isBatteryMonitoringEnabled does not change value

I need to know the battery state in my watch app, but I can not set isBatteryMonitoringEnabled to true.

Is there anything else I should do beforehand? Or rules on where to change the property?

It looks like on watchOS and UIKit you could set up battery monitoring like so.

@IBOutlet weak var battery: WKInterfaceLabel!

@IBAction func getLevel() {
        let device = WKInterfaceDevice.current()
        device.isBatteryMonitoringEnabled = true

        let level = device.batteryLevel

        // Do something with the battery monitoring.
        print("Battery level is: \(level)")
        battery.setText("Level: \(level)")
    }

On the watchOS simulator it appears battery level monitoring will always return a value of -1.0, you'll need to run this on an actual device.

Here's a link to the documentation.