为什么场景工具包的PhysicsWorld didBeginContact会因为一次碰撞而多次开火？

Why is SceneKit#39;s physicsWorld didBeginContact firing multiple times for a single collision?(为什么场景工具包的PhysicsWorld didBeginContact会因为一次碰撞而多次开火？)

问题描述

我正在使用SceneKit的PhysiicsBody系统来检测对象之间的碰撞，并获得了一些非常奇怪的结果。为了说明这一点，我举了一个最小的例子，它生成了两个带有运动学物理的球体，并使它们沿直线移动，从而使它们短暂重叠。

我期望看到PhysicsWorld(：didBeginContact：)在球体第一次重叠时被调用一次，而PhysicsWorld(：didEndContact：)在它们停止重叠时调用一次。相反，我看到每个函数都被调用了25次！

以下是要重现的代码：在Xcode8.0中，使用"Game"模板创建一个全新的项目。将GameViewController.swft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正在上演。如果我稍微更改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，一个调用开始，一个调用结束！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行为？

推荐答案

SCN呈现器在每一帧运行物理模拟，调用其SCNSceneRendererDelegate的renderer(_:didSimulatePhysicsAtTime:)方法。

在两个球体交叉的过程中，将渲染多个帧，每次运行物理模拟时都会检测到碰撞。

这是意料之中的。这取决于您处理碰撞，并将两个球体置于不再碰撞的状态。例如，在游戏中，一旦投射物击中玩家，它就会消失。冲突已处理，因此不再激发。

---

在XCode8.0测试版5(8S193k)作为OS X应用程序运行时，我以以下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
    }
}

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