Android凌空图像缓存问题

Question

in the google's own volley image cache tutorial

// Returns a cache size equal to approximately three screens worth of images.
public static int getCacheSize(Context ctx) {
    final DisplayMetrics displayMetrics = ctx.getResources().
            getDisplayMetrics();
    final int screenWidth = displayMetrics.widthPixels;
    final int screenHeight = displayMetrics.heightPixels;
    // 4 bytes per pixel
    final int screenBytes = screenWidth * screenHeight * 4;

    return screenBytes * 3;
}

the recommended cache is three screens worth of images which equals to 7mb. I have an social media app and there is a newsfeed inside it.

1-) My first question is what will happen after the cache is full?

2-) I am thinking about removing cache every one hour and thus the cache will include the newer content. Is that reasonable ? What is the image caching logic behind the apps which includes something like newsfeed(for example, instagram)?

3-) How can i remove the old cache of specific item and force it to download it again? I tried this solution but it did not work:

VolleySingleton.getInstance().getRequestQueue().getCache().remove(IMAGE_URL);

mNetworkImageView = (NetworkImageView) getView().findViewById(R.id.networkImageView);
mImageLoader = VolleySingleton.getInstance().getImageLoader();
mNetworkImageView.setImageUrl(IMAGE_URL, mImageLoader);

There are a lots of clone question of my third question but none of them has been answered.

Thanks for your helps. :)

Answer 1

1.) There are 2 layers of cache in Volley, one is the in-memory cache (in RAM) and the other one is a disk cache. Once a cache is full, the oldest image (meaning the image that hasn't been accessed the longest) in that cache will be evicted when a new image is about to be cached to make room for the new items. When something is evicted from the in-memory cache, it is still present in the disk cache and can be loaded very quickly from disk if it is needed again. If an image is evicted from the disk cache, it would have to be redownloaded if it's needed again.

2.) This doesn't sound reasonable once you understood the answer to question 1. The cache automatically makes room for newer content and there is no reason to evict content manually. Manual eviction will in fact lower your cache's efficiency.

3.) Broadly speaking, this is not possible (without hacks), because it should not be needed. If an image resource (almost) always expires after a certain time, the server should announce this using HTTP headers when sending the resource to the client. For example using the max-age property of the cache-control header. There are lots of websites explaining this in detail, for example: http://www.mobify.com/blog/beginners-guide-to-http-cache-headers/ . If an image resource almost never expires, you can consider changing its filename upon change and store that filename as a property. For example a user can have an avatar property containing the URL to the avatar. The avatar image can be cached indefinitely and you change the URL of the image if a new avatar gets uploaded.

Answer 2

For you 3rd question, I suggest that you read the following Google's documentation:

Request an Image

ImageRequest —a canned request for getting an image at a given URL and calling back with a decoded bitmap. It also provides convenience features like specifying a size to resize to. Its main benefit is that Volley's thread scheduling ensures that expensive image operations (decoding, resizing) automatically happen on a worker thread.

So if you use a ImageRequest only, you can refer to my answer at the following question:

Tell Volley not to use cached data but to initiate a new request?

Also in the Google's documentation:

ImageLoader —a helper class that handles loading and caching images from remote URLs. ImageLoader is a an orchestrator for large numbers of ImageRequests, for example when putting multiple thumbnails in a ListView. ImageLoader provides an in-memory cache to sit in front of the normal Volley cache ,...

If you use NetworkImageView, you can refer to my answer at the following question:

Disable or Remove Cache in NetworkImageView- Volley

In which you will find that I use the following code inside VolleySingleton class:

    mImageLoader = new ImageLoader(mRequestQueue, new ImageLoader.ImageCache() {
        @Override
        public Bitmap getBitmap(String url) {
            return null;
        }

        @Override
        public void putBitmap(String url, Bitmap bitmap) {
        }
    });

Hope it helps!