# Add and return a directional pad (`DPad`) to a default location
	#
	# The 4 buttons fire events with the corresponding name in `names`.
	# Items in `names` should be in order of top, left, down and right.
	# If `null`, defaults to WASD.
	#
	# If this method is called, it should be before `add_button` to
	# avoid overlapping controls.
	#
	# A maximum of 2 `DPad` may be added using this method.
	# The first `DPad` is placed on the left of the screen.
	# The second `DPad` is on the right and replaces some buttons
	# added by `add_button`.
	#
	# Require: `names == null or names.length == 4`
	fun add_dpad(names: nullable Array[String]): nullable DPad
	do
		if names == null then names = ["w","a","s","d"]
		assert names.length == 4

		if n_dpads == 0 then
			var dpad = new DPad(app.ui_camera.bottom_left.offset(200.0, 100.0, 0.0), names)
			controls.add dpad
			return dpad
		else if n_dpads == 1 then
			var dpad = new DPad(app.ui_camera.bottom_right.offset(-200.0, 100.0, 0.0), names)
			controls.add dpad
			return dpad
		else
			print_error "Too many DPad ({n_dpads}) in {self}"
			return null
		end
	end