SDWebImage的简单使用

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
，sd_setImageWithURL获取网络图片
，placeholderImage占位图片
，progress 下载进度 用法： NSLog(@"下载进步：%f",(double)receivedSize / expectedSize);
, *image *error *imageURL分别完成后返回的图片，错误和下载地址
，SDImageCacheType cacheType 是枚举类型，图片存储位置在内存、磁盘或无
，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;

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：

 - (void)clearCaches
 {
       //使用了'MBProgressHUD'提示框框架
 [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]; 
 } 
 - (void)cleanCacheSuccess 
11
 { 

 dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t) * NSEC_PER_SEC), dispatch_get_main_queue(), ^{ 

 self.cacheLab.text = @"0.00M"; 

 return; 
19
  }); 

 }

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

,设置定期清理缓存时间    

//设置100天，默认是7天
[SDWebImageManager sharedManager].imageCache.maxCacheAge =  *  *  *
，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。