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。
一、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
//导入头文件
#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;
提示: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。
//包含了多种功能 1，sd_setImageWithURL获取网络图片 2，placeholderImage占位图片 3，progress 下载进度 用法： NSLog(@"下载进步：%f",(double)receivedSize / expectedSize); 4, *image *error *imageURL分别完成后返回的图片，错误和下载地址 5，SDImageCacheType cacheType 是枚举类型，图片存储位置在内存、磁盘或无 6，SDWebImageOptions 枚举类型 用法：SDWebImageOptions options = SDWebImageRetryFailed | SDWebImageLowPriority SDWebImageRetryFailed 下载失败重复下载 常用 SDWebImageLowPriority 当UI交互的时候暂停下载 常用 SDWebImageCacheMemoryOnly 只存图片在内存 SDWebImageProgressiveDownload 可以像浏览器那样从上往下下载刷新图片 SDWebImageRefreshCached 刷新缓存 SDWebImageHighPriority 高优先级 SDWebImageDelayPlaceholder 不加载占位图
//示例tableview的cell: [cell.imageView sd_setImageWithURL:(NSURL *)placeholderImage:(UIImage *)options:(SDWebImageOptions)progress:^(NSInteger receivedSize, NSInteger expectedSize) { //'receivedSize'已经接收了多少数据大小
//'expectedSize'服务器上文件大小
} completed:^(UIImage *image,NSError *error,SDImageCacheType cacheType,NSURL *imageURL) {
// 'image'下载完成后自动转成的image图片
// 'error'返回错误
// 'cacheType'缓存类型
// 'imageURL'
}];
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：
1 #import "SDImageCache.h" 2 然后就可以进行相关的操作了。让我们直接看看SDImageCache.h文件： 3 4 5 @property (assign, nonatomic) NSUInteger maxMemoryCost; 6 7 /** 8 * The maximum length of time to keep an image in the cache, in seconds 9 */ 10 @property (assign, nonatomic) NSInteger maxCacheAge; 11 12 /** 13 * The maximum size of the cache, in bytes. 14 */ 15 @property (assign, nonatomic) NSUInteger maxCacheSize; 16 17 /** 18 * Returns global shared cache instance 19 * 20 * @return SDImageCache global instance 21 */ 22 + (SDImageCache *)sharedImageCache; 23 24 /** 25 * Init a new cache store with a specific namespace 26 * 27 * @param ns The namespace to use for this cache store 28 */ 29 - (id)initWithNamespace:(NSString *)ns; 30 31 /** 32 * Add a read-only cache path to search for images pre-cached by SDImageCache 33 * Useful if you want to bundle pre-loaded images with your app 34 * 35 * @param path The path to use for this read-only cache path 36 */ 37 - (void)addReadOnlyCachePath:(NSString *)path; 38 39 /** 40 * Store an image into memory and disk cache at the given key. 41 * 42 * @param image The image to store 43 * @param key The unique image cache key, usually it's image absolute URL 44 */ 45 - (void)storeImage:(UIImage *)image forKey:(NSString *)key; 46 47 /** 48 * Store an image into memory and optionally disk cache at the given key. 49 * 50 * @param image The image to store 51 * @param key The unique image cache key, usually it's image absolute URL 52 * @param toDisk Store the image to disk cache if YES 53 */ 54 - (void)storeImage:(UIImage *)image forKey:(NSString *)key toDisk:(BOOL)toDisk; 55 56 /** 57 * Store an image into memory and optionally disk cache at the given key. 58 * 59 * @param image The image to store 60 * @param recalculate BOOL indicates if imageData can be used or a new data should be constructed from the UIImage 61 * @param imageData The image data as returned by the server, this representation will be used for disk storage 62 * instead of converting the given image object into a storable/compressed image format in order 63 * to save quality and CPU 64 * @param key The unique image cache key, usually it's image absolute URL 65 * @param toDisk Store the image to disk cache if YES 66 */ 67 - (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk; 68 69 /** 70 * Query the disk cache asynchronously. 71 * 72 * @param key The unique key used to store the wanted image 73 */ 74 - (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock; 75 76 /** 77 * Query the memory cache synchronously. 78 * 79 * @param key The unique key used to store the wanted image 80 */ 81 - (UIImage *)imageFromMemoryCacheForKey:(NSString *)key; 82 83 /** 84 * Query the disk cache synchronously after checking the memory cache. 85 * 86 * @param key The unique key used to store the wanted image 87 */ 88 - (UIImage *)imageFromDiskCacheForKey:(NSString *)key; 89 90 /** 91 * Remove the image from memory and disk cache synchronously 92 * 93 * @param key The unique image cache key 94 */ 95 - (void)removeImageForKey:(NSString *)key; 96 97 98 /** 99 * Remove the image from memory and disk cache synchronously 100 * 101 * @param key The unique image cache key 102 * @param completionBlock An block that should be executed after the image has been removed (optional) 103 */ 104 - (void)removeImageForKey:(NSString *)key withCompletition:(void (^)())completion; 105 106 /** 107 * Remove the image from memory and optionally disk cache synchronously 108 * 109 * @param key The unique image cache key 110 * @param fromDisk Also remove cache entry from disk if YES 111 */ 112 - (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk; 113 114 /** 115 * Remove the image from memory and optionally disk cache synchronously 116 * 117 * @param key The unique image cache key 118 * @param fromDisk Also remove cache entry from disk if YES 119 * @param completionBlock An block that should be executed after the image has been removed (optional) 120 */ 121 - (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk withCompletition:(void (^)())completion; 122 123 /** 124 * Clear all memory cached images 125 */ 126 - (void)clearMemory; 127 128 /** 129 * Clear all disk cached images. Non-blocking method - returns immediately. 130 * @param completionBlock An block that should be executed after cache expiration completes (optional) 131 */ 132 - (void)clearDiskOnCompletion:(void (^)())completion; 133 134 /** 135 * Clear all disk cached images 136 * @see clearDiskOnCompletion: 137 */ 138 - (void)clearDisk; 139 140 /** 141 * Remove all expired cached image from disk. Non-blocking method - returns immediately. 142 * @param completionBlock An block that should be executed after cache expiration completes (optional) 143 */ 144 - (void)cleanDiskWithCompletionBlock:(void (^)())completionBlock; 145 146 /** 147 * Remove all expired cached image from disk 148 * @see cleanDiskWithCompletionBlock: 149 */ 150 - (void)cleanDisk; 151 152 /** 153 * Get the size used by the disk cache 154 */ 155 - (NSUInteger)getSize; 156 157 /** 158 * Get the number of images in the disk cache 159 */ 160 - (NSUInteger)getDiskCount; 161 162 /** 163 * Asynchronously calculate the disk cache's size. 164 */ 165 - (void)calculateSizeWithCompletionBlock:(void (^)(NSUInteger fileCount, NSUInteger totalSize))completionBlock; 166 167 /** 168 * Check if image exists in cache already 169 */ 170 - (BOOL)diskImageExistsWithKey:(NSString *)key; 171 172 /** 173 * Get the cache path for a certain key (needs the cache path root folder) 174 * 175 * @param key the key (can be obtained from url using cacheKeyForURL) 176 * @param path the cach path root folder 177 * 178 * @return the cache path 179 */ 180 - (NSString *)cachePathForKey:(NSString *)key inPath:(NSString *)path; 181 182 /** 183 * Get the default cache path for a certain key 184 * 185 * @param key the key (can be obtained from url using cacheKeyForURL) 186 * 187 * @return the default cache path 188 */ 189 - (NSString *)defaultCachePathForKey:(NSString *)key;
不要看着想吐就行。先看看使用吧。
存图:
SDImageCache *imageCache = [SDImageCache sharedImageCache]; [imageCache storeImage:image forKey:@"myphoto" toDisk:YES];
取图:
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。
场景二、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：
1 - (void)clearCaches 2 {
//使用了'MBProgressHUD'提示框框架
3 [MBProgressHUD showMessage:@"正在清理.."];
4
5 [[SDImageCache sharedImageCache] clearMemory];
6 [[SDImageCache sharedImageCache] clearDisk];
7
8 [self performSelectorOnMainThread:@selector(cleanCacheSuccess) withObject:nil waitUntilDone:YES];
9 }
10 - (void)cleanCacheSuccess
11
12 {
13
14 dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)2 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
15
16 self.cacheLab.text = @"0.00M";
17
18 return;
19
20 });
21
22 }
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。
- (void)sd_setImageWithPreviousCachedImageWithURL:(NSURL *)url andPlaceholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
五、其他
- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application { SDWebImageManager *mrg = [SDWebImageManager sharedManager]; //1,取消下载操作 [mrg cancelAll]; //2,清除内存缓存 [mrg.imageCache clearMemory]; }
3其他功能 1,设置定期清理缓存时间 //设置100天，默认是7天 [SDWebImageManager sharedManager].imageCache.maxCacheAge = 100 * 24 * 60 * 60 2，设置最大缓存容量 //无默认值，单位目前不清楚 [SDWebImageManager sharedManager].imageCache.maxCacheSize = ;
未完待续。