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Cancelling Periodic Updates

“What are periodic updates” you might ask? Well, you probably know that you can update live tiles in code, from background services and using Push Notification Services. But sometimes, none of these are the right approach.

For a shipping company, Push Notification Services makes sense because each package is tied to a particular user. But a company that offers flight information might have hundreds of users, all with multiple devices, interested in the status of the same flight. For a flight app, push doesn’t make much sense. Especially when you consider that a large number of devices might be turned off when the server tries to deliver the information! Weather applications face the same problem: lots of users interested in the exact same data wanting updates at the same time.

What these companies need is an easy way to publish a live tile to a “central” area, then let devices update after an amount of time has passed or whenever the device wakes up. That’s what periodic updates do.

You can start a periodic update by calling TileUpdater.StartPeriodicUpdate. This method takes a Uri and a recurrence (HalfHour, Hour, etc.) You can also pass a DateTime to start updating if you don’t want to start immediately. When the time elapses (or the device wakes up and the time has already elapsed), Windows will automatically connect and download the Uri. What Windows expects to receive is an XML document that matches the Tile schema.

To stop periodic updates from code, simply call TileUpdater.StopPeriodicUpdate. But wait, Windows performs these updates automatically without the app actually running. What if the user never launches the app? For a weather application that’s perfectly alright, but for a flight tracker the data isn’t really useful after the flight has landed. It doesn’t make sense to pay for bandwidth (or battery) when the data won’t change again.

What we need is the ability for the server to tell Windows to stop polling for updates. Unfortunately there’s no option for that in the tile schema, but it is possible to cancel the subscription by setting an HTTP header in the response. From this answer on Stack Overflow:

The endpoint that you specify for the periodic notification needs to serve the XML content and set the X-WNS-Expires header (and perhaps X-WNS-Tag). If, for example, you’re using Windows Azure blob storage or Amazon S3 to host the template XML and sending that URI to startPeriodicUpdate, then you don’t have the opportunity to set those headers. You’ll need to set up a lightweight service that passes through the XML content and sets the headers appropriately.

[…] That URI will be checked until you call stopPeriodicUpdate; see Periodic Notification Overview:

Polling continues until you explicitly stop it or your app is uninstalled. Otherwise, Windows will continue to poll for updates to your tile or badge even if your app is never launched again.

Lastly, the Guidelines and checklist for periodic notifications does recommend:

Call the StartPeriodicUpdate or StartPeriodicUpdateBatch method each time your app is launched or brought into focus. This ensures that the tile content will be updated each time the user launches or switches to the app.

So there you have it.

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