SpriteKit Physics - 不停留在屏幕范围内的节点

Question

下面的简单代码创建一个球（触摸时）并设置物理体以使其下降和反弹。我设置了与框架匹配的界限，希望球在屏幕内保持100％。但是，当运行代码时，球将从屏幕左右移出屏幕。如何将精灵保持在屏幕范围内？

import SpriteKit

class GameScene: SKScene {

    override func didMoveToView(view: SKView) {
        /* Setup your scene here */
        physicsBody = SKPhysicsBody(edgeLoopFromRect: frame)
    }

    override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
        /* Called when a touch begins */
        if let touch = touches.first {
            let location = touch.locationInNode(self)

            let ball = SKShapeNode(circleOfRadius: 20)
            ball.physicsBody = SKPhysicsBody(circleOfRadius: 20)
            ball.physicsBody!.restitution = 0.4
            ball.position = location
            addChild(ball)
        }
    }
}