SDWebImage下载图片有时候无法成功显示出来

之前用下面的方法现在图片,有时候会出现图片没有下载成功显示:

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;

后来研究下,还有下面的方法:

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options;

重点就在这个SDWebImageOptions上,文档提示如下:

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.

* This flag disable this blacklisting.

*/

SDWebImageRetryFailed = 1 << 0,

/**

* By default, image downloads are started during UI interactions, this flags disable this feature,

* leading to delayed download on UIScrollView deceleration for instance.

*/

SDWebImageLowPriority = 1 << 1,

/**

* This flag disables on-disk caching

*/

SDWebImageCacheMemoryOnly = 1 << 2,

/**

* This flag enables progressive download, the image is displayed progressively during download as a browser would do.

* By default, the image is only displayed once completely downloaded.

*/

SDWebImageProgressiveDownload = 1 << 3,

/**

* Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.

* The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.

* This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.

* If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.

*

* Use this flag only if you can't make your URLs static with embedded cache busting parameter.

*/

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

* extra time in background to let the request finish. If the background task expires the operation will be cancelled.

*/

SDWebImageContinueInBackground = 1 << 5,

/**

* Handles cookies stored in NSHTTPCookieStore by setting

* NSMutableURLRequest.HTTPShouldHandleCookies = YES;

*/

SDWebImageHandleCookies = 1 << 6,

/**

* Enable to allow untrusted SSL certificates.

* Useful for testing purposes. Use with caution in production.

*/

SDWebImageAllowInvalidSSLCertificates = 1 << 7,

/**

* By default, image are loaded in the order they were queued. This flag move them to

* the front of the queue and is loaded immediately instead of waiting for the current queue to be loaded (which

* could take a while).

*/

SDWebImageHighPriority = 1 << 8,

/**

* By default, placeholder images are loaded while the image is loading. This flag will delay the loading

* 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,

* as most transformation code would mangle it.

* Use this flag to transform them anyway.

*/

SDWebImageTransformAnimatedImage = 1 << 10,

/**

* By default, image is added to the imageView after download. But in some cases, we want to

* have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)

* Use this flag if you want to manually set the image in the completion when success

*/

SDWebImageAvoidAutoSetImage = 1 << 11

};

这个文档解释的很详细,然后将options设置为SDWebImageRetryFailed(默认下载图片失败后就不会再次下载了,该选项会让下载失败的重新下载) ,解决了问题.还有其他很多选项,在相应的场合都会有用的.