Sprite Kit physicsBody.resting behavior - swift

Question

I am using Swift and Sprite Kit to develop a game on XCode Beta 6.
In order to detect if all nodes are sleeping, i check their physicsBody.resting property.
In update method i print out the result.
import SpriteKit
class GameScene: SKScene, SKPhysicsContactDelegate {
var hero:SKSpriteNode!
override func didMoveToView(view: SKView) {
self.physicsWorld.gravity = CGVectorMake(0, 0)
self.physicsWorld.contactDelegate = self
self.physicsBody = SKPhysicsBody(edgeLoopFromRect:self.frame)
hero = SKSpriteNode(imageNamed: "Spaceship")
hero.position = CGPoint(x:CGRectGetMidX(self.frame), y:CGRectGetMidY(self.frame))
hero.zPosition = 10.0
hero.physicsBody = SKPhysicsBody(circleOfRadius: hero.size.width/2)
hero.physicsBody.allowsRotation = false
hero.physicsBody.linearDamping = 0.5
self.addChild(hero)
}
override func update(currentTime: CFTimeInterval) {
if hero.physicsBody.resting {
println("resting")
} else {
println("moving")
}
}
}
To my surprise, the results are:
moving
resting
moving
(n times the same)
moving
resting
So why the hero is moving, although i didn't do anything. The node moves N times and takes a break(resting), after that goes on moving.
Can anyone explain that behaviour? Is that a bug or do i miss something? Thanks in advance.

Answer 1

If you examine the velocity of a physics body, you'll see that it is indeed moving but at a rate that is not perceivable. That's why the resting property is not set. A more reliable way to check if a SKPhysicsBody is at rest is to test if its linear and angular speeds are nearly zero. Here's an example of how to do that:
func speed(velocity:CGVector) -> Float {
let dx = Float(velocity.dx);
let dy = Float(velocity.dy);
return sqrtf(dx*dx+dy*dy)
}
func angularSpeed(velocity:CGFloat) -> Float {
return abs(Float(velocity))
}
// This is a more reliable test for a physicsBody at "rest"
func nearlyAtRest(node:SKNode) -> Bool {
return (self.speed(node.physicsBody.velocity)<self.verySmallValue
&& self.angularSpeed(node.physicsBody.angularVelocity) < self.verySmallValue)
}
override func update(_ currentTime: TimeInterval) {
/* Enumerate over child nodes with names starting with "circle" */
enumerateChildNodesWithName("circle*") {
node, stop in
if (node.physicsBody.resting) {
println("\(node.name) is resting")
}
if (self.nearlyAtRest(node)) {
println("\(node.name) is nearly resting")
}
}
}