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Re: Persisting ContentCache to disk between sessions

Hi Alex:

The icons and logos are loaded from a server on startup, and then kept up to date via a 20 second polling pattern where the app checks if anything has changed on the server. We timestamp all images and when one does change we get latest and update the image in the app.

The way a ContentCache is used for BitmapImage and Image is to set the loader like this:

public static var imageCache:ImageCache = new ImageCache(); // subclass of ContentCache
_image.contentLoader = Model.imageCache;
_image.source = "https/someImageUrl"

When the image is first retrieved from the URL, the ContentCache loader will save the image into a Dictionary (key/value pairs) where the key is the URL and the value is the bitmapData of the image.

Then when using ItemRenderers, and they get recycled, the when the image URL changes, ContentCache will look up the URL in it's Dictionary and if it's already been downloaded it will simply display it without having to make an HTTP request. 

ContentCache is a very cool thing the Adobe engineers designed into Flex/Flash and it makes lists with icons and pictures far more responsive.

What I want to do is persist the cache to disk and load it when the user logs in again. If any image have changed since the last time they launched the app, the first 20 second poll will indicate a timestamp has changed and the app will retrieve images with a timestamp after the one returned when polling. So they will always be up to date.

I said the images don't change but that's not actually true, they just don't change often, maybe once a week we'll onboard new organizations and events with images. 

Embedding images is not an option because every new onboarded entity will require a new app build, full regression testing, submission to Apple for review and the entire release process. That's a bad business model.

Anyway, there must be a way in FileStream to save a Dictionary with key as String and value as ByteArray, but I haven't figured it out yet. Was hoping someone else knows how.

Thanks, Alex.


PS: I know how I can do this, but it's too much work the way I know how, I would have to take each cached image, use pngencoder to encode the bitmapData (ByteArray) to a PNG and save each image as a separate file. Last count our image cache has about 1500 images so that means 1500 files, plus a hash file to match the image URL to a generated filename for the image. Loading will then require reversing the process. If you've used image encoders in Flash/AIR you know they're  pretty slow. If we don't gain any speed, it's not worth the effort.

On Oct 26, 2018, at 3:44 PM, Alex Harui <aharui@xxxxxxxxx.INVALID> wrote:


How do the icons and logos get into the cache in the first place?  If they don't change, is there any reason not to embed them in the application?


On 10/26/18, 3:36 PM, "Erik J. Thomas" <erik@xxxxxxxxxx> wrote:

   Hey all:

   Have any of you come across an example of persisting a ContentCache to disk and reading it back on app launch? We have lots of icons and logos for hundreds of companies and they don't change between sessions. 

   Like this (doesn't work though). ImageCache is a subclass of ContentCache to get access to the protected cachedData:Dictionary which is key/value pairs, with key as URL to image, and value is bitmapData.

   public function saveImageCache(imageCache:ImageCache):void {
      var cacheFile:File = File.applicationStorageDirectory;
      cacheFile = cacheFile.resolvePath(IMAGE_CACHE_FILE_NAME);
      if (cacheFile.exists) {
      var fileStream:FileStream = new FileStream();, FileMode.WRITE);
      fileStream.writeObject(imageCache.getEntries()); // this is a Dictionary with byte array for image as value

   The writeObject API of FileStream does not marshal the bitmapData to disk. 

    * Loads a persisted image cache from disk.
    * @return ContentCache
   public function loadImageCache():ImageCache {
      var cacheFile:File = File.applicationStorageDirectory;
      cacheFile = cacheFile.resolvePath(IMAGE_CACHE_FILE_NAME);
      if (cacheFile.exists) {
         var fileStream:FileStream = new FileStream();, FileMode.READ);
         var entries:Dictionary = fileStream.readObject() as Dictionary;
         var imageCache:ImageCache = new ImageCache();
         return imageCache;
      return null;

   The entries variable does populate with all the keys as URLs, but the values are null. FileStream won't read just raw binary data in this way.

   I don't want to have to save every image using an encoder into separate files and then load them all back if I can help it. 

   Just seems FileStream should be able to just write a blob of binary data to disk and retrieve it "as-is" but it doesn't or I can't find the way.

   Thanks for your suggestions.