ipad sleep wake up event

When the ipad goes to sleep i.e. the screen turns off, the user will turn it back on and the current application at the time is still open. Is there a way to detect this event that the ipad has woken up from sleep?

I want to be show an UIAlert whenever it goes to sleep an开发者_运维百科d wakes up.

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Take a look at the - (void)applicationDidBecomeActive:(UIApplication *)application delegate method of the application delegate.