Displaying an incoming call screen using CallKit.

You can use CallKit to use the UI on incoming calls. I used it to create an app that would call me.

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	import UIKit

	class UISliderRoundValue_20190523: UIViewController {

	var sliderValue: UILabel!

	override func viewDidLoad() {
	super.viewDidLoad()

	view.backgroundColor = .white

	let initialValue: Float = 8.0
	let width = view.frame.width
	let height = view.frame.height

	let slider = UISlider()
	slider.minimumValue = 0.0
	slider.maximumValue = 10.0
	slider.value = initialValue
	slider.tintColor = .orange
	slider.frame = CGRect(x: width * 0.1, y: height * 0.2, width: width * 0.8, height: height * 0.1)
	slider.addTarget(self, action: #selector(sliderDidChangeValue(_:)), for: .valueChanged)
	view.addSubview(slider)

	sliderValue = UILabel()
	sliderValue.frame = CGRect(x: width * 0.1, y: height * 0.4, width: width * 0.8, height: height * 0.1)
	sliderValue.textAlignment = .center
	sliderValue.text = String(initialValue)
	view.addSubview(sliderValue)
	}

	@objc func sliderDidChangeValue(_ sender: UISlider) {
	let roundValue = roundf(sender.value * 2.0) * 0.5

	// set round value
	sender.value = roundValue
	sliderValue.text = String(roundValue)
	}
	}