从SKNode地板类中移除重力

我在比赛开始时有两个矩形,但一旦开始,重力就会把它们取下来。我把下面的代码放在一个单独的“Floor.swift”类中。我还有一个“BitMaskCategories.swift”文件,其中包含以下信息。

let ballCategory : UInt32 = 0x1 << 1
let avoidCategory : UInt32 = 0x1 << 2
let floorCategory : UInt32 = 0x1 << 3
let pointCategory : UInt32 = 0x1 << 4
let lifeCategory : UInt32 = 0x1 << 5 

这是我的“Floor.swift”课程

import Foundation
import SpriteKit

class Floor: SKNode {
    override init() {
        super.init()

        let leftWall = SKSpriteNode(color: UIColor.brown, size: CGSize(width: 50, height: 300))
        leftWall.position = CGPoint(x: 0, y: 100)
        leftWall.physicsBody!.isDynamic = false
        leftWall.physicsBody = SKPhysicsBody(rectangleOf: leftWall.size)
        self.addChild(leftWall)

        let rightWall = SKSpriteNode(color: UIColor.brown, size: CGSize(width: 50, height: 300))
        rightWall.position = CGPoint(x: 100, y: 200)
       rightWall.physicsBody!.isDynamic = false
        rightWall.physicsBody = SKPhysicsBody(rectangleOf: rightWall.size)
        self.addChild(rightWall)

        // self.physicsBody?.affectedByGravity = false
        // self.physicsBody?.isDynamic = false

        // Set the bit mask properties
        self.physicsBody?.categoryBitMask = balloonCategory
        self.physicsBody?.contactTestBitMask = nailDropCategory | pointCategory | lifeCategory
        //self.physicsBody?.collisionBitMask = balloonCategory
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemted")
    }
}

为什么会出现错误,“线程1:致命错误:在展开可选值时意外发现零”但使用“”时而不是“!”该应用程序可以工作,但矩形会下降。我需要那条线,让他们不要那样做,我甚至不明白这有什么意义。