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copied!<p>I know another answer is accepted already, but I came across this working on a similar problem and think I'm going to try an alternate much simpler approach that I figured I may as well document if anyone else wants to try to go down the same path.enter code here</p> <p>The general idea is just to track the system clock time in a SharedPreference whenever any Activity pauses - sounds simple enough, but alas, there's a security hole if that's all you use, since that clock resets on reboot. To work around that:</p> <ul> <li>Have an <code>Application</code> subclass or shared static singleton class with a global unlocked-since-boot state (initially false). This value should live as long as your Application's process.</li> <li>Save the system time (<a href="http://developer.android.com/reference/android/os/SystemClock.html#elapsedRealtime%28%29" rel="noreferrer">realtime</a> since boot) in every relevant <code>Activity</code>'s <code>onPause</code> into a <code>SharedPreference</code> if the current app state is unlocked.</li> <li>If the appwide unlocked-since-boot state is false (clean app start - either the app or the phone restarted), show the lock screen. Otherwise, check the <code>SharedPreference</code>'s value at the lockable activity's onResume; if it's nonexistent or greater than the <code>SharedPreference</code> value + the timeout interval, also show the lock screen.</li> <li>When the app is unlocked, set the appwide unlocked-since-boot state to true.</li> </ul> <p>Besides the timeout, this approach will also automatically lock your app if your app is killed and restarts or if your phone restarts, but I don't think that's an especially bad problem for most apps. It's a little over-safe and may lock unecessarily on users who task switch a lot, but I think it's a worthwhile tradeoff for reduced code and complexity by a total removal of any background process / wakelock concerns (no services, alarms, or receivers necessary). </p> <p>To work around process-killing locking the app regardless of time, instead of sharing an appwide singleton for unlocked-since-boot, you could use a SharedPreference and register a listener for the system boot broadcast intent to set that Preference to false. That re-adds some of the complexity of the initial solution with the benefit being a little more convenience in the case that the app's process is killed while backgrounded within the timeout interval, although for most apps it's probably overkill.</p>

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