SDWebImage 简介
1.提供了一个UIImageView的category用来加载网络图片并且对网络图片的缓存进行管理
2.采用异步方式来下载网络图片
3.采用异步方式，使用memory＋disk来缓存网络图片，自动管理缓存
4.支持GIF动画
5.支持WebP格式
6.同一个URL的网络图片不会被重复下载
7.失效的URL不会被无限重试
8.耗时操作都在子线程，确保不会阻塞主线程
9.使用GCD和ARC
10.支持Arm64
SDWebImage 使用
1.使用IImageView+WebCache category来加载一张远程图片
let url = URL(string: "http://f.hiphotos.baidu.com/image/h%3D200/sign=1e6c01ac9b13b07ea2bd57083cd69113/5fdf8db1cb134954ecbd6ee95e4e9258d0094a98.jpg")
imageV.sd_setImage(with: url)
2.加载一张远程图片,在未完成加载时预留一张图片
let placeholderImage = UIImage(named: "placeholderImage.png")
imageV.sd_setImage(with: url, placeholderImage: placeholderImage)
3.加载一张远程图片,加载完成是执行某些操作
imageV.sd_setImage(with: url) { (image, error, SDImageCacheTypeNone, url) in
print("OK")
}
SDWebImage 流程
下载选项
typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
SDWebImageDownloaderLowPriority = 1 << 0,
SDWebImageDownloaderProgressiveDownload = 1 << 1,
/**
* By default, request prevent the use of NSURLCache. With this flag, NSURLCache
* is used with default policies.
*/
SDWebImageDownloaderUseNSURLCache = 1 << 2,
/**
* Call completion block with nil image/imageData if the image was read from NSURLCache
* (to be combined with `SDWebImageDownloaderUseNSURLCache`).
*/
SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
/**
* In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
* extra time in background to let the request finish. If the background task expires the operation will be cancelled.
*/
SDWebImageDownloaderContinueInBackground = 1 << 4,
/**
* Handles cookies stored in NSHTTPCookieStore by setting
* NSMutableURLRequest.HTTPShouldHandleCookies = YES;
*/
SDWebImageDownloaderHandleCookies = 1 << 5,
/**
* Enable to allow untrusted SSL certificates.
* Useful for testing purposes. Use with caution in production.
*/
SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,
/**
* Put the image in the high priority queue.
*/
SDWebImageDownloaderHighPriority = 1 << 7,
};
可以看出，这些选项主要涉及到下载的优先级、缓存、后台任务执行、cookie处理以认证几个方面。
缓存
为了减少网络流量的消耗，我们都希望下载下来的图片缓存到本地，下次再去获取同一张图片时，可以直接从本地获取，而不再从远程服务器获取。这样做的另一个好处是提升了用户体验，用户第二次查看同一幅图片时，能快速从本地获取图片直接呈现给用户。
SDWebImage提供了对图片缓存的支持，而该功能是由SDImageCache类来完成的。该类负责处理内存缓存及一个可选的磁盘缓存。其中磁盘缓存的写操作是异步的，这样就不会对UI操作造成影响。
查询图片
- (UIImage *)imageFromMemoryCacheForKey:(NSString *)key;
- (UIImage *)imageFromDiskCacheForKey:(NSString *)key;
移除图片
- (void)removeImageForKey:(NSString *)key;
- (void)removeImageForKey:(NSString *)key withCompletion:(SDWebImageNoParamsBlock)completion;
- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk;
- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(SDWebImageNoParamsBlock)completion;
清空缓存图片
- (void)clearMemory;
清空磁盘
- (void)clearDisk;
- (void)clearDiskOnCompletion:(SDWebImageNoParamsBlock)completion;
清理磁盘
删除无效过期的图片
- (void)cleanDisk;
- (void)cleanDiskWithCompletionBlock:(SDWebImageNoParamsBlock)completionBlock;
