dynamic box 会停在static floor上，不会穿过。

let scene1 = SCNScene()
        gameView.scene = scene1
        
        let box = SCNNode(geometry: SCNBox(width: 10, height: 10, length: 10, chamferRadius: 0))
        scene1.rootNode.addChildNode(box)
        box.physicsBody = SCNPhysicsBody.dynamic()
        box.geometry?.firstMaterial?.diffuse.contents = "WoodCubeA.jpg"
        
        let floor = SCNNode(geometry: SCNFloor())
        scene1.rootNode.addChildNode(floor)
        floor.physicsBody = SCNPhysicsBody.static()
        floor.geometry?.firstMaterial?.diffuse.contents = "wood.png"
        
        let camera = SCNNode()
        camera.camera = SCNCamera()
        camera.position = SCNVector3(0, 20, 20)
        camera.rotation = SCNVector4(1, 0, 0, -Double.pi / 4)
        scene1.rootNode.addChildNode(camera)

一个项目中box是dynamic，floor是static，设置正常，可box总是穿过floor，经过艰辛的回到最简单状态，再一一比较，发现原来问题在这：

原来错误的：
floor.physicsBody? = SCNPhysicsBody.static()
正确的：
floor.physicsBody = SCNPhysicsBody.static()

发现细节没？physicsBody后面不能有问号！！有了问号，这句话其实没走。