iOS 9 introduces Low Power Mode. When the user enables Low Power Mode, NSProcessInfo.processInfo().lowPowerModeEnabled returns true. We can observe the NSProcessInfoPowerStateDidChangeNotification notification and check against the lowPowerModeEnabled flag.

func someMethod() {
    NSNotificationCenter.defaultCenter().addObserver(self,
        selector: #selector(lowPowerModeChanged),
        name: NSProcessInfoPowerStateDidChangeNotification,
        object: nil)
}

func lowPowerModeChanged(notification: NSNotification) {
    if NSProcessInfo.processInfo().lowPowerModeEnabled {
        //Do less
    } else {
        //OK
    }
}

When Low Power Mode is enabled, iOS switches off several system features such as background fetch and motion effects, and reduces CPU and GPU performance. As developers, we can play a part too. Switch off expensive visual effects (such as complex animations) or delay unnecessary operations that consumes significant battery such as network operations via cellular connectivity. Be a good iOS citizen app.

#181 talks more about Swift selectors.

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