Adding code to the update function to control the movement of the invaders including when they drop down a row and head in the opposite direction.

KotlinInvadersView.kt

private fun update(fps: Long) {
	// Update the state of all the game objects

	// Move the player's ship
	playerShip.update(fps)

	// Did an invader bump into the side of the screen
	var bumped = false

	// Has the player lost
	var lost = false

	// Update all the invaders if visible
	for (invader in invaders) {

		if (invader.isVisible) {
			// Move the next invader
			invader.update(fps)

			// If that move caused them to bump
			// the screen change bumped to true
			if (invader.position.left > size.x - invader.width
					|| invader.position.left < 0) {

				bumped = true

			}
		}
	}

	// Did an invader bump into the edge of the screen
	if (bumped) {

		// Move all the invaders down and change direction
		for (invader in invaders) {
			invader.dropDownAndReverse(waves)
			// Have the invaders landed
			if (invader.position.bottom >= size.y && invader.isVisible) {
				lost = true;
			}
		}
	}
}