How to create a slingshot mechanism in Swift, SpriteKit - swift

Question

I am trying to create a Sling shot mechanism where I rotate a node with an object attached at the end. The slingshot starts off with a touch and hold interaction to rotate the object and on release, launches the object attached at the end - A similar effect is seen here
This is my code for the rotation which has worked. Not sure how to start the joining and slinging part.
var touchingScreen = false
override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
super.touchesBegan(touches, withEvent: event)
touchingScreen = true
println("Screen Touched")
}
override func touchesCancelled(touches: Set<NSObject>!, withEvent event: UIEvent!) {
super.touchesCancelled(touches, withEvent: event)
touchingScreen = false
println("Screen Not Touched")
}
override func touchesEnded(touches: Set<NSObject>, withEvent event: UIEvent) {
super.touchesEnded(touches, withEvent: event)
touchingScreen = false
println("Screen Not Touched")
}
override func update(currentTime: CFTimeInterval) {
if touchingScreen {
var RotatingAngle = CGFloat(M_PI)
var rotationDuration = 20.0
var rotateCanon = SKAction.rotateByAngle(CGFloat(RotatingAngle), duration: rotationDuration)
canon.runAction(rotateCanon)
} else if !touchingScreen {
var RotatingAngleTwo = CGFloat(0)
var rotationDurationTwo = 0.0
var rotateCanon = SKAction.rotateByAngle(CGFloat(RotatingAngleTwo), duration: rotationDurationTwo)
canon.runAction(rotateCanon)
}
}
Thanks for the help.

Answer 1

Some thoughts...
It's fairly straightforward to implement a slingshot as shown in the video if you break it down into smaller steps
If you represent the ball's position on the circle with polar coordinates, you can rotate the ball by simply incrementing the angle over time and then converting from polar to cartesian coordinates
When launched, the ball's trajectory should be tangent to the ball's position on the circle and its speed should be the same as its angular velocity.
and some code...
enum State {
case Stopped
case Rotating
case Launched
}
let two_pi = CGFloat(M_PI*2.0)
let pi = CGFloat(M_PI)
// These are useful vector/point operators
func * (left:CGPoint, right:CGFloat) -> CGPoint {
return CGPointMake(left.x*right, left.y*right)
}
func += (inout left:CGPoint, right:CGPoint) {
left = CGPointMake(left.x+right.x, left.y+right.y)
}
func * (left:CGVector, right:CGFloat) -> CGVector {
return CGVectorMake(left.dx*right, left.dy*right)
}
func / (left:CGVector, right:CGFloat) -> CGVector {
return CGVectorMake(left.dx/right, left.dy/right)
}
class GameScene: SKScene {
let shape = SKShapeNode(circleOfRadius: 7)
let radius:CGFloat = 30
var center = CGPointZero
var currentAngle = -pi/2
let angleIncr = two_pi / 60.0
var state:State = .Stopped
override func didMoveToView(view: SKView) {
scaleMode = .ResizeFill
// Set the center of the sling
center = CGPointMake (CGRectGetMidX(view.frame),CGRectGetMidY(view.frame))
addBall()
let circle = SKShapeNode(circleOfRadius: radius)
circle.position = CGPointMake (CGRectGetMidX(view.frame),CGRectGetMidY(view.frame))
addChild(circle)
}
// Adds a circle shape node at the bottom of the sling
func addBall() {
currentAngle = -pi/2
shape.fillColor = SKColor.blueColor()
shape.position = CGPointMake (center.x, center.y-radius)
shape.physicsBody = SKPhysicsBody(circleOfRadius: 7)
shape.physicsBody?.affectedByGravity = false
shape.physicsBody?.mass = 0.5
shape.zPosition = 1
addChild(shape)
}
override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
if let touch = touches.first as? UITouch {
let location = touch.locationInNode(self)
if (state == .Stopped) {
// Start rotating the ball around the sling
state = .Rotating
}
}
}
override func touchesEnded(touches: Set<NSObject>, withEvent event: UIEvent) {
if (state == .Rotating) {
// Launch the ball on a vector tangent to its current position on the circle
state = .Launched
// Normal vector
var normal = CGVectorMake(shape.position.x-center.x, shape.position.y-center.y)
normal = normal / magnitude(normal)
// Tangent vector
let vector = CGVectorMake(normal.dy, -normal.dx)
// Convert angular to linear speed
let speed = angleIncr * 60.0 * radius
shape.physicsBody?.velocity = vector*speed
runAction(SKAction.waitForDuration(1.0)) {
self.shape.removeFromParent()
self.state = .Stopped
self.addBall()
}
}
}
override func update(currentTime: CFTimeInterval) {
switch (state) {
case .Rotating:
var point = angleToPoint(currentAngle) * radius
point += center
shape.position = point
currentAngle -= angleIncr
// Wrap at 2 pi
currentAngle %= two_pi
default:
break
}
}
func angleToPoint(angle:CGFloat) -> CGPoint {
return CGPointMake(cos(angle), sin(angle))
}
func magnitude(v1:CGVector) -> CGFloat {
return sqrt(v1.dx*v1.dx+v1.dy*v1.dy)
}
}
and a video...