MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

efore setting up new defaults. As mentioned before, it’s also possible to migrate state from a
background task. (See Chapter 13.)
Generally speaking, local and temp app data are the same—they have the same APIs and are stored
in parallel folders. Temp, however, doesn’t support settings and settings containers. The other difference
is that the contents of your temp folder (along with the HTML5 app cache) are subject to the Windows
Disk Cleanup tool. This means that your temp data could disappear at any time when the user wants to
free up some disk space. You could also employ a background task with a maintenance trigger for
doing cleanup on your own (again see Chapter 13, in the section “Tasks for Maintenance Triggers.”)
For these reasons, temp should be used for storage that optimizes your apps performance but not
for anything that’s critical to its operation. For example, if you have a JSON file in your package that you
parse or decompress on first startup such that the app runs more quickly afterwards, and you don’t
make any changes to that data from the app, you might elect to store that in temp. The same is true for
graphical resources that you might have fine-tuned for the specific device you’re running on; you can
always repeat that process from the original resources, so it’s another good candidate for temp data.
Similarly, if you’ve acquired data from an online service as an optimization (that is, so that you can just
update the local copy incrementally), you can always reacquire it. This is especially helpful for providing
an offline experience for your app, though in some cases you might want to let the user choose to save
it in local instead of temp (an option that would appear in Settings along with the ability to clear the
cache).
Sidebar: HTML5 App Cache
Store apps can employ the HTML 5 app cache as part of an offline/caching strategy. It is most
useful in iframe web context elements where it can be used for any kind of content. For example,
an app that reads online books can show such content in an iframe, and if those pages include
app cache tags, they’ll be saved and available offline. In the local context, the app cache works for
nonexecutable resources like images, audio, and video, but not for HTML or JavaScript.
IndexedDB and Other Database Options
Many forms of local app data are well suited to being managed in a database. In Windows Store apps,
the IndexedDB API is available through the window.indexedDB and worker.indexedDB objects. For
complete details on using this feature, I’ll refer you to the W3C specifications, the Indexed Database API
reference for Store apps, and the IndexedDB sample.
Although an IndexedDB database is stored within your app’s local app data, be aware that there are
some limitations because there isn’t a means through which the app or the system can shrink a database
file and reclaim unused space:
329