說道用sdWebImage下載圖片，那就先簡單介紹下SDWebImage中下載圖片的幾個工具類了。SDWebImageManager是SDWebImage的核心類，他有兩個屬性SDImageCache *imageCache（圖片緩存）、 SDWebImageDownloader *imageDownloader（圖片下載）和一個代理id <SDWebImageManagerDelegate> delegate。
1、協議SDWebImageManagerDelegate
@protocol SDWebImageManagerDelegate <NSObject>
@optional
/**
* Controls which image should be downloaded when the image is not found in the cache.
* 控制在緩存中找不到圖像時應下載的圖像。
*
* @param imageManager The current `SDWebImageManager` 當前的“ SDWebImageManager”，也就是當前的圖片管理類
* @param imageURL The url of the image to be downloaded 要下載圖片的URL
*
* @return Return NO to prevent the downloading of the image on cache misses. If not implemented, YES is implied.
也就是判斷緩存中有沒有圖片，控制是否去下載圖片
這個方法就是判斷本地緩存中是否有某個url的圖片，根據是否存在返回YES/NO
*/
- (BOOL)imageManager:(nonnull SDWebImageManager *)imageManager shouldDownloadImageForURL:(nullable NSURL *)imageURL;
/**
* Controls the complicated logic to mark as failed URLs when download error occur.
* 控制發生下載錯誤時將複雜的邏輯標記爲失敗的URL。
* If the delegate implement this method, we will not use the built-in way to mark URL as failed based on error code;
* 如果委託實現此方法，則不會基於錯誤代碼使用內置方法將URL標記爲失敗；
@param imageManager The current `SDWebImageManager` 當前的“ SDWebImageManager”
@param imageURL The url of the image 這個圖片的URL
@param error The download error for the url 這個url下載出現錯誤的error
@return Whether to block this url or not. Return YES to mark this URL as failed. 是否阻止該URL。
這個方法的作用是，判斷某個url是否已經被標記過爲下載失敗，被標記失敗的url將不會被下載圖片，直接返回錯誤
*/
- (BOOL)imageManager:(nonnull SDWebImageManager *)imageManager shouldBlockFailedURL:(nonnull NSURL *)imageURL withError:(nonnull NSError *)error;
/**
* Allows to transform the image immediately after it has been downloaded and just before to cache it on disk and memory.
* 允許在下載圖像之後立即轉換圖像，然後再將其緩存在磁盤和內存中。
* NOTE: This method is called from a global queue in order to not to block the main thread.
* 注意：爲了不阻塞主線程，從全局隊列中調用此方法。
*
*
* @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.
*
*
* 該方法的作用是，在下載完圖片後，首先做的是將圖片賦值，顯示到屏幕上之後再將圖片保存到緩存和磁盤中
*/
- (nullable UIImage *)imageManager:(nonnull SDWebImageManager *)imageManager transformDownloadedImage:(nullable UIImage *)image withURL:(nullable NSURL *)imageURL;
@end
2、SDImageCache
SDImageCache提供了對圖片的內存緩存、異步磁盤緩存、圖片緩存查詢等功能，下載過的圖片會被緩存到內存，也可選擇保存到本地磁盤，當再次請求相同圖片時直接從緩存中讀取圖片，從而大大提高了加載速度。在SDImageCache中，內存緩存是通過 NSCache的子類AutoPurgeCache來實現的；磁盤緩存是通過 NSFileManager 來實現文件的存儲(默認路徑爲/Library/Caches/default/com.hackemist.SDWebImageCache.default)，是異步實現的。
解讀SDWebImage提供的方法
#pragma mark - Properties - 屬性
/**
* Cache Config object - storing all kind of settings
* 緩存配置對象-存儲所有類型的設置
*/
@property (nonatomic, nonnull, readonly) SDImageCacheConfig *config;
/**
* The maximum "total cost" of the in-memory image cache. The cost function is the number of pixels held in memory.
* 內存中圖像緩存的最大“總成本”。 代價函數是內存中保留的像素數。，其實就是內存最大容量
*/
@property (assign, nonatomic) NSUInteger maxMemoryCost;
/**
* The maximum number of objects the cache should hold.
* 緩存應容納的最大對象數。和NSCache相關的
*/
@property (assign, nonatomic) NSUInteger maxMemoryCountLimit;
#pragma mark - Singleton and initialization - 單例和初始化
/**
* Returns global shared cache instance
*
* @return SDImageCache global instance
*
* -- 返回單例
*/
+ (nonnull instancetype)sharedImageCache;
/**
* Init a new cache store with a specific namespace - 初始化具有特定名稱空間的新緩存存儲
*
* @param ns The namespace to use for this cache store - 用於此緩存存儲的名稱空間
*
* 使用指定的命名空間實例化一個新的緩存存儲
*/
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns;
/**
* Init a new cache store with a specific namespace and directory - 初始化具有特定名稱空間和目錄的新緩存存儲
*
* @param ns The namespace to use for this cache store - 用於此緩存存儲的名稱空間
* @param directory Directory to cache disk images in - 用於緩存磁盤映像的目錄
* 使用指定的命名空間實例化一個新的緩存存儲和目錄
*/
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
diskCacheDirectory:(nonnull NSString *)directory NS_DESIGNATED_INITIALIZER;
#pragma mark - Cache paths - 緩存路徑
/**
* 圖片緩存目錄
*/
- (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace;
/**
* Add a read-only cache path to search for images pre-cached by SDImageCache - 添加只讀緩存路徑以搜索SDImageCache預緩存的圖像
* Useful if you want to bundle pre-loaded images with your app - 如果您想將預加載的圖像與應用捆綁在一起，則很有用
*
* @param path The path to use for this read-only cache path - 用於此只讀緩存路徑的路徑
* 添加只讀的緩存路徑
*/
- (void)addReadOnlyCachePath:(nonnull NSString *)path;
#pragma mark - Store Ops - 商店運營
/**
* Asynchronously store an image into memory and disk cache at the given key. - 以給定的密鑰將映像異步存儲到內存和磁盤緩存中。
*
* @param image The image to store - 要存儲的圖像
* @param key The unique image cache key, usually it's image absolute URL - 唯一的圖片緩存鍵，通常是圖片的絕對URL
* @param completionBlock A block executed after the operation is finished - 操作完成後執行的塊
*/
- (void)storeImage:(nullable UIImage *)image
forKey:(nullable NSString *)key
completion:(nullable SDWebImageNoParamsBlock)completionBlock;
/**
* Asynchronously store an image into memory and disk cache at the given key. - 以給定的密鑰將映像異步存儲到內存和磁盤緩存中。
*
* @param image The image to store
* @param key The unique image cache key, usually it's image absolute URL
* @param toDisk（到磁盤） Store the image to disk cache if YES - 如果是，將映像存儲到磁盤緩存
* @param completionBlock A block executed after the operation is finished
*/
- (void)storeImage:(nullable UIImage *)image
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock;
/**
* Asynchronously store an image into memory and disk cache at the given key. - 以給定的密鑰將映像異步存儲到內存和磁盤緩存中。
*
* @param image The image to store
* @param imageData The image data as returned by the server, this representation will be used for disk storage - 服務器返回的圖像數據，此表示形式將用於磁盤存儲
* instead of converting the given image object into a storable/compressed image format in order - 而不是按順序將給定的圖像對象轉換爲可存儲/壓縮的圖像格式
* to save quality and CPU - 節省質量和CPU
* @param key The unique image cache key, usually it's image absolute URL - 唯一的圖片緩存鍵，通常是圖片的絕對URL
* @param toDisk Store the image to disk cache if YES
* @param completionBlock A block executed after the operation is finished
*/
- (void)storeImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock;
/**
* Synchronously store image NSData into disk cache at the given key.
* 將映像NSData同步存儲到給定鍵的磁盤緩存中。
*
* @param imageData The image data to store
* @param key The unique image cache key, usually it's image absolute URL
*/
- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key;
#pragma mark - Query and Retrieve Ops - 查詢和檢索操作
/**
* Async check if image exists in disk cache already (does not load the image)
* 異步檢查磁盤高速緩存中是否已存在映像（不加載映像）
*
* @param key the key describing the url
* @param completionBlock the block to be executed when the check is done.
* @note the completion block will be always executed on the main queue
* 完成塊將始終在主隊列上執行
*/
- (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
* Sync check if image data exists in disk cache already (does not load the image)
* 同步檢查磁盤緩存中是否已存在映像數據（不加載映像）
*
* @param key the key describing the url 描述網址的鍵
*/
- (BOOL)diskImageDataExistsWithKey:(nullable NSString *)key;
/**
* Query the image data for the given key synchronously.
* 同步查詢給定鍵的圖像數據。
*
* @param key The unique key used to store the wanted image
* @return The image data for the given key, or nil if not found.
* 如果找不到圖片返回nil，找的情況下返回該圖片
*/
- (nullable NSData *)diskImageDataForKey:(nullable NSString *)key;
/**
* Operation that queries the cache asynchronously and call the completion when done.
* 異步查詢緩存並在完成時調用完成的操作。
*
* @param key The unique key used to store the wanted image
* @param doneBlock The completion block. Will not get called if the operation is cancelled
*
* @return a NSOperation instance containing the cache op - 一個包含緩存操作的NSOperation實例
*
*這個方法是在緩存中查詢對應key的數據
*/
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock;
/**
* Operation that queries the cache asynchronously and call the completion when done.
* 異步查詢緩存並在完成時調用完成的操作。
*
* @param key The unique key used to store the wanted image
* @param options A mask to specify options to use for this cache query
* @param doneBlock The completion block. Will not get called if the operation is cancelled
*
* @return a NSOperation instance containing the cache op
* 同上，增加了options參數
*/
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options done:(nullable SDCacheQueryCompletedBlock)doneBlock;
/**
* Query the memory cache synchronously.
* 同步查詢內存緩存。
*
* @param key The unique key used to store the image
* @return The image for the given key, or nil if not found.
*/
- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key;
/**
* Query the disk cache synchronously.
* 同步查詢磁盤緩存。
*
* @param key The unique key used to store the image
* @return The image for the given key, or nil if not found.
*/
- (nullable UIImage *)imageFromDiskCacheForKey:(nullable NSString *)key;
/**
* Query the cache (memory and or disk) synchronously after checking the memory cache.
* 檢查內存緩存後，同步查詢緩存（內存和/或磁盤）。
*
* @param key The unique key used to store the image
* @return The image for the given key, or nil if not found.
*/
- (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key;
#pragma mark - Remove Ops - 刪除操作
/**
* Remove the image from memory and disk cache asynchronously
* 異步從內存和磁盤緩存中刪除映像
*
* @param key The unique image cache key
* @param completion A block that should be executed after the image has been removed (optional) 刪除圖像後應執行的Block，可選參數
*/
- (void)removeImageForKey:(nullable NSString *)key withCompletion:(nullable SDWebImageNoParamsBlock)completion;
/**
* Remove the image from memory and optionally disk cache asynchronously
* 從內存和可選的磁盤高速緩存中異步刪除映像
*
* @param key The unique image cache key
* @param fromDisk Also remove cache entry from disk if YES - 如果傳入YES，還從磁盤中刪除緩存條目
* @param completion A block that should be executed after the image has been removed (optional)
*/
- (void)removeImageForKey:(nullable NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(nullable SDWebImageNoParamsBlock)completion;
#pragma mark - Cache clean Ops - 緩存清理操作
/**
* Clear all memory cached images
* 清除所有內存緩存的圖像
*/
- (void)clearMemory;
/**
* Async clear all disk cached images. Non-blocking method - returns immediately.
* 異步清除所有磁盤緩存的映像。 非阻塞方法-立即返回。
* @param completion A block that should be executed after cache expiration completes (optional) 可選Block參數，緩存過期後的回調
*/
- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion;
/**
* Async remove all expired cached image from disk. Non-blocking method - returns immediately.
* 異步從磁盤中刪除所有過期的緩存映像。 非阻塞方法-立即返回。
* @param completionBlock A block that should be executed after cache expiration completes (optional)
*/
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock;
#pragma mark - Cache Info - 緩存信息
/**
* Get the size used by the disk cache
* 獲取磁盤緩存使用的大小
*/
- (NSUInteger)getSize;
/**
* Get the number of images in the disk cache
* 獲取磁盤緩存中的映像數
*/
- (NSUInteger)getDiskCount;
/**
* Asynchronously calculate the disk cache's size.
* 異步計算磁盤緩存的大小。
*/
- (void)calculateSizeWithCompletionBlock:(nullable SDWebImageCalculateSizeBlock)completionBlock;
#pragma mark - Cache Paths - 緩存路徑
/**
* Get the cache path for a certain key (needs the cache path root folder)
* 根據key獲取緩存路徑（需要緩存路徑根文件夾）
*
* @param key the key (can be obtained from url using cacheKeyForURL)
* @param path the cache path root folder 緩存根文件夾的路徑
*
* @return the cache path
*/
- (nullable NSString *)cachePathForKey:(nullable NSString *)key inPath:(nonnull NSString *)path;
/**
* Get the default cache path for a certain key
* 根據key獲取默認的緩存路徑
*
* @param key the key (can be obtained from url using cacheKeyForURL)
*
* @return the default cache path
*/
- (nullable NSString *)defaultCachePathForKey:(nullable NSString *)key;
3、SDWebImageDownloaderSDWebImageDownloader完成了對網絡圖片的異步下載工作，就是一個文件下載的工具類，網絡請求是在繼承於NSOperation的SDWebImageDownloaderOperation類實現的。SDWebImageDownloader的主要任務是下載相關配置項的管理，包括下載隊列的先後順序、最大下載任務數量控制、下載隊列中的任務創建、取消、暫停等任務管理，以及其他 HTTPS 和 HTTP Header的設置。
- SDWebImageDownloaderOptions：
typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
//默認模式
SDWebImageDownloaderLowPriority = 1 << 0,
//本模式在返回進度Block的同時，同時返回completedBlock，裏面的UIImage就是當前下載時的圖片，可以實現將圖片一點點顯示出來的功能
SDWebImageDownloaderProgressiveDownload = 1 << 1,
//默認情況下，http請求阻止使用NSURLCache對象。如果設置了這個標記，則NSURLCache會被http請求使用。
SDWebImageDownloaderUseNSURLCache = 1 << 2,
//如果image/imageData是從NSURLCache返回的，則completion這個回調會返回nil
SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
//如果app進入後臺模式，是否繼續下載，這個是通過在後臺申請時間來完成這個操作。如果指定的時間範圍內沒有完成，則直接取消下載。
SDWebImageDownloaderContinueInBackground = 1 << 4,
//處理緩存在`NSHTTPCookieStore`對象裏面的cookie，通過設置`NSMutableURLRequest.HTTPShouldHandleCookies = YES`來實現的。
SDWebImageDownloaderHandleCookies = 1 << 5,
//允許非信任的SSL證書請求。在測試的時候很有用，但是正式環境要小心使用。
SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,
//默認情況下，圖片加載的順序是根據加入隊列的順序加載的。但是這個標記會把任務加入隊列的最前面。
SDWebImageDownloaderHighPriority = 1 << 7,
//默認情況下，圖片會按照它的原始大小來解碼顯示。這個屬性會根據設備的內存限制調整圖片的尺寸到合適的大小。如果`SDWebImageProgressiveDownload`標記被設置了，則這個flag不起作用。
SDWebImageDownloaderScaleDownLargeImages = 1 << 8,
};
在使用的時候，當判斷options是否是SDWebImageDownloaderIgnoreCachedResponse選項時，應該這樣來判斷：
self.option & SDWebImageDownloaderIgnoreCachedResponse
SDWebImageDownloaderExecutionOrder
typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
/**
* Default value. All download operations will execute in queue style (first-in-first-out).
* 默認值。 所有下載操作將以隊列方式（先進先出）執行。
*/
SDWebImageDownloaderFIFOExecutionOrder,
/**
* All download operations will execute in stack style (last-in-first-out).
* 所有下載操作將以堆棧樣式（後進先出）執行。
*/
SDWebImageDownloaderLIFOExecutionOrder
};
- SDWebImageDownloadToken
/**
The download's URL. This should be readonly and you should not modify
下載的URL。 只讀屬性
*/
@property (nonatomic, strong, nullable) NSURL *url;
/**
The cancel token taken from `addHandlersForProgress:completed`. This should be readonly and you should not modify
取消令牌取自`addHandlersForProgress：completed`。 只讀屬性
@note use `-[SDWebImageDownloadToken cancel]` to cancel the token
*/
@property (nonatomic, strong, nullable) id downloadOperationCancelToken;
SDWebImageDownloadToken爲每一個下載任務的唯一身份標識，SDWebImageDownloader和我們平時開發中的下載有一些不同，它弱化了下載過程，比較強調的是下載結果，不支持斷點下載。
- SDWebImageDownloader的屬性
/**
* Decompressing images that are downloaded and cached can improve performance but can consume lot of memory.
* 對下載並緩存的圖像進行解壓縮可以提高性能，但會佔用大量內存。
* Defaults to YES. Set this to NO if you are experiencing a crash due to excessive memory consumption.
* 默認爲是。 如果由於過多的內存消耗而導致崩潰，請將其設置爲NO。
*/
@property (assign, nonatomic) BOOL shouldDecompressImages;
/**
* The maximum number of concurrent downloads
* 併發下載的最大數量
*/
@property (assign, nonatomic) NSInteger maxConcurrentDownloads;
/**
* Shows the current amount of downloads that still need to be downloaded
* 顯示當前仍需要下載的下載量
*/
@property (readonly, nonatomic) NSUInteger currentDownloadCount;
/**
* The timeout value (in seconds) for the download operation. Default: 15.0.
* 下載操作的超時值（以秒爲單位）。 默認值：15.0。
*/
@property (assign, nonatomic) NSTimeInterval downloadTimeout;
/**
* The configuration in use by the internal NSURLSession.
* 內部NSURLSession使用的配置。
* Mutating this object directly has no effect.
*直接更改此對象無效。
*
* @see createNewSessionWithConfiguration:
* 使用配置創建新會話：
*/
@property (readonly, nonatomic, nonnull) NSURLSessionConfiguration *sessionConfiguration;
/**
* Changes download operations execution order. Default value is `SDWebImageDownloaderFIFOExecutionOrder`.
* 更改下載操作執行順序。 默認值爲“ SDWebImageDownloaderFIFOExecutionOrder”。
*/
@property (assign, nonatomic) SDWebImageDownloaderExecutionOrder executionOrder;
4、SDWebImageManager根據文檔中的說明，就是對圖片下載的管理類
//對SDWebImageManager的一個簡單應用
SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager downloadImageWithURL:imageURL
options:0
progress:nil
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
if (image) {
// 得到image，展示或者乾點其他的事情
}
}];
SDWebImageManager最主要的三個屬性在上面已經講過了，還有幾個其他的屬性和方法：
@property (weak, nonatomic) id <SDWebImageManagerDelegate> delegate;
@property (strong, nonatomic, readonly) SDImageCache *imageCache;
@property (strong, nonatomic, readonly) SDWebImageDownloader *imageDownloader;
//------------上面已經講過了---------------
/**
* The cache filter is a block used each time SDWebImageManager need to convert an URL into a cache key. This can be used to remove dynamic part of an image URL.
* 緩存過濾器是SDWebImageManager每次需要將URL轉換爲緩存鍵時使用的block。 這可用於刪除圖像URL的動態部分。
*
* The following example sets a filter in the application delegate that will remove any query-string from the URL before to use it as a cache key:
*下面的示例在應用程序委託中設置一個過濾器，該過濾器將在使用URL作爲緩存鍵之前從URL中刪除所有查詢字符串：
* @code
[[SDWebImageManager sharedManager] setCacheKeyFilter:^(NSURL *url) {
url = [[NSURL alloc] initWithScheme:url.scheme host:url.host path:url.path];
return [url absoluteString];
}];
* @endcode
*/
@property (nonatomic, copy) SDWebImageCacheKeyFilterBlock cacheKeyFilter;
/**
* Returns global SDWebImageManager instance.
*
* @return SDWebImageManager shared instance
*/
+ (SDWebImageManager *)sharedManager;
/**
* Allows to specify instance of cache and image downloader used with image manager.
* 允許指定與圖像管理器一起使用的緩存和圖像下載器的對象
* @return new instance of `SDWebImageManager` with specified cache and downloader.
*/
- (instancetype)initWithCache:(SDImageCache *)cache downloader:(SDWebImageDownloader *)downloader;
/**
* Downloads the image at the given URL if not present in cache or return the cached version otherwise.
* 如果圖片不在內存中，則用給定的url下載圖片，否則返回內存中的圖片
*
* @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
* @param completedBlock A block called when operation has been completed.
*
* This parameter is required. 以上的參數是必須的
*
* This block has no return value and takes the requested UIImage as first parameter. 沒有返回值的block，返回請求得到的image作爲第一個返回值
* In case of error the image parameter is nil and the second parameter may contain an NSError. 返回圖片爲nil的時候，第二個參數爲error
*
* The third parameter is an `SDImageCacheType` enum indicating if the image was retrieved from the local cache
* or from the memory cache or from the network.
* 返回SDImageCacheType這個枚舉值，說明圖片是從服務端還是從緩存或者磁盤中獲取的
* The last 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.
* 重複回調的一個block，顯示下載進度，如果沒有下載完成返回NO，下載完成後返回YES
*
* @return Returns an NSObject conforming to SDWebImageOperation. Should be an instance of SDWebImageDownloaderOperation
*/
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock;
/**
* Saves image to cache for given URL
* 保存圖片的url
* @param image The image to cache
* @param url The URL to the image
*
*/
- (void)saveImageToCache:(UIImage *)image forURL:(NSURL *)url;
/**
* Cancel all current operations
* 取消當前所有操作，所有下載任務、線程、資源全部置空
*/
- (void)cancelAll;
/**
* Check one or more operations running
* 檢查一項或多項正在運行的操作
*/
- (BOOL)isRunning;
/**
* Check if image has already been cached
* 檢查圖像是否已經被緩存
* @param url image url
*
* @return if the image was already cached
*/
- (BOOL)cachedImageExistsForURL:(NSURL *)url;
/**
* Check if image has already been cached on disk only
* 檢查圖像是否僅已緩存在磁盤上
* @param url image url
*
* @return if the image was already cached (disk only)
*/
- (BOOL)diskImageExistsForURL:(NSURL *)url;
/**
* Async check if image has already been cached
* 異步檢查圖像是否已經被緩存
* @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
*/
- (void)cachedImageExistsForURL:(NSURL *)url
completion:(SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
* Async check if image has already been cached on disk only
* 異步檢查圖像是否被存在磁盤中
* @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
*/
- (void)diskImageExistsForURL:(NSURL *)url
completion:(SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
*Return the cache key for a given URL
* 獲取url在存儲圖片的key
*/
- (NSString *)cacheKeyForURL:(NSURL *)url;
