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[操作系统]SDWebImage的简单使用


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。