Run timer in ‘background’ when app is closed

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The correct approach is to maintain the illusion that the timer is running, without actually keeping it running. There are two aspects to this:

  1. Before the app leaves the foreground, it should save information about the timer (e.g. either the time the timer started if you’re counting up or the time the timer is supposed to stop if you’re counting down). You can use a variety of different technologies to save this information (e.g. a plist, keyed archiver, Core Data, etc.) in persistent storage.

    Then when the app starts up again, check the saved time and reconstruct the UI to start the timer again from the appropriate state.

  2. If you’re counting down to some time (i.e. you want to let the user know when the particular point in time has been reached, even if the app isn’t running), you could use a local notification. See the Local and Remote Notification Programming Guide, focusing on the local notifications.

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