SpriteKit中ApplyImpulse的问题

Question

我是SpriteKit的新手，我不知道为什么我的代码根本不起作用。在我创建所有节点之后，我想要的唯一受物理影响的是球。我想对球节点施加冲动，但我可以“让它起作用。”

以下是代码：

import SpriteKit
import GameplayKit

class GameScene: SKScene {

    var ramp = SKSpriteNode()
    var label = SKLabelNode()
    var ball = SKSpriteNode()


    override func didMove(to view: SKView) {
        ball = self.childNode(withName: "ball") as! SKSpriteNode
        ramp = self.childNode(withName: "ramp") as! SKSpriteNode
        label = self.childNode(withName: "label") as! SKLabelNode

        ball.physicsBody?.isDynamic = true
        ball.physicsBody?.affectedByGravity = true

        let border = SKPhysicsBody(edgeLoopFrom: self.frame)
        border.friction = 0
        border.restitution = 0
        self.physicsBody = border

        ball.physicsBody?.applyImpulse(CGVector(dx: 0, dy: 50))
    }

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    }
    override func update(_ currentTime: TimeInterval) {
        // Called before each frame is rendered
    }


}