Monday, September 13, 2010

Palm Shaking

This is a small point, but I could find no one else who has ever commented on this issue before: minor filtering with Palm shake events via the Accelerometer.

It's pretty easy to setup an event listener for the "shaking" event, just follow the instructions here. However there are two partially tricky points to getting it right.

First, you don't want to listen to every shake event. When listening to the "shaking" event, there's a parameter event.magnitude that you should use to filter. If you listen to all shaking events, then it's easy for your app to pick up shakes that are unintentional, such as someone putting their phone on a table or back in their pocket. I've found that a good number to pick is 2.08 - the reason for this being that the emulator, when you press "f5", simulates a shake event of magnitude ~2.09. Set your filter any higher and you won't be able to test with the emulator, but with a magnitude of at least 2, the user's shake has to be relatively intentional.

The second catch is that the shake event, unlike most events you might listen to in a scene, can happen in any scene on the stack. With buttons and the like, you can be lazy and not disable their listeners when you add another scene to the stack because those buttons are not accessible. You cannot be so lazy with the "shaking" event; even if you push another scene, your previous event listener will still fire off whenever the phone shakes. So make sure to disable any "shaking" handlers before you launch a new scene.

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