Moving a SKSpriteNode in a downward loop, using Swift

问题

Using Swift and SpriteKit, I'd like to move a SKSpritenode in a spiral pattern, but didn't find the right resources to get me started. To be more precise, I'd like to move a sprite node in a downward loop. I've checked a sequence of SKActions, but as they are not executed parallel, a circular movement combined with move-to will not work. I'd be glad for any hints, tutorials or snippets to set me on the right track.

Thanx in advance, Marcus

回答1:

I've put together some sample code which you can adapt for your purposes. I've based the code off the equation for an Archimedean Spiral:

r = a + bθ

Where a is the starting radius; b is the radius the spiral will increase by per revolution and θ is the current angle.

A spiral is basically a glorified circle (IMO), so to move your node in a spiral you need to be able to calculate point on a circle using an angle, radius and center point:

func pointOnCircle(#angle: CGFloat, #radius: CGFloat, #center: CGPoint) -> CGPoint {
    return CGPoint(x: center.x + radius * cos(angle),
                   y: center.y + radius * sin(angle))
}

Next, extend SKAction so you can easily create a spiral action:

extension SKAction {
    static func spiral(#startRadius: CGFloat, endRadius: CGFloat, angle 
         totalAngle: CGFloat, centerPoint: CGPoint, duration: NSTimeInterval) -> SKAction {

        // The distance the node will travel away from/towards the 
        // center point, per revolution.
        let radiusPerRevolution = (endRadius - startRadius) / totalAngle

        let action = SKAction.customActionWithDuration(duration) { node, time in
            // The current angle the node is at.
            let θ = totalAngle * time / CGFloat(duration)

            // The equation, r = a + bθ
            let radius = startRadius + radiusPerRevolution * θ

            node.position = pointOnCircle(angle: θ, radius: radius, center: centerPoint)
        }

        return action
    }
}

Finally, an example of use. In didMoveToView:

let node = SKSpriteNode(color: UIColor.redColor(), size: CGSize(width: 10, height: 10))
node.position = CGPoint(x: size.width / 2, y: size.height / 2)
addChild(node)

let spiral = SKAction.spiral(startRadius: size.width / 2,
                             endRadius: 0,
                             angle: CGFloat(M_PI) * 2,
                             centerPoint: node.position,
                             duration: 5.0)

node.runAction(spiral)

回答2:

While the above mentioned solution is brilliantly thought of, there is a much easier way.

Add a spinnerNode, and repetitively spin it using rotateBy
Add your sprite as a child to the spinnerNode (your sprite will by default rotate with the spinnerNode)
Move your spriteNode (using moveTo) to the centre of the wheelNode (your sprite will appear traveling in a spiral path to the center)

来源：https://stackoverflow.com/questions/30427482/moving-a-skspritenode-in-a-downward-loop-using-swift

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swift

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