转载：https://www.cnblogs.com/xjy-123/p/5205339.html
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：
#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;
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：
- (void)clearCaches
{
//使用了'MBProgressHUD'提示框框架
[MBProgressHUD showMessage:@"正在清理.."];
[[SDImageCache sharedImageCache] clearMemory];
[[SDImageCache sharedImageCache] clearDisk];
[self performSelectorOnMainThread:@selector(cleanCacheSuccess) withObject:nil waitUntilDone:YES];
}
- (void)cleanCacheSuccess
12 {
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)2 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
self.cacheLab.text = @"0.00M";
return;
20 });
}
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 = ;