CODEWITHSUNDEEP
iosswiftsprite-kitgame-physicsskphysicsbody
Sid Mani
Sid Mani

Reputation: 369

How do I maintain child SKNode velocities when moving the parent?

I'm working on a game in Swift with SpriteKit. I have a parent node with an SKPhysicsBody and several subnodes with their own physics bodies. All of the subnodes have some velocity relative to the parent node. How do I keep that relative velocity when giving the parent node a velocity? Here's pseudocode to show what I'm trying to do:

let parent:SKNode = SKNode()
let child:SKNode = SKNode()
parent.physicsBody = SKPhysicsBody()
child.physicsBody = SKPhysicsBody()
parent.addChild(child)

child.physicsBody!.velocity = CGVectorMake(dx: 5, dy: 5)
// child now has velocity
parent.physicsBody!.velocity = CGVectorMake(dx: 10, dy:10)
// parent has velocity, but child velocity is still (5,5)

How would I get the child velocity to be set such that it maintains a velocity relative to the parent? (e.g. the child's absolute velocity should become (15,15) once the parent is given a velocity of (10,10) so that it maintains (5,5) relative to the parent). I tried using SKPhysicsJoint but that seems to fix the node and not allow velocity. Any solutions? (I'm sure I'm overlooking something obvious, but I'm new to SpriteKit)
Thanks!

Upvotes: 3

Views: 610

Answers (1)

Epsilon
Epsilon

Reputation: 1030

If you want the child's velocity to be relative to its parent, add the parent's velocity to the child's constant velocity in the update method. For example,

override func update(currentTime: CFTimeInterval) {
    child.physicsBody?.velocity = CGVectorMake(parentNode.physicsBody!.velocity.dx + 5.0, parentNode.physicsBody!.velocity.dy + 5.0)
}

BTW, you shouldn't use parent as a variable name in Sprite Kit, since it's a property of SKNode.

Update

You can loop over all children of the parent node with the following.

override func update(currentTime: CFTimeInterval) {
    for (i, child) in parentNode.children.enumerate() {
        child.physicsBody?.velocity = CGVectorMake(parentNode.physicsBody!.velocity.dx + velocity[i].dx, parentNode.physicsBody!.velocity.dy + velocity[i].dy)
    }
}

velocity is an array of CGVectors that contains the velocity of each child.

Upvotes: 0

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