“Even longer battery life.” - Apple says. Low Power Mode is a new feature of iOS 9 which lets iOS users extend their devices battery life. For now up to one hour, but hey, this is additional one hour, right?

About Low Power Mode

This feature uses ambient light and proximity sensors to know if device is facedown so system does not turn on device’s screen e.g. when notification came in. But this is not all about the feature. System changes behaviour itself as much as it can, including:

  1. Reduce CPU and GPU performance
  2. Pause background activities including networking
  3. Reduce screen brightness
  4. Reduce the timeout for auto-locking the device
  5. Disable Mail fetch
  6. Disable motion effects
  7. Disable animated wallpapers

Apple recommends to take additional steps and reduce animations (including frame rate), stop location updates or just reduce number of those updates, disable syncs and backups.

Usage

You can determine Low Power Mode state changes by observing NSProcessInfoPowerStateDidChangeNotification and current state by checking lowPowerModeEnabled property of NSProcessInfo class.

override func viewWillAppear(animated: Bool) {
    super.viewWillAppear(animated)
    startObservingLowPowerModeChanges()
}

override func viewWillDisappear(animated: Bool) {
    super.viewWillDisappear(animated)
    stopObservingLowPowerModeChanges()
}

func startObservingLowPowerModeChanges() {
    NSNotificationCenter.defaultCenter().addObserver(self,
        selector: Selector("lowPowerModeDidChangeNotification"),
        name: NSProcessInfoPowerStateDidChangeNotification,
        object: nil)
}

func stopObservingLowPowerModeChanges() {
    NSNotificationCenter.defaultCenter().removeObserver(self,
        name: NSProcessInfoPowerStateDidChangeNotification,
        object: nil)
}

func lowPowerModeDidChangeNotification() {
    let state = NSProcessInfo.processInfo().lowPowerModeEnabled
    print("low power mode did change notification, current state: \(state)")
}

You can enable Low Power Mode on your actual device in Settings > Battery > Low Power Mode.

Unfortunately this is not possible on simulator, at least for now - rdar://21945440.