Stretch SKSpriteNode between two points/touches - ios

Question

I have an SKSpriteNode in which I've set the centerRect property so that the node can be stretched to appear like a styled line. My intention is for the user to touch the screen, and draw/drag a straight line with the node. The line would pivot around an anchor point to remain straight.
In touchesBegan:, the node is added:
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
guard let touch = touches.first else { return }
let positionInScene = touch.location(in: self)
if let _ = fgNode.childNode(withName: "laser") {
print("already there")
} else {
laser.centerRect = CGRect(x: 0.42857143, y: 0.57142857, width: 0.14285714, height: 0.14285714)
laser.anchorPoint = CGPoint(x: 0, y: 0.5)
laser.position = positionInScene
fgNode.addChild(laser)
}
}
And adjusted in touchesMoved::
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
guard let touch = touches.first else { return }
let positionInScene = touch.location(in: self)
stretchLaserTo(positionInScene)
}
The node is stretched and rotated with two functions:
func stretchLaserTo(_ point: CGPoint) {
let offset = point - laser.anchorPoint
let length = offset.length()
let direction = offset / CGFloat(length)
laser.xScale = length
rotate(sprite: laser, direction: direction)
}
func rotate(sprite: SKSpriteNode, direction: CGPoint) {
sprite.zRotation = atan2(direction.y, direction.x)
}
I think I'm somewhat on the right track. The line rotates with my touch and expands, however, it's extremely sensitive and doesn't stay with my touch. Maybe I'm going about it wrong. Is there a standard technique for doing something like this?
An example of this working can be seen here: https://imgur.com/A83L45i

Answer 1

I suggest you set anchor point of the sprite to (0, 0), set the sprite's scale to the distance between the sprite's position and the current touch location, and then rotate the sprite.
First, create a sprite and set its anchor point.
let laser = SKSpriteNode(color: .white, size: CGSize(width: 1, height: 1))
override func didMove(to view: SKView) {
laser.anchorPoint = CGPoint(x: 0, y: 0)
addChild(laser)
}
In touchesBegan, set the position of the sprite to the location of the touch. In this case, it's also the start of the line.
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
guard let touch = touches.first else { return }
let positionInScene = touch.location(in: self)
laser.position = positionInScene
laser.setScale(1)
}
Update the sprite so that it forms a line that starts at the position of the sprite and ends at the current touch location.
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
guard let touch = touches.first else { return }
let positionInScene = touch.location(in: self)
stretchLaserTo(positionInScene)
}
Stretch the sprite by setting its xScale to the distance from the start of the line to the location of the current touch and then rotate the sprite.
func stretchLaserTo(_ point: CGPoint) {
let dx = point.x - laser.position.x
let dy = point.y - laser.position.y
let length = sqrt(dx*dx + dy*dy)
let angle = atan2(dy, dx)
laser.xScale = length
laser.zRotation = angle
}