imgcache.js

The purpose of this JS library is to provide a nice interface for locally storing images for offline apps using PhoneGap/Cordova or browsers supporting the new html5 File API (e.g. Chrome).

This library is especially useful for mobile web applications using Phonegap/Cordova where the normal browser cache cannot be relied upon and where offline navigation is quite common.

Used with imagesloaded as shown in examples/example2.html, you can see that it can automatically:

store images in cache
replace images with cached version if they fail to load (offline / busy server..)

This is the best solution I have found so far to provide easy caching of images within a phonegap web app.

This library works with Phonegap/Cordova (v >= 1.7) so the supported platforms should be:

Android [TESTED]
iOS [TESTED]
BlackBerry WebWorks (OS 5.0 and higher)
Windows Phone 7 ( Mango )

All methods are asynchronous : use callbacks if required.

Using imgcache.js

Requirements

jQuery (any version from 1.6 should do) or Zepto
Phonegap/Cordova optional (v >= v1.7)
[imagesloaded] (http://desandro.github.com/imagesloaded/) optional

Installation

To use imgcache.js, you need to copy js/imgcache.js and add it to your application's Javascript. You can then load it like so:

<script src="imgcache.js" type="application/javascript"></script>

Using with PhoneGap/Cordova:

Requires the File API and Device plugins in config.xml
Remember to allow access to remote files by adding your domain in config.xml - or all domains using a wildcard: <access origin="*" />

Using with PhoneGap/Cordova (v >= 3.0):

Check Issue #15 for more information

Using with Chrome or other browsers that support the [html5 filesystem API]:

Beware of cross domain ajax issue! retrieve image from the same domain or set CORS solutions with the server...
If the page is opened locally (file:// ..), Chrome needs to be loaded with the following flags: --allow-file-access-from-files --allow-file-access otherwise the local filesystem will not be accessible (security error)
To navigate through the local filesystem open a new tab with filesystem:http://yourSiteDomain/persistent/ or filesystem:http://yourSiteDomain/temporary/

Setup your cache

Before initializing the cache, you must specify any default options you wish to configure:

// write log to console
ImgCache.options.debug = true;

// increase allocated space on Chrome to 50MB, default was 10MB
ImgCache.options.chromeQuota = 50*1024*1024;

See ImgCache.options at the top of the source file for more settings.

After setting any custom configuration, initialize the cache:

ImgCache.init(function(){
  alert('cache created successfully!');
}, function(){
  alert('check the log for errors');
});

If the cache successfully initializes, ImgCache.ready will be set to true.

If you're using imgcache.js with PhoneGap/Cordova, ImgCache.init() must be called in onDeviceReady, not before!

Note that in Chrome, the user will be prompted to give permission to the page for accessing the local filesystem (which will run the error callback if they decline).

Storing images

Images are stored into the local folder specified by ImgCache.options.localCacheFolder. To add a file to the cache:

ImgCache.cacheFile('http://my-cdn.com/users/2/profile.jpg');

TODO: add documentation for ImgCache.cacheBackground

Using cached images

Once an image is stored in the cache, you can replace the file displayed by the image tag:

target = $('img#profile');
ImgCache.cacheFile(target.attr('src'), function(){
  ImgCache.useCachedFile(target, function(){
    alert('now using local copy');
  }, function(){
    alert('could not load from cache');
  })
});

To check if a file is stored locally:

ImgCache.isCached(target.attr('src'), function(path, success){
  if(success){
    // already cached
    ImgCache.useCachedFile(target);
  } else {
    // not there, need to cache the image
    ImgCache.cacheFile(target.attr('src'), function(){
      ImgCache.useCachedFile(target);
    });
  }
});

When you no longer want to use the locally cached file:

target = $('img#profile');
ImgCache.useOnlineFile(target);

Clearing the cache

To remove all cached files, clear the local cache folder:

ImgCache.clearCache(function(){
  // continue cleanup...
}, function(){
  // something went wrong
});

There is currently no way to invalidate single images from the cache.

High level API

ImgCache.init() initialises the local cache
ImgCache.cacheFile() writes a copy of a file into the local cache
ImgCache.isCached() checks if a the given image exists in the cache - does not check if the latest version of that file is cached
ImgCache.useCachedFile() replaces the img src with the cached version
ImgCache.useCachedFileWithSource() similar to useCachedFile but with the image source url as extra parameter
ImgCache.useOnlineFile() replaces back the img src with the original (online) version
ImgCache.clearCache() clears the local cache folder
ImgCache.cacheBackground() caches the background image of an element
ImgCache.useCachedBackground() replaces the background image source of the given element with the cached version

Options

See ImgCache.options at the top of the source file for the list of options. Options can be overriden from your own script, no need to modify the library!

Code samples

See html files in the examples/ folder.

Release Notes

See CHANGELOG.md for recent updates.

License

Apache License - see LICENSE.md

Code from http://code.google.com/p/tiny-sha1/ is being used which is under the MIT License. The copyright for this part belongs to the creator of this work.

Todo

Find a solution for cache invalidation in case an image changes
When Chrome finally supports canvas.toBlob(), possibly replace download method with new one that draws an Image into a canvas and then retrieves its content using the toBlob() method -- or use [canvas-toBlob.js] (https://github.com/eligrey/canvas-toBlob.js)

airamrguez/imgcache.js