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简介
在iOS的图片加载框架中，SDWebImage可谓是占据大半壁江山。它支持从网络中下载且缓存图片，并设置图片到对应的UIImageView控件或者UIButton控件。在项目中使用SDWebImage来管理图片加载相关操作可以极大地提高开发效率，让我们更加专注于业务逻辑实现。
SDWebImage核心方法
//SDWebImage
/**
@param url 图像的URL
@param placeholder 占位图
@param options 选项（枚举）
@param progressBlock 进度块代码
@param completedBlock 完成的块代码
*/
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownLoaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlocak)completedBlock;| receivedSize | 已经接收到的大小 |
| expectdSize | 期望的大小，总大小 |
进度条：
progress:^(NSInteger receivedSize,NSInteger expectdSize){
float progress = (float)receivedSize/expectdSize;
NSLog(@"下载进度 %f",progress);
}
options: SDWebImageRetryFailed | SDWebImageLowPriority //下载失败，重新下载
SDWebImageRetryFailed//低优先级。UI交互的时候，暂停下载图片。在滚动的时候，延迟下载
SDWebImageLowPriority//仅在内存中缓存，不在沙河里面缓存
SDWebImageCacheMemoryOnly//即使图像被缓存，遵守 HTPP 响应的缓存控制，如果需要，从远程刷新图像;仅在无法使用嵌入式缓存清理参数确定图像 URL 时，使用此标记
SDWebImageProgressiveDownload| SDWebImageManager | SDWebImage核心类 |
| SDWebImageDownloader | 图片下载类，管理图像下载操作 |
| SDImageCache | 图片缓存类，支持内存缓存和磁盘缓存 |
| SDWebImageDownloaderOperation | 图像下载操作，处理一个单一的图像下载 |
内存处理：当APP接收到内存警告时，SDWebImage做了什么？
SDWebImage会监听系统的UIApplicationDidReceiceMemoryWarningNotification通知，一旦收到通知，就会清理内存
| UIApplicationWillterminateNotification | 应用程序将要终止的通知；接收到该通知，清理磁盘 |
| UIApplicationDidEnterBackgroundNotification | 应用程序进入后台的通知；也会清理磁盘 |
SDWebImage.h
typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
/**
* By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
* 默认情况下，当一个 URL 下载失败，会将该 URL 放在黑名单中，不再尝试下载
*
* This flag disable this blacklisting.
* 此标记取消黑名单
*/
SDWebImageRetryFailed = 1 << 0,
/**
* By default, image downloads are started during UI interactions, this flags disable this feature,
* 默认情况下，在 UI 交互时也会启动图像下载，此标记取消这一特性
*
* leading to delayed download on UIScrollView deceleration for instance.
* 会推迟到滚动视图停止滚动之后再继续下载(这个注释是假的！)
*/
SDWebImageLowPriority = 1 << 1,
/**
* This flag disables on-disk caching
* 此标记取消磁盘缓存，只有内存缓存
*/
SDWebImageCacheMemoryOnly = 1 << 2,
/**
* This flag enables progressive download, the image is displayed progressively during download as a browser would do.
* 此标记允许渐进式下载，就像浏览器中那样，下载过程中，图像会逐步显示出来
*
* By default, the image is only displayed once completely downloaded.
* 默认情况下，图像会在下载完成后一次性显示
*/
SDWebImageProgressiveDownload = 1 << 3,
/**
* Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
* 即使图像被缓存，遵守 HTPP 响应的缓存控制，如果需要，从远程刷新图像
*
* The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
* 磁盘缓存将由 NSURLCache 处理，而不是 SDWebImage，这会对性能有轻微的影响
*
* This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
* 此选项有助于处理同一个请求 URL 的图像发生变化
*
* If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
* 如果缓存的图像被刷新，会调用一次 completion block，并传递最终的图像
*
* Use this flag only if you can't make your URLs static with embeded cache busting parameter.
* 仅在无法使用嵌入式缓存清理参数确定图像 URL 时，使用此标记
*/
SDWebImageRefreshCached = 1 << 4,
/**
* In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
* 在 iOS 4+，当 App 进入后台后仍然会继续下载图像。这是向系统请求额外的后台时间以保证下载请求完成的
*
* extra time in background to let the request finish. If the background task expires the operation will be cancelled.
* 如果后台任务过期，请求将会被取消
*/
SDWebImageContinueInBackground = 1 << 5,
/**
* Handles cookies stored in NSHTTPCookieStore by setting
* 通过设置
* NSMutableURLRequest.HTTPShouldHandleCookies = YES;
* 处理保存在 NSHTTPCookieStore 中的 cookies
*/
SDWebImageHandleCookies = 1 << 6,
/**
* Enable to allow untrusted SSL ceriticates.
* 允许不信任的 SSL 证书
*
* Useful for testing purposes. Use with caution in production.
* 可以出于测试目的使用，在正式产品中慎用
*/
SDWebImageAllowInvalidSSLCertificates = 1 << 7,
/**
* By default, image are loaded in the order they were queued. This flag move them to
* 默认情况下，图像会按照在队列中的顺序被加载，此标记会将它们移动到队列前部立即被加载
*
* the front of the queue and is loaded immediately instead of waiting for the current queue to be loaded (which
* 而不是等待当前队列被加载，等待队列加载会需要一段时间
* could take a while).
*/
SDWebImageHighPriority = 1 << 8,
/**
* By default, placeholder images are loaded while the image is loading. This flag will delay the loading
* 默认情况下，在加载图像时，占位图像已经会被加载。而此标记会延迟加载占位图像，直到图像已经完成加载
*
* of the placeholder image until after the image has finished loading.
*/
SDWebImageDelayPlaceholder = 1 << 9,
/**
* We usually don't call transformDownloadedImage delegate method on animated images,
* 通常不会在可动画的图像上调用 transformDownloadedImage 代理方法，因为大多数转换代码会破坏动画文件
*
* as most transformation code would mangle it.
* Use this flag to transform them anyway.
* 使用此标记尝试转换
*/
SDWebImageTransformAnimatedImage = 1 << 10,
};