What happens to SDWebImage Cached Images in my app when the image file on the server changes?

Question

I am using the SDWebImage library to cache web images in my app:

https://github.com/rs/SDWebImage/blob/master/README.md

Current Usage:

[imageView setImageWithURL:[NSURL URLWithString:profilePictureUrl] placeholderImage:[UIImage imageNamed:@"placeholder.png"]];

My question is what happens once the image has been cached and then a couple of days later that image file on the server has been updated with a new image?

At the moment my application is still displaying the cached image.

I can't see in any of the documentation on setting a cache timeout or something that recognises that the file size has changed.

If anyone has experience using this particular library then any help would be greatly appreciated.

Thanks in advance.

Answer 1

I had a look at the source code. It processes the setImageWithURL method like this:

1. Ask the memory cache if the image is there, if yes return the image and don't go any further
2. Ask the disk cache if the image is there, if yes return the image and don't go any further
3. Try to download the image, return image on success else keep the placeholder image

There is no request sent to ask the remote server if there is a new version while there is something old on disk, like using ETags of the HTTP protocol.

Digging a bit deeper the cache time is set to a static value in SDImageCache.m

static NSInteger cacheMaxCacheAge = 60*60*24*7; // 1 week

it cannot be changed with a setter.

So as long as the image in the cache is valid the SDWebImage lib won't download anything new. After a week it'll download your changed image.