Use the appCache event listeners
function handleCacheEvent(e) {
// Do your thing
}
function handleCacheError(e) {
alert('Error: Cache failed to update!');
};
// Fired after the first cache of the manifest.
appCache.addEventListener('cached', handleCacheEvent, false);
// Checking for an update. Always the first event fired in the sequence.
appCache.addEventListener('checking', handleCacheEvent, false);
// An update was found. The browser is fetching resources.
appCache.addEventListener('downloading', handleCacheEvent, false);
// The manifest returns 404 or 410, the download failed,
// or the manifest changed while the download was in progress.
appCache.addEventListener('error', handleCacheError, false);
// Fired after the first download of the manifest.
appCache.addEventListener('noupdate', handleCacheEvent, false);
// Fired if the manifest file returns a 404 or 410.
// This results in the application cache being deleted.
appCache.addEventListener('obsolete', handleCacheEvent, false);
// Fired for each resource listed in the manifest as it is being fetched.
appCache.addEventListener('progress', handleCacheEvent, false);
// Fired when the manifest resources have been newly redownloaded.
appCache.addEventListener('updateready', handleCacheEvent, false);