SDWebImage的类主要有以下三个：
SDWebImageManager：图片工具管理类
SDWebImageDownloader：图片下载工具类
SDImageCache：cache管理类
1.SDWebImageManager
提供了单例方法
+ (nonnull instancetype)sharedManager;
也提供了指定cache和downloader的初始化方法
- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader NS_DESIGNATED_INITIALIZER;
一般我们使用单例方法即可。
我们可以使用其对象方法去加载图片，方法如下：
- (nullable id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock;
也可以单独调用方法缓存图片
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url;
持有两个属性：
//通过提供的url来生成cache的key的block，默认为nil
@property (nonatomic, copy, nullable) SDWebImageCacheKeyFilterBlock cacheKeyFilter;
//一些图片下载过程中的调用的代理对象
@property (weak, nonatomic, nullable) id <SDWebImageManagerDelegate> delegate;
//管理图片缓存的对象
@property (strong, nonatomic, readonly, nullable) SDImageCache *imageCache;
//管理图像下载的对象
@property (strong, nonatomic, readonly, nullable) SDWebImageDownloader *imageDownloader;
//管理图片下载后保存缓存的block，默认为nil，使用imageCache的缓存策略
@property (nonatomic, copy, nullable) SDWebImageCacheSerializerBlock cacheSerializer;
2.SDWebImageDownloader
属性如下：
/**
* Decompressing images that are downloaded and cached can improve performance but can consume lot of memory.
* Defaults to YES. Set this to NO if you are experiencing a crash due to excessive memory consumption.
*/
@property (assign, nonatomic) BOOL shouldDecompressImages;
/**
* The maximum number of concurrent downloads
*/
@property (assign, nonatomic) NSInteger maxConcurrentDownloads;
/**
* Shows the current amount of downloads that still need to be downloaded
*/
@property (readonly, nonatomic) NSUInteger currentDownloadCount;
/**
* The timeout value (in seconds) for the download operation. Default: 15.0.
*/
@property (assign, nonatomic) NSTimeInterval downloadTimeout;
/**
* The configuration in use by the internal NSURLSession.
* Mutating this object directly has no effect.
*
* @see createNewSessionWithConfiguration:
*/
@property (readonly, nonatomic, nonnull) NSURLSessionConfiguration *sessionConfiguration;
/**
* Changes download operations execution order. Default value is `SDWebImageDownloaderFIFOExecutionOrder`.
*/
@property (assign, nonatomic) SDWebImageDownloaderExecutionOrder executionOrder;
/**
* Set the default URL credential to be set for request operations.
*/
@property (strong, nonatomic, nullable) NSURLCredential *urlCredential;
/**
* Set username
*/
@property (strong, nonatomic, nullable) NSString *username;
/**
* Set password
*/
@property (strong, nonatomic, nullable) NSString *password;
/**
* Set filter to pick headers for downloading image HTTP request.
*
* This block will be invoked for each downloading image request, returned
* NSDictionary will be used as headers in corresponding HTTP request.
*/
@property (nonatomic, copy, nullable) SDWebImageDownloaderHeadersFilterBlock headersFilter;
主要可以设置最大下载数，最长的下载超时时间等。
SDWebImageDownloader也提供了单例方法供程序员使用：
/**
* Singleton method, returns the shared instance
*
* @return global shared instance of downloader class
*/
+ (nonnull instancetype)sharedDownloader;
也可以自己根据需求来对请求头进行设置：
/**
* Set a value for a HTTP header to be appended to each download HTTP request.
*
* @param value The value for the header field. Use `nil` value to remove the header.
* @param field The name of the header field to set.
*/
- (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field;
/**
* Returns the value of the specified HTTP header field.
*
* @return The value associated with the header field field, or `nil` if there is no corresponding header field.
*/
- (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field;
主要给程序员使用的下载方法如下：
/**
* Creates a SDWebImageDownloader async downloader instance with a given URL
*
* The delegate will be informed when the image is finish downloaded or an error has happen.
*
* @see SDWebImageDownloaderDelegate
*
* @param url The URL to the image to download
* @param completedBlock A block called once the download is completed.
* If the download succeeded, the image parameter is set, in case of error,
* error parameter is set with the error. The last parameter is always YES
* if SDWebImageDownloaderProgressiveDownload isn't use. With the
* SDWebImageDownloaderProgressiveDownload option, this block is called
* repeatedly with the partial image object and the finished argument set to NO
* before to be called a last time with the full image and finished argument
* set to YES. In case of error, the finished argument is always YES.
*/
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;
/**
* Creates a SDWebImageDownloader async downloader instance with a given URL
*
* The delegate will be informed when the image is finish downloaded or an error has happen.
*
* @see SDWebImageDownloaderDelegate
*
* @param url The URL to the image to download
* @param options The options to be used for this download
* @param progressBlock A block called repeatedly while the image is downloading
* @note the progress block is executed on a background queue
* @param completedBlock A block called once the download is completed.
* If the download succeeded, the image parameter is set, in case of error,
* error parameter is set with the error. The last parameter is always YES
* if SDWebImageDownloaderProgressiveDownload isn't use. With the
* SDWebImageDownloaderProgressiveDownload option, this block is called
* repeatedly with the partial image object and the finished argument set to NO
* before to be called a last time with the full image and finished argument
* set to YES. In case of error, the finished argument is always YES.
*
* @return A token (SDWebImageDownloadToken) that can be passed to -cancel: to cancel this operation
*/
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;
3.SDImageCache
主要属性如下：
/**
* Cache Config object - storing all kind of settings
*/
@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.
*/
@property (assign, nonatomic) NSUInteger maxMemoryCost;
/**
* The maximum number of objects the cache should hold.
*/
@property (assign, nonatomic) NSUInteger maxMemoryCountLimit;
可以设置缓存在内存中的最大数量和最大占用内存大小，设置图片在硬盘上保存的时间，在硬盘上最大缓存的大小。超过设置的最大数时会进行删除操作。默认最长时间为7天，硬盘缓存无空间限制。
SDImageCache提供了单例对象方法，也提供了自定义对象方法，自定义对象方法可以程序员自己设置缓存图片的文件夹。
/**
* Returns global shared cache instance
*
* @return SDImageCache global instance
*/
+ (nonnull instancetype)sharedImageCache;
/**
* Init a new cache store with a specific namespace
*
* @param ns The namespace to use for this cache store
*/
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns;
/**
* Init a new cache store with a specific namespace and directory
*
* @param ns The namespace to use for this cache store
* @param directory Directory to cache disk images in
*/
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
diskCacheDirectory:(nonnull NSString *)directory NS_DESIGNATED_INITIALIZER;
SDImageCache提供了保存缓存图片、查询缓存图片、移除及清空缓存图片的方法，如下：
保存缓存图片
/**
* Asynchronously store an image into memory and disk cache at the given key.
*
* @param image The image to store
* @param key The unique image cache key, usually it's image absolute URL
* @param completionBlock A block executed after the operation is finished
*/
- (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.
*
* @param image The image to store
* @param key The unique image cache key, usually it's image absolute 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
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock;
/**
* Asynchronously store an image into memory and disk cache at the given key.
*
* @param image The image to store
* @param imageData The image data as returned by the server, this representation will be used for disk storage
* instead of converting the given image object into a storable/compressed image format in order
* to save quality and CPU
* @param key The unique image cache key, usually it's image absolute 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.
*
*
* @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;
查询缓存图片
/**
* Async check if image exists in disk cache already (does not load the image)
*
* @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
*/
- (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
* Sync check if image data exists in disk cache already (does not load the image)
*
* @param key the key describing the url
*/
- (BOOL)diskImageDataExistsWithKey:(nullable NSString *)key;
/**
* Query the image data for the given key synchronously.
*
* @param key The unique key used to store the wanted image
* @return The image data for the given key, or nil if not found.
*/
- (nullable NSData *)diskImageDataForKey:(nullable NSString *)key;
/**
* Operation that queries the cache asynchronously and call the completion when done.
*
* @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
*/
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock;
/**
* Operation that queries the cache asynchronously and call the completion when done.
*
* @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
*/
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options done:(nullable SDCacheQueryCompletedBlock)doneBlock;
/**
* Query the memory cache synchronously.
*
* @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.
*
* @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.
*
* @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;
移除缓存图片
/**
* Remove the image from memory and disk cache asynchronously
*
* @param key The unique image cache key
* @param completion A block that should be executed after the image has been removed (optional)
*/
- (void)removeImageForKey:(nullable NSString *)key withCompletion:(nullable SDWebImageNoParamsBlock)completion;
/**
* Remove the image from memory and optionally disk cache asynchronously
*
* @param key The unique image cache key
* @param fromDisk Also remove cache entry from disk if YES
* @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;
清空缓存图片
/**
* Clear all memory cached images
*/
- (void)clearMemory;
/**
* Async clear all disk cached images. Non-blocking method - returns immediately.
* @param completion A block that should be executed after cache expiration completes (optional)
*/
- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion;
/**
* Async remove all expired cached image from disk. Non-blocking method - returns immediately.
* @param completionBlock A block that should be executed after cache expiration completes (optional)
*/
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock;
还提供了获取硬盘缓存大小的方法
/**
* Get the size used by the disk cache
*/
- (NSUInteger)getSize;
/**
* Get the number of images in the disk cache
*/
- (NSUInteger)getDiskCount;
/**
* Asynchronously calculate the disk cache's size.
*/
- (void)calculateSizeWithCompletionBlock:(nullable SDWebImageCalculateSizeBlock)completionBlock;
SDImageCache在初始化时，添加了两个监听方法
// Subscribe to app events
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationWillTerminate:)
name:UIApplicationWillTerminateNotification
object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationDidEnterBackground:)
name:UIApplicationDidEnterBackgroundNotification
object:nil];
在两个监听方法里最终会调用removeExpiredData方法，清除过期的图片缓存。
[self.diskCache removeExpiredData];
