使用node.position.x的Swift SKAction意外行为

Question

我定义了几个SKActions，例如下面的SKActions，将各个节点移动到给定位置。我将目标X坐标定义为platform.position.x，将目标Y坐标定义为platform.position.y - 900.预期的行为是节点将向下移动900，但保持在相同的x位置。相反，节点以一定角度向下移动，朝向彼此，最终在中心处终止，然后最终消失在屏幕外。我无法弄清楚为什么会这样。

import SpriteKit
import GameplayKit

class GameScene: SKScene, SKPhysicsContactDelegate {

    override func didMove(to view: SKView) {

    let platform1 = SKSpriteNode(color: UIColor.yellow, size: CGSize(width: 400, height: 60))

    platform1.xScale = 0.8
    platform1.yScale = 0.8
    platform1.physicsBody = SKPhysicsBody(rectangleOf: CGSize(width: platform1.size.width, height: platform1.size.width / 6))

    platform1.position = CGPoint(x: -170, y: -270)
    platform1.physicsBody?.categoryBitMask = 1
    platform1.physicsBody?.collisionBitMask = 0
    platform1.physicsBody?.isDynamic = false
    platform1.physicsBody?.affectedByGravity = false
    platform1.physicsBody?.friction = 0

    addChild(platform1)


    let platform1GlideOff = SKAction.move(to: CGPoint(x: platform1.position.x, y: platform1.position.y - 900), duration: TimeInterval(5))

    var firstActionplatform1 = SKAction.run({
        platform1.run(platform1GlideOff)
    })

        let platform2 = SKSpriteNode(color: UIColor.green, size: CGSize(width: 400, height: 60))

        platform2.xScale = 0.8
        platform2.yScale = 0.8
        platform2.physicsBody = SKPhysicsBody(rectangleOf: CGSize(width: platform2.size.width, height: platform2.size.width / 6))

        platform2.position = CGPoint(x: 200, y: -270)
        platform2.physicsBody?.categoryBitMask = 1
        platform2.physicsBody?.collisionBitMask = 0
        platform2.physicsBody?.isDynamic = false
        platform2.physicsBody?.affectedByGravity = false
        platform2.physicsBody?.friction = 0

        addChild(platform2)


        let platform2GlideOff = SKAction.move(to: CGPoint(x: platform1.position.x, y: platform1.position.y - 900), duration: TimeInterval(5))

        var firstActionplatform2 = SKAction.run({
            platform2.run(platform2GlideOff)
        })


    var sequence = SKAction.sequence([SKAction.wait(forDuration: 1), firstActionplatform1, firstActionplatform2])

    run(sequence)
    }
}