swift - 为什么 SceneKit 的 physicsWorld didBeginContact 会为一次碰撞触发多次？

Question

我正在使用 SceneKit 的 physicsBody 系统来检测对象之间的碰撞，并得到一些非常奇怪的结果。为了说明这一点，我有一个最小的示例，它生成两个具有运动学 physicsBodies 的球体并沿直线移动它们，以便它们短暂重叠。

我希望看到 physicsWorld(:didBeginContact:) 在球体第一次重叠时被调用一次，而 physicsWorld(:didEndContact:) 在它们停止重叠时被调用一次。相反，我看到每个函数被调用了 25 次!

下面是要重现的代码:在 Xcode 8.0 中，使用“游戏”模板创建一个全新的项目。将 GameViewController.swift 的内容替换为:

import UIKit
import SceneKit

class GameViewController: UIViewController, SCNSceneRendererDelegate, SCNPhysicsContactDelegate {

    var scnScene: SCNScene!
    var scnView: SCNView!
    var cameraNode: SCNNode!

    var nodeA: SCNNode!
    var nodeB: SCNNode!

    var countBeginnings: Int = 0
    var countEndings: Int = 0

    override func viewDidLoad() {
        super.viewDidLoad()

        setupScene()
        setupNodes()
    }

    func setupScene() {
        // create a new SCNScene and feed it to the view
        scnView = self.view as! SCNView
        scnScene = SCNScene()
        scnView.scene = scnScene

        // assign self as SCNView delegate to get access to render loop
        scnView.delegate = self
        // assign self as contactDelegate to handle collisions
        scnScene.physicsWorld.contactDelegate = self

        // create the camera and position it at origin
        cameraNode = SCNNode()
        cameraNode.camera = SCNCamera()
        cameraNode.position = SCNVector3Zero
        scnScene.rootNode.addChildNode(cameraNode)

        // tell scnView to update every frame
        scnView.isPlaying = true
    }

    func setupNodes() {
        // create two spheres with physicsBodies, one inside the other
        nodeA = SCNNode()
        nodeA.name = "Node A"
        nodeA.geometry = SCNSphere(radius: 1.0)
        nodeA.geometry!.firstMaterial?.diffuse.contents = UIColor.yellow.withAlphaComponent(0.6)
        // expected behavior
        // nodeA.position = SCNVector3(x: 0.0, y: -0.8, z: -10.0)
        // weird behavior
        nodeA.position = SCNVector3(x: 0.0, y: -0.9, z: -10.0)
        nodeA.physicsBody = SCNPhysicsBody(type: .kinematic, shape: SCNPhysicsShape(geometry: nodeA.geometry!, options: nil))
        scnScene.rootNode.addChildNode(nodeA)

        nodeB = SCNNode()
        nodeB.name = "Node B"
        nodeB.geometry = SCNSphere(radius: 0.5)
        nodeB.geometry!.firstMaterial?.diffuse.contents = UIColor.red
        nodeB.position = SCNVector3(x: -2.0, y: 0.0, z: -10.0)
        nodeB.physicsBody = SCNPhysicsBody(type: .kinematic, shape: SCNPhysicsShape(geometry: nodeB.geometry!, options: nil))
        scnScene.rootNode.addChildNode(nodeB)

        // node A can collide with node B but not the other way around
        nodeA.physicsBody!.categoryBitMask = 2
        nodeB.physicsBody!.categoryBitMask = 1
        nodeA.physicsBody!.contactTestBitMask = 1
    }

    func physicsWorld(_ world: SCNPhysicsWorld, didBegin contact: SCNPhysicsContact) {
        countBeginnings += 1
        print("(" + String(countBeginnings) + ") " + contact.nodeA.name! + " began contact with " + contact.nodeB.name!)
    }
    func physicsWorld(_ world: SCNPhysicsWorld, didEnd contact: SCNPhysicsContact) {
        countEndings += 1
        print("(" + String(countEndings) + ") " + contact.nodeA.name! + " ended contact with " + contact.nodeB.name!)
    }

    var frameNumber = 0
    func renderer(_ renderer: SCNSceneRenderer, updateAtTime time: TimeInterval) {
        nodeB.position.x += 0.01
        nodeB.position.y -= 0.01
    }

}

还有其他奇怪的事情发生。如果我稍微改变其中一个球体的初始位置，将 y 位置从 -0.9 移动到 -0.8:

nodeA.position = SCNVector3(x: 0.0, y: -0.8, z: -10.0)

现在我得到了预期的行为，一次调用开始，一次调用结束!稍微不同的碰撞角度会导致完全不同的行为。

这可能是 SceneKit 错误还是这实际上是预期的行为？

Answer 1

SCNRenderer 在每一帧运行物理模拟，调用 renderer(_:didSimulatePhysicsAtTime:)其方法SCNSceneRendererDelegate .

在两个球体相交的过程中，会渲染几帧，每次运行物理模拟时，都会检测到碰撞。

这是预料之中的。由您来处理碰撞并将两个球体置于不再碰撞的状态。例如，在游戏中，一旦弹丸击中玩家，它就会消失。碰撞已处理，因此不再触发。

---

我在 XCode Version 8.0 beta 5 (8S193k) 作为 OS X 应用程序运行时以下列方式使用了您的代码。我在控制台中看到以下跟踪。 begin 和 end 方法只调用一次!

(1) Node A began contact with Node B
(1) Node A ended contact with Node B


import SceneKit
import QuartzCore

class GameViewController: NSViewController, SCNSceneRendererDelegate, SCNPhysicsContactDelegate {

    @IBOutlet weak var gameView: GameView!

    // MARK: Properties

    var cameraNode: SCNNode!

    var nodeA: SCNNode!
    var nodeB: SCNNode!

    var countBeginnings: Int = 0
    var countEndings: Int = 0

    // MARK: Initialization

    override func awakeFromNib(){
        super.awakeFromNib()

        // create a new scene
        let scene = SCNScene(named: "art.scnassets/scene.scn")!

        // create and add a camera to the scene
        let cameraNode = SCNNode()
        cameraNode.camera = SCNCamera()
        scene.rootNode.addChildNode(cameraNode)

        // place the camera
        cameraNode.position = SCNVector3(x: 0, y: 0, z: 15)

        // create and add a light to the scene
        let lightNode = SCNNode()
        lightNode.light = SCNLight()
        lightNode.light!.type = SCNLightTypeOmni
        lightNode.position = SCNVector3(x: 0, y: 10, z: 10)
        scene.rootNode.addChildNode(lightNode)

        // create and add an ambient light to the scene
        let ambientLightNode = SCNNode()
        ambientLightNode.light = SCNLight()
        ambientLightNode.light!.type = SCNLightTypeAmbient
        ambientLightNode.light!.color = NSColor.darkGray
        scene.rootNode.addChildNode(ambientLightNode)

        // set the scene to the view
        self.gameView!.scene = scene

        // allows the user to manipulate the camera
        self.gameView!.allowsCameraControl = true

        // show statistics such as fps and timing information
        self.gameView!.showsStatistics = true

        // configure the view
        self.gameView!.backgroundColor = NSColor.black

        self.gameView!.delegate = self

        setupScene()
        setupNodes()
    }

    func setupScene() {

        // assign self as contactDelegate to handle collisions
        self.gameView!.scene?.physicsWorld.contactDelegate = self

        // create the camera and position it at origin
        cameraNode = SCNNode()
        cameraNode.camera = SCNCamera()
        cameraNode.position = SCNVector3Zero
        self.gameView!.scene?.rootNode.addChildNode(cameraNode)

        // tell scnView to update every frame
        self.gameView.isPlaying = true
    }

    func setupNodes() {
        // create two spheres with physicsBodies, one inside the other
        nodeA = SCNNode()
        nodeA.name = "Node A"
        nodeA.geometry = SCNSphere(radius: 1.0)
        nodeA.geometry!.firstMaterial?.diffuse.contents = NSColor.yellow.withAlphaComponent(0.6)
        nodeA.position = SCNVector3(x: 0.0, y: -0.8, z: -10.0)
        nodeA.physicsBody = SCNPhysicsBody(type: .kinematic, shape: SCNPhysicsShape(geometry: nodeA.geometry!, options: nil))
        self.gameView!.scene?.rootNode.addChildNode(nodeA)

        nodeB = SCNNode()
        nodeB.name = "Node B"
        nodeB.geometry = SCNSphere(radius: 0.5)
        nodeB.geometry!.firstMaterial?.diffuse.contents = NSColor.red
        nodeB.position = SCNVector3(x: -2.0, y: 0.0, z: -10.0)
        nodeB.physicsBody = SCNPhysicsBody(type: .kinematic, shape: SCNPhysicsShape(geometry: nodeB.geometry!, options: nil))
        self.gameView!.scene?.rootNode.addChildNode(nodeB)

        // node A can collide with node B but not the other way around
        nodeA.physicsBody!.categoryBitMask = 2
        nodeB.physicsBody!.categoryBitMask = 1
        nodeA.physicsBody!.contactTestBitMask = 1
    }

    // MARK: SCNPhysicsContactDelegate

    func physicsWorld(_ world: SCNPhysicsWorld, didBegin contact: SCNPhysicsContact) {
        countBeginnings += 1
        print("(" + String(countBeginnings) + ") " + contact.nodeA.name! + " began contact with " + contact.nodeB.name!)
    }
    func physicsWorld(_ world: SCNPhysicsWorld, didEnd contact: SCNPhysicsContact) {
        countEndings += 1
        print("(" + String(countEndings) + ") " + contact.nodeA.name! + " ended contact with " + contact.nodeB.name!)
    }

    // MARK: SCNSceneRendererDelegate

    var frameNumber = 0
    func renderer(_ renderer: SCNSceneRenderer, updateAtTime time: TimeInterval) {
        nodeB.position.x += 0.01
        nodeB.position.y -= 0.01
    }
}