Glide : How to find if the image is already cached and use the cached version?

Question

Scenario:

I have a large GIF image which I want to cache the first time user opens the app using Glide - Image Loading and Caching library. After that whenever user opens the app, I want to show the cached version if present. This GIF URL will expire after a given interval. When it expires, I fetch the new GIF URL and display/cache that for future use.

What I tried:

I went through Caching and Cache Invalidation on Glide's github page. I also went though the Google Group thread Ensuring That Images Loaded Only Come From Disk Cache, which shows how to get the image form cache. I also went through How to invalidate Glide cache for some specific images question.

From the links above I see the following code sniplet which shows how to load the image from cache. However this only tries to get the image from cache. If its not present in cache, it doesn't try to get from the network and fails:

Glide.with(TheActivity.this)
        .using(new StreamModelLoader<String>() {
            @Override
            public DataFetcher<InputStream> getResourceFetcher(final String model, int i, int i1) {
                return new DataFetcher<InputStream>() {
                    @Override
                    public InputStream loadData(Priority priority) throws Exception {
                        throw new IOException();
                    }

                    @Override
                    public void cleanup() {
                    }

                    @Override
                    public String getId() {
                        return model;
                    }

                    @Override
                    public void cancel() {
                    }
                };
            }
        })
       .load("http://sampleurl.com/sample.gif")
       .diskCacheStrategy(DiskCacheStrategy.SOURCE)
       .into(theImageView);

Questions:

1. Is there a cleaner way to achieve the following: Show the GIF image from the cache if present else download the GIF, cache it for later use and show it in the ImageView.

2. The caching article above mentions the following:

   In practice, the best way to invalidate a cache file is to change your identifier when the content changes (url, uri, file path etc)

   The server sends a different URL to the app when the previous one expires. In this case, I believe the old image will eventually be Garbage Collected? Is there a way to force remove the image from the cache?

3. On a similar note, is there a way to prevent the Garbage Collection of an image with specific key (to prevent downloading the large file again) and then later instruct to delete the old image from cache when the URL changes?

Answer 1

1. You don't need a custom ModelLoader to show the GIF from cache if present and fetch it otherwise, that's actually Glide's default behavior. Just using a standard load line should work fine:

   Glide.with(TheActivity.this)
      .load("http://sampleurl.com/sample.gif")
      .diskCacheStrategy(DiskCacheStrategy.SOURCE)
      .into(theImageView);
   

Your code will prevent Glide from downloading the GIF and will only show the GIF if it is already cached, which it sounds like you don't want.

2. Yes, the old image will eventually be removed. By default Glide uses an LRU cache, so when the cache is full, the least recently used image will be removed. You can easily customize the size of the cache to help this along if you want. See the Configuration wiki page for how to change the cache size.

3. Unfortunately there isn't any way to influence the contents of the cache directly. You cannot either remove an item explicitly, or force one to be kept. In practice with an appropriate disk cache size you usually don't need to worry about doing either. If you display your image often enough, it won't be evicted. If you try to cache additional items and run out of space in the cache, older items will be evicted automatically to make space.