ios-battery-level.swift

First just enable battery monitoring:

UIDevice.current.isBatteryMonitoringEnabled = true
Then you can create a computed property to return the battery level:

var batteryLevel: Float {
    return UIDevice.current.batteryLevel
}
To monitor your device battery level you can add an observer for the UIDeviceBatteryLevelDidChange notification:

NotificationCenter.default.addObserver(self, selector: #selector(batteryLevelDidChange), name: .UIDeviceBatteryLevelDidChange, object: nil)
func batteryLevelDidChange(_ notification: Notification) {
    print(batteryLevel)
}
You can also verify the battery state:

var batteryState: UIDeviceBatteryState {
    return UIDevice.current.batteryState
}
case .unknown   //  "The battery state for the device cannot be determined."
case .unplugged //  "The device is not plugged into power; the battery is discharging"
case .charging  //  "The device is plugged into power and the battery is less than 100% charged."
case .full      //   "The device is plugged into power and the battery is 100% charged."
and add an observer for UIDeviceBatteryStateDidChange notification:

NotificationCenter.default.addObserver(self, selector: #selector(batteryStateDidChange), name: .UIDeviceBatteryStateDidChange, object: nil)
func batteryStateDidChange(_ notification: Notification) {
    switch batteryState {
    case .unplugged, .unknown:
        print("not charging")
    case .charging, .full:
        print("charging or full")
    }
}