Does the iOS let developers do this?

Let's say the user leaves the phone on the table. (probably on home screen)

A few hours later, he p开发者_如何学Goicks it up. And when it does, the iPhone detects the accelerometer, and it rings.

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Unfortunately, this is not possible. Once the OS goes to the home screen, applications running in the background have a limited set of options.

Read this article for more information: http://developer.apple.com/library/ios/#DOCUMENTATION/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html

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Not through officially accepted means (i.e. you could do it on a jailbroken phone conceivably). Primarily, you can't run in the background for that purpose. If the app was running in the foreground, you can easily detect motion and perform an activity - but if the phone is left for too long a period without activity, it will sleep.

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Unfortunately, the accelerometer is not one of the specified keys in the UIBackgroundModes option in your apps info.plist.

auido, location and voip are the only ones available for now.

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short answer: YES

The trick is to make sure the app doesn't get backgrounded when the phone goes to sleep. The other elements like detecting changes in the accelerometer and playing a sound are all standard features.

As for the no sleep solution, it has been asked before, basically is you play a music sample the app will continue running, so perhaps a loop of silence continuously played.

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Only if you left an app designed for that purpose running in the foreground.

It won't be possible if your app is running in the background.