UIDeviceBatteryLevelDidChange在后台不起作用？

Question

ViewController.swift

NotificationCenter.default.addObserver(self, selector: #selector(batteryLevelDidChange(_:)), name: .UIDeviceBatteryLevelDidChange, object: nil)
@objc func batteryLevelDidChange(_ notification: Notification) {
    self.infoLabel.text = ":\(Int(batteryLevel*100))%"
}

AppDelegate.swift

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

    UNUserNotificationCenter.current().requestAuthorization(options: [.sound, .alert, .carPlay, .badge]) { (success, error) in
    }
    UIDevice.current.isBatteryMonitoringEnabled = true
    return true
}

为什么它在后台不起作用？