下
2013-01-31 15:28:29.923 NSURLCacheDemo[27848:907] 即将发送请求
2013-01-31 15:28:30.043 NSURLCacheDemo[27848:907] 将接收输出
2013-01-31 15:28:30.045 NSURLCacheDemo[27848:907] 接受数据
2013-01-31 15:28:30.047 NSURLCacheDemo[27848:907] 数据长度为 = 30047
2013-01-31 15:28:30.095 NSURLCacheDemo[27848:907] 接受数据
2013-01-31 15:28:30.098 NSURLCacheDemo[27848:907] 数据长度为 = 3575
2013-01-31 15:28:30.102 NSURLCacheDemo[27848:907] 接受数据
2013-01-31 15:28:30.104 NSURLCacheDemo[27848:907] 数据长度为 = 1482
2013-01-31 15:28:30.105 NSURLCacheDemo[27848:907] 将缓存输出
2013-01-31 15:28:30.107 NSURLCacheDemo[27848:907] 请求完成
第二次点击打印结果如下
2013-01-31 15:28:31.599 NSURLCacheDemo[27848:907] 如果有缓存输出，从缓存中获取数据
2013-01-31 15:28:31.607 NSURLCacheDemo[27848:907] 即将发送请求
2013-01-31 15:28:31.840 NSURLCacheDemo[27848:907] 将接收输出
2013-01-31 15:28:31.843 NSURLCacheDemo[27848:907] 接受数据
2013-01-31 15:28:31.845 NSURLCacheDemo[27848:907] 数据长度为 = 35104
2013-01-31 15:28:31.846 NSURLCacheDemo[27848:907] 请求完成