SDWebImageManager是SDWebImage的核心类，也是我们经常接触到的类，我们将一起看下是如何实现的
准备知识：位运算
1.SDWebImageOptions
typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
/**
* By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
* This flag disable this blacklisting.
不会重新下载已经失败过的图片
*/
SDWebImageRetryFailed = 1 << 0,
/**
* By default, image downloads are started during UI interactions, this flags disable this feature,
* leading to delayed download on UIScrollView deceleration for instance.
*默认情况下，图像下载是在UI交互期间启动的，此标志禁用此功能，
*导致延迟下载UIScrollView减速为例。
*/
SDWebImageLowPriority = 1 << 1,
/**
* This flag disables on-disk caching
禁止在磁盘缓存，只在缓存中存在
*/
SDWebImageCacheMemoryOnly = 1 << 2,
/**
* This flag enables progressive download, the image is displayed progressively during download as a browser would do.
* By default, the image is only displayed once completely downloaded.
此标志支持逐行下载，图像在下载过程中逐步显示，就像浏览器所做的那样。
*默认情况下，图像只显示一次，完全下载。
*/
SDWebImageProgressiveDownload = 1 << 3,
/**
* Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
* The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
* This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
* If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
*
* Use this flag only if you can't make your URLs static with embedded cache busting parameter.
任何图片都从新下载，不使用http cache，比如一个图片发送变化，但是url没有变化，用改options刷新数据
*/
SDWebImageRefreshCached = 1 << 4,
/**
* In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
* extra time in background to let the request finish. If the background task expires the operation will be cancelled.
后台下载
*/
SDWebImageContinueInBackground = 1 << 5,
/**
* Handles cookies stored in NSHTTPCookieStore by setting
* NSMutableURLRequest.HTTPShouldHandleCookies = YES;
存储在cookie nshttpcookiestore
*/
SDWebImageHandleCookies = 1 << 6,
/**
* Enable to allow untrusted SSL certificates.
* Useful for testing purposes. Use with caution in production.
允许不信任的ssl证书
*/
SDWebImageAllowInvalidSSLCertificates = 1 << 7,
/**
* By default, images are loaded in the order in which they were queued. This flag moves them to
* the front of the queue.
默认情况下，图像按它们排队的顺序加载。这个标志是将图片放在队列的最前面
*/
SDWebImageHighPriority = 1 << 8,
/**
* 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.
占位符图像是在图像加载时加载的，完全加载完才暂时
*/
SDWebImageDelayPlaceholder = 1 << 9,
/**
* We usually don't call transformDownloadedImage delegate method on animated images,
* as most transformation code would mangle it.
* Use this flag to transform them anyway.
转换图像
*/
SDWebImageTransformAnimatedImage = 1 << 10,
/**
* By default, image is added to the imageView after download. But in some cases, we want to
* have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)
* Use this flag if you want to manually set the image in the completion when success
默认情况下，image是在下载完成后加载，但是在一些情况下，我们想要在设置图像之前使用（例如应用过滤器或添加交叉淡入淡出动画），如果您想在成功完成时手动设置图像，请使用此标志
*/
SDWebImageAvoidAutoSetImage = 1 << 11,
/**
* By default, images are decoded respecting their original size. On iOS, this flag will scale down the
* images to a size compatible with the constrained memory of devices.
* If `SDWebImageProgressiveDownload` flag is set the scale down is deactivated.
默认情况下，图像是进行解码的，这个标志是按比例缩小images的尺寸，来缩小占用的手机内存，如果` sdwebimageprogressivedownload `标志设置的情况下被停用。，压缩大的图片
*/
SDWebImageScaleDownLargeImages = 1 << 12
};
SDWebImageManagerDelegate
@protocol SDWebImageManagerDelegate
@optional
/**
* Controls which image should be downloaded when the image is not found in the cache.
*
* @param imageManager The current `SDWebImageManager`
* @param imageURL The url of the image to be downloaded
*
* @return Return NO to prevent the downloading of the image on cache misses. If not implemented, YES is implied.
//当缓存没有发现当前图片，那么会查看调用者是否实现改方法，如果return一个no，则不会继续下载这张图片
*/
- (BOOL)imageManager:(nonnull SDWebImageManager *)imageManager shouldDownloadImageForURL:(nullable NSURL *)imageURL;
/**
* Allows to transform the image immediately after it has been downloaded and just before to cache it on disk and memory.
* NOTE: This method is called from a global queue in order to not to block the main thread.
*
* @param imageManager The current `SDWebImageManager`
* @param image The image to transform
* @param imageURL The url of the image to transform
*
* @return The transformed image object.
//当图片下载完成但是未添加到缓存里面，这时候调用该方法可以给图片旋转方向，注意是异步执行， 防止组织主线程
*/
- (nullable UIImage *)imageManager:(nonnull SDWebImageManager *)imageManager transformDownloadedImage:(nullable UIImage *)image withURL:(nullable NSURL *)imageURL;
@end
属性
一个值得学习的地方
在h文件中
@interface SDWebImageManager : NSObject
//SDWebImageManagerDelegate的delegate
@property (weak, nonatomic, nullable) id delegate;
//缓存中心
@property (strong, nonatomic, readonly, nullable) SDImageCache *imageCache;
//下载中心
@property (strong, nonatomic, readonly, nullable) SDWebImageDownloader *imageDownloader;
在m文件中
@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;//缓存对象
@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader *imageDownloader;//下载对象
这样做的目的是为了外包访问是只读属性，但是实际在本类中缺可以修改，我们平常应该学习这种设计方式
其他属性方法的含义
//这个缓存block的作用是，在block内部进行缓存key的生成并return，key就是根据图片url根据规则生成，sd的缓存策略就是key是图片url，value就是image
* @endcode
*/
@property (nonatomic, copy, nullable) SDWebImageCacheKeyFilterBlock cacheKeyFilter;
/**
* Returns global SDWebImageManager instance.
*
* @return SDWebImageManager shared instance
*/
+ (nonnull instancetype)sharedManager;
/**
* Allows to specify instance of cache and image downloader used with image manager.
* @return new instance of `SDWebImageManager` with specified cache and downloader.
*/
//根据特定的cache和downloader生成一个新的SDWebImageManager
- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader NS_DESIGNATED_INITIALIZER;
/**
* Downloads the image at the given URL if not present in cache or return the cached version otherwise.
*
* @param url The URL to the image
* @param options A mask to specify options to use for this request
* @param progressBlock A block called while image is downloading
* @note the progress block is executed on a background queue
* @param completedBlock A block called when operation has been completed.
*
* This parameter is required.
*
* This block has no return value and takes the requested UIImage as first parameter and the NSData representation as second parameter.
* In case of error the image parameter is nil and the third parameter may contain an NSError.
*
* The forth parameter is an `SDImageCacheType` enum indicating if the image was retrieved from the local cache
* or from the memory cache or from the network.
*
* The fith parameter is set to NO when the SDWebImageProgressiveDownload option is used and the image is
* downloading. This block is thus called repeatedly with a partial image. When image is fully downloaded, the
* block is called a last time with the full image and the last parameter set to YES.
*
* The last parameter is the original image URL
*
* @return Returns an NSObject conforming to SDWebImageOperation. Should be an instance of SDWebImageDownloaderOperation
*/
//下载图片的关键方法，第一个参数图片url，第二个参数设置下载多样操作，第三个参数下载中进度block，第四个参数下载完成后回调
- (nullable id )loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock;
/**
* Saves image to cache for given URL
*
* @param image The image to cache
* @param url The URL to the image
*
*/
//缓存图片根据指定的url和image
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url;
/**
* Cancel all current operations
*/
//取消所有当前的operation
- (void)cancelAll;
/**
* Check one or more operations running
*/
//检查是否有图片正在下载
- (BOOL)isRunning;
/**
* Async check if image has already been cached
*
* @param url image url
* @param completionBlock the block to be executed when the check is finished
*
* @note the completion block is always executed on the main queue
*/
//异步检查图片是否已经缓存
- (void)cachedImageExistsForURL:(nullable NSURL *)url
completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
* Async check if image has already been cached on disk only
*
* @param url image url
* @param completionBlock the block to be executed when the check is finished
*
* @note the completion block is always executed on the main queue
*/
//检查图片是否缓存 在磁盘中
- (void)diskImageExistsForURL:(nullable NSURL *)url
completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
*Return the cache key for a given URL
*/
//给定一个url返回缓存的字符串key
- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url;
单例和初始化方法
+ (nonnull instancetype)sharedManager {
static dispatch_once_t once;
static id instance;
dispatch_once(&once, ^{
instance = [self new];
});
return instance;
}
- (nonnull instancetype)init {
SDImageCache *cache = [SDImageCache sharedImageCache];
SDWebImageDownloader *downloader = [SDWebImageDownloader sharedDownloader];
return [self initWithCache:cache downloader:downloader];
}
- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader {
if ((self = [super init])) {
_imageCache = cache;
_imageDownloader = downloader;
_failedURLs = [NSMutableSet new];
_runningOperations = [NSMutableArray new];
}
return self;
}
SDWebImage 中，用url来作为缓存image的key
//根据URL获取缓存中的key
- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url {
if (!url) {
return @"";
}
if (self.cacheKeyFilter) {
return self.cacheKeyFilter(url);
} else {
return url.absoluteString;
}
}
根据图片的scale或图片中的图片组 重新计算返回一张新图片
//根据图片的scale或图片中的图片组 重新计算返回一张新图片
- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image {
return SDScaledImageForKey(key, image);
}
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 {
NSString *key = [self cacheKeyForURL:url];
BOOL isInMemoryCache = ([self.imageCache imageFromMemoryCacheForKey:key] != nil);
if (isInMemoryCache) {
// making sure we call the completion block on the main queue
dispatch_async(dispatch_get_main_queue(), ^{
if (completionBlock) {
completionBlock(YES);
}
});
return;
}
[self.imageCache diskImageExistsWithKey:key completion:^(BOOL isInDiskCache) {
// the completion block of checkDiskCacheForImageWithKey:completion: is always called on the main queue, no need to further dispatch
if (completionBlock) {
completionBlock(isInDiskCache);
}
}];
}
根据URL判断磁盘缓存中是否存在图片
- (void)diskImageExistsForURL:(nullable NSURL *)url
completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
NSString *key = [self cacheKeyForURL:url];
[self.imageCache diskImageExistsWithKey:key completion:^(BOOL isInDiskCache) {
// the completion block of checkDiskCacheForImageWithKey:completion: is always called on the main queue, no need to further dispatch
if (completionBlock) {
completionBlock(isInDiskCache);
}
}];
}
下面介绍下SDWebImageManager中最重要的一个方法：进行图片下载操作 返回一个operation
- (id )loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
// Invoking this method without a completedBlock is pointless
//completedBlock为nil，则触发断言，程序crash
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.
//判断类型是否为string类型，如果是转换为url类型
if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
// Prevents app crashing on argument type error like sending NSNull instead of NSURL
//防止NSNull类型
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}
//封装下载操作的对象
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;
BOOL isFailedUrl = NO; // self.failedURLs 保存了一个失败列表，防止失败连接多次调用消耗资源
if (url) {
@synchronized (self.failedURLs) { //synchronized关键字是用来控制线程同步的，就是在多线程的环境下，控制synchronized代码段不被多个线程同时执行。synchronized既可以加在一段代码上，也可以加在方法上。也就是说是一个互斥锁
isFailedUrl = [self.failedURLs containsObject:url];
}
}
//如果url为空，或者 options == SDWebImageRetryFailed 失败重新尝试，并且这个url是失败列表里的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; // operation 设置为 isFinish = YES;
}
//operation 加入 队列 ,用互斥锁，防止多个线程同时添加
@synchronized (self.runningOperations) {
[self.runningOperations addObject:operation];
}
NSString *key = [self cacheKeyForURL:url]; //生成url的key，为了以后缓存
//异步查询图片是否在缓存里, 使用缓存对象，根据key去寻找查找
operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
if (operation.isCancelled) {
//线程安全的移除下载operation
[self safelyRemoveOperationFromRunning:operation];
return;
}
//没有缓存数据 或者 options & SDWebImageRefreshCached 需要重新刷新缓存
if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
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
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;
}
//下载图片
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) { //操作取消，不做任何处理
// 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) { //下载错误
[self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];
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) { //下载失败则添加图片url到failedURLs集合，添加互斥锁
[self.failedURLs addObject:url];
}
}
}
else {
if ((options & SDWebImageRetryFailed)) { //虽然下载失败，但是如果设置了可以重新下载失败的url则remove该url
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly); //是否需要缓存在磁盘
// We've done the scale process in SDWebImageDownloader with the shared manager, this is used for custom manager and avoid extra scale.
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:)]) {
//图片下载成功并且判断是否需要转换图片
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
//根据代理获取转换后的图片 ，由用户自己去实现该方法
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]; //把图片缓存
}
//
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
});
} else {//下载完成且有image则缓存图片
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) { //如果下载和缓存都完成了则删除操作队列中的operation
[self safelyRemoveOperationFromRunning:strongOperation];
}
}];
@synchronized(operation) {
// Need same lock to ensure cancelBlock called because cancel method can be called in different queue
operation.cancelBlock = ^{
[self.imageDownloader cancel:subOperationToken];
__strong __typeof(weakOperation) strongOperation = weakOperation;
[self safelyRemoveOperationFromRunning:strongOperation];
};
}
} else if (cachedImage) { //有缓存数据
__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
__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];
}
}];
return operation;
}
这个方法的思路大概是这样的：（1）首先先判断url是否正确，（2）如果正确，封装一个下载操作的对象，这个对象主要有cancell的方法，通过self.runningOperations，和self.failedURLs 两个数组来记录正在下载的对象和失败的url （3）到缓存当中异步查询图片是否在缓存里，有缓存直接返回图片，没有缓存区下载。在后面的文章中，我们会一块一块详细剖析SDWebImage是如何拿到图片的
下面是一些功能方法
将图片存入缓存
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url {
if (image && url) {
NSString *key = [self cacheKeyForURL:url];
[self.imageCache storeImage:image forKey:key toDisk:YES completion:nil];
}
}
取消所有的下载操作
- (void)cancelAll {
@synchronized (self.runningOperations) {
NSArray *copiedOperations = [self.runningOperations copy];
[copiedOperations makeObjectsPerformSelector:@selector(cancel)];//让数组中的每个元素 都调用 cancel
[self.runningOperations removeObjectsInArray:copiedOperations];
}
}
关于 makeObjectsPerformSelector
- makeObjectsPerformSelector:@select（aMethod）
简介：让数组中的每个元素 都调用 aMethod - makeObjectsPerformSelector:@select（aMethod）
withObject:oneObject
简介：让数组中的每个元素 都调用 aMethod 并把 withObject 后边的 oneObject 对象做为参数传给方法aMethod
查看当前是否有下载图片
- (BOOL)isRunning {
BOOL isRunning = NO;
@synchronized (self.runningOperations) {
isRunning = (self.runningOperations.count > 0);
}
return isRunning;
}
在线程安全的情况下的移除下载operation
- (void)safelyRemoveOperationFromRunning:(nullable SDWebImageCombinedOperation*)operation {
@synchronized (self.runningOperations) {
if (operation) {
[self.runningOperations removeObject:operation];
}
}
}
SDWebImageCombinedOperation 类
@interface SDWebImageCombinedOperation : NSObject
也就是说 遵守 SDWebImageOperation 的 cancel 方法
@protocol SDWebImageOperation
- (void)cancel;
@end
@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;//是否取消当前所有操作
@property (copy, nonatomic, nullable) SDWebImageNoParamsBlock cancelBlock;//没有参数取消回调
@property (strong, nonatomic, nullable) NSOperation *cacheOperation;//执行缓存的操作
- (void)setCancelBlock:(nullable SDWebImageNoParamsBlock)cancelBlock {
// check if the operation is already cancelled, then we just call the cancelBlock
if (self.isCancelled) {
if (cancelBlock) {
cancelBlock();
}
_cancelBlock = nil; // don't forget to nil the cancelBlock, otherwise we will get crashes将block置为空，防止崩溃
} else {
_cancelBlock = [cancelBlock copy];
}
}
//重写cancel方法
- (void)cancel {
@synchronized(self) {//同步锁，保证线程安全
self.cancelled = YES;
if (self.cacheOperation) {
[self.cacheOperation cancel];
self.cacheOperation = nil;
}
if (self.cancelBlock) { //回调cancelblock
self.cancelBlock();
self.cancelBlock = nil;
}
}
}