一、前言
第一次使用SDWebImage是在自己做《康复医生》的时候。记得那时有一个表格要显示所有的用户头像。不使用它的时候每次加载该table都会很卡顿，后来使用后发现不卡顿了。瞬间感觉这个世界好有爱。
二、安装
首先，SDWebImage的git地址是：https://github.com/rs/SDWebImage。我们可以直接到这里进行下载，然后添加到自己的项目中去。
三、使用场景（前提是已经导入了SDWebImage这个库）
1、场景一、加载图片
使用SDWebImage可以去加载远程图片，而且还会缓存图片，下次请求会看一下是否已经存在于缓存中，如果是的话直接取本地缓存，如果不是的话则重新请求。使用方法很简单，在需要使用该场景的类中导入
#import "UIImageView+WebCache.h"
然后调用：
- (void)sd_setImageWithPreviousCachedImageWithURL:(NSURL *)url andPlaceholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
即可。
我们还可以在UIImageView+WebCache.h中看到其他的方法，和上边的方法类似，读者自行查看即可。
2、场景二、使用它做本地缓存
很多时候我们可能拍照得到的一张图片要多个地方使用，那么我们就希望可以把这张图片放到缓存里面，然后每次用这张图片的时候就去通过特定的方式取即可。SDWebImage就有这样的一个类：SDImageCache。该类完美地帮助了我们解决了这个问题。
使用的时候，我们首先要在使用的类里面做导入：
#import "SDImageCache.h"
然后就可以进行相关的操作了。让我们直接看看SDImageCache.h文件：
@property (assign, nonatomic) NSUInteger maxMemoryCost;/** * The maximum length of time to keep an image in the cache, in seconds */@property (assign, nonatomic) NSInteger maxCacheAge;/** * The maximum size of the cache, in bytes. */@property (assign, nonatomic) NSUInteger maxCacheSize;/** * Returns global shared cache instance * * @return SDImageCache global instance */+ (SDImageCache *)sharedImageCache;/** * Init a new cache store with a specific namespace * * @param ns The namespace to use for this cache store */- (id)initWithNamespace:(NSString *)ns;/** * Add a read-only cache path to search for images pre-cached by SDImageCache * Useful if you want to bundle pre-loaded images with your app * * @param path The path to use for this read-only cache path */- (void)addReadOnlyCachePath:(NSString *)path;/** * 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 */- (void)storeImage:(UIImage *)image forKey:(NSString *)key;/** * Store an image into memory and optionally 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 */- (void)storeImage:(UIImage *)image forKey:(NSString *)key toDisk:(BOOL)toDisk;/** * Store an image into memory and optionally disk cache at the given key. * * @param image The image to store * @param recalculate BOOL indicates if imageData can be used or a new data should be constructed from the UIImage * @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 */- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk;/** * Query the disk cache asynchronously. * * @param key The unique key used to store the wanted image */- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock;/** * Query the memory cache synchronously. * * @param key The unique key used to store the wanted image */- (UIImage *)imageFromMemoryCacheForKey:(NSString *)key;/** * Query the disk cache synchronously after checking the memory cache. * * @param key The unique key used to store the wanted image */- (UIImage *)imageFromDiskCacheForKey:(NSString *)key;/** * Remove the image from memory and disk cache synchronously * * @param key The unique image cache key */- (void)removeImageForKey:(NSString *)key;/** * Remove the image from memory and disk cache synchronously * * @param key The unique image cache key * @param completionBlock An block that should be executed after the image has been removed (optional) */- (void)removeImageForKey:(NSString *)key withCompletition:(void (^)())completion;/** * Remove the image from memory and optionally disk cache synchronously * * @param key The unique image cache key * @param fromDisk Also remove cache entry from disk if YES */- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk;/** * Remove the image from memory and optionally disk cache synchronously * * @param key The unique image cache key * @param fromDisk Also remove cache entry from disk if YES * @param completionBlock An block that should be executed after the image has been removed (optional) */- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk withCompletition:(void (^)())completion;/** * Clear all memory cached images */- (void)clearMemory;/** * Clear all disk cached images. Non-blocking method - returns immediately. * @param completionBlock An block that should be executed after cache expiration completes (optional) */- (void)clearDiskOnCompletion:(void (^)())completion;/** * Clear all disk cached images * @see clearDiskOnCompletion: */- (void)clearDisk;/** * Remove all expired cached image from disk. Non-blocking method - returns immediately. * @param completionBlock An block that should be executed after cache expiration completes (optional) */- (void)cleanDiskWithCompletionBlock:(void (^)())completionBlock;/** * Remove all expired cached image from disk * @see cleanDiskWithCompletionBlock: */- (void)cleanDisk;/** * 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:(void (^)(NSUInteger fileCount, NSUInteger totalSize))completionBlock;/** * Check if image exists in cache already */- (BOOL)diskImageExistsWithKey:(NSString *)key;/** * Get the cache path for a certain key (needs the cache path root folder) * * @param key the key (can be obtained from url using cacheKeyForURL) * @param path the cach path root folder * * @return the cache path */- (NSString *)cachePathForKey:(NSString *)key inPath:(NSString *)path;/** * Get the default cache path for a certain key * * @param key the key (can be obtained from url using cacheKeyForURL) * * @return the default cache path */- (NSString *)defaultCachePathForKey:(NSString *)key;
不要看着想吐就行。先看看使用吧。
存图片：
SDImageCache *imageCache = [SDImageCache sharedImageCache];
[imageCache storeImage:image forKey:@"myphoto" toDisk:YES];
取图片：
SDImageCache *imageCache = [SDImageCache sharedImageCache]; UIImage *image = [imageCache imageFromDiskCacheForKey:@"myphoto"];
这样就可以取到自己存的图片了。可以看一下SDImageCache.h这个类了，里面提供了存图片到内存和磁盘的方法，还有取图片的方法，以及判断该图片是否存在的方法。还有就是删除图片、清空磁盘缓存、得到缓存大小等方法。
场景三、做设置中的清除缓存功能
简单来说，当我们点击清除缓存按钮的时候会触发的消息如下：
- (void)clearCaches{ [MBProgressHUD showMessage:@"正在清理.."]; [[SDImageCache sharedImageCache] clearMemory]; [[SDImageCache sharedImageCache] clearDisk]; [self performSelectorOnMainThread:@selector(cleanCacheSuccess) withObject:nil waitUntilDone:YES];}
- (void)cleanCacheSuccess
{
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)2 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
self.cacheLab.text = @"0.00M";
return;
});
}
很简单，就是把使用SDWebImage缓存的图片都清空就可以了，这里包括了内存中的缓存图片以及磁盘上的缓存图片。
还有其他的使用场景，这里就不再赘述了。
四、实现原理
其实SDWebImage之所以能够实现缓存的原理关键就是在哪个key值。
比如我们在使用
- (void)sd_setImageWithPreviousCachedImageWithURL:(NSURL *)url andPlaceholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
的时候，其实就是把url当做了一个图片的key值，然后存储对应的图片，如果下次请求的url和这次请求的url一样，那么就直接根据url（这个key）来取图片，如果url作为key的图片缓存不存在，就去请求远程服务器，然后请求过来之后再次将url和图片对应，然后存储。
五、其他
未完待续。
原标题：SDWebImage的简单使用
关键词：web
我知道了 