如何解决 Swift Playgrounds 中的“运行此页面时出现问题”消息？

Question

我正在尝试使用 SceneKit 为 WWDC 学生挑战赛制作一些东西，这要求我的项目位于 Swift 操场上。我不断收到“运行此页面时出现问题”的消息。它不提供错误消息。它只是建议检查我的代码或重新开始。我曾尝试单独删除代码片段，但找不到此问题的根源。我还尝试在 man Xcode playground 中运行它，它提供了警告，我引用了“操场无法继续运行，因为操场源代码已经运行”。我被困住了。我的代码有什么问题。

import UIKit
import SceneKit
import QuartzCore 
import PlaygroundSupport

class GameScene: UIViewController, SCNSceneRendererDelegate {
    var primaryView: SCNView!
    var primaryScene: SCNScene!
    var cameraNode: SCNNode!
    
    override func viewDidLoad() {
        sceneAndViewInit()
        cameraInit()
        createGround()
        moonInit()
    }
    
    func createGround() {
        var ground = SCNBox(width: 200, height: 1, length: 200, chamferRadius: 0)
        var groundPhysicsShape = SCNPhysicsShape(geometry: ground, options: nil) 
        var groundNode = SCNNode(geometry: ground)
        ground.firstMaterial?.diffuse.contents = UIColor.orange
        ground.firstMaterial?.specular.contents = UIColor.white
        groundNode.position = SCNVector3(0, -6, 0)
        groundNode.physicsBody = SCNPhysicsBody(type: .static, shape: groundPhysicsShape)
        primaryScene.rootNode.addChildNode(groundNode)
    }
    
    func sceneAndViewInit() {
        primaryView = SCNView(frame: CGRect(x: 0, y: 0, width: 500, height: 300))
        primaryView.allowsCameraControl = true
        primaryView.autoenablesDefaultLighting = true
        
        primaryScene = SCNScene()
        
        primaryView.scene = primaryScene
        primaryView.isPlaying = true
    }
    
    func cameraInit() {
        var cameraNode = SCNNode()
        cameraNode.camera = SCNCamera()
        cameraNode.position = SCNVector3(x: 0, y: 0, z: 2)
        cameraNode.camera?.fieldOfView = 65
        cameraNode.camera?.wantsHDR = false
        primaryScene.rootNode.addChildNode(cameraNode)
    }
    
    func moonInit() {
        let moonScene = SCNScene(named: "Moon.scn")
        var moonNode = moonScene?.rootNode.childNode(withName: "Sphere", recursively: true)
        var moonPhysicsShape = SCNPhysicsShape(node: moonNode!, options: nil)
        moonNode?.position = SCNVector3(0, 0, 0)
        moonNode?.scale = SCNVector3(1, 1, 1)
        moonNode?.name = "Moon"
        moonNode?.physicsBody = SCNPhysicsBody(type: .static, shape: moonPhysicsShape)
        primaryScene.rootNode.addChildNode(moonNode!)
    }
    
    func renderer(_ aRenderer: SCNSceneRenderer, updateAtTime time: TimeInterval) {
        print(time)
    }
}

let gameScene = GameScene()
PlaygroundPage.current.liveView = gameScene

（“Moon.scn”是我在资源/文件部分中的一个文件及其纹理，但没有任何与月球相关的代码我仍然遇到问题。）

Answer 1

要查看完整的错误消息，请转到设置应用 -> 游乐场 -> 开启创作调试模式。这并不能直接解决问题，但对定位错误更有帮助。

Answer 2

我找到了解决此问题的方法。但是，我只能为您提供一个关于问题是什么以及如何解决它的大致想法。
问题似乎出在类顶部的变量声明上。

var primaryView: SCNView!
var primaryScene: SCNScene!
var cameraNode: SCNNode!

类型后面的感叹号表示该变量是implicitly-unwrapped optional（我引用this Stack Overflow response，它更深入地解释了这是什么），但或多或​​少，该变量将自动强制展开用来。我对 Swift 的了解多于技术知识，所以我的猜测是，当创建 GameScene 对象时，合成初始化程序会尝试设置这些变量，出于某种原因，它需要从变量中读取，获取nil，无法继续。

解决方案似乎是将变量的值放在它们的初始声明中（可能有更好的解决方案，但这至少在我的测试中有效。），因此它们不是 nil 的初始化程序。现在我不能在这里完全解释这个问题的原因是这似乎只对SCNScene 变量是必要的。我真的不知道为什么会这样。

代码（***标记重要的cmets）：

import UIKit
import SceneKit
import QuartzCore 
import PlaygroundSupport

class GameScene: UIViewController, SCNSceneRendererDelegate {
    var primaryView: SCNView!// = SCNView(frame: CGRect(x: 0, y: 0, width: 500, height: 300)) //***This seems to work for reasons beyond me with the implicitly-unwrapped optional***
    var primaryScene = SCNScene() //***The implicitly unwrapped optional is removed here***
    var cameraNode = SCNNode() //***The implicitly unwrapped optional is removed here (I am not sure if that is necessary but it probably good practice not to use it anywhere intros code.  I use it earlier to demonstrate its weird complexities.)***
    
    override func viewDidLoad() {
        sceneAndViewInit()
        cameraInit()
        createGround()
        //moonInit() ***I removed this for simplicity in the answer***
    }
    
    func createGround() {
        var ground = SCNBox(width: 200, height: 1, length: 200, chamferRadius: 0)
        var groundPhysicsShape = SCNPhysicsShape(geometry: ground, options: nil) 
        var groundNode = SCNNode(geometry: ground)
        ground.firstMaterial?.diffuse.contents = UIColor.orange
        ground.firstMaterial?.specular.contents = UIColor.white
        groundNode.position = SCNVector3(0, -6, 0)
        groundNode.physicsBody = SCNPhysicsBody(type: .static, shape: groundPhysicsShape)
        primaryScene.rootNode.addChildNode(groundNode)
    }
    
    func sceneAndViewInit() {
        primaryView = SCNView(frame: CGRect(x: 0, y: 0, width: 500, height: 300))
        primaryView.allowsCameraControl = true
        primaryView.autoenablesDefaultLighting = true
        
        //primaryScene = SCNScene() ***This is not necessary when it is declared above***
        
        self.primaryView.scene = primaryScene
        //primaryView.isPlaying = true
        
        self.view = primaryView //***I realized later you need this bit of code or the program will only display a blank screen***
    }
    
    func cameraInit() {
        //var cameraNode = SCNNode()
        cameraNode.camera = SCNCamera()
        cameraNode.position = SCNVector3(x: 0, y: 0, z: 2)
        cameraNode.camera?.fieldOfView = 65
        cameraNode.camera?.wantsHDR = false
        primaryScene.rootNode.addChildNode(cameraNode)
    }
    
    func moonInit() {
        let moonScene = SCNScene(named: "Moon.scn")
        var moonNode = moonScene?.rootNode.childNode(withName: "Sphere", recursively: true)
        var moonPhysicsShape = SCNPhysicsShape(node: moonNode!, options: nil)
        moonNode?.position = SCNVector3(0, 0, 0)
        moonNode?.scale = SCNVector3(1, 1, 1)
        moonNode?.name = "Moon"
        moonNode?.physicsBody = SCNPhysicsBody(type: .static, shape: moonPhysicsShape)
        primaryScene.rootNode.addChildNode(moonNode!)
    }
    
    func renderer(_ aRenderer: SCNSceneRenderer, updateAtTime time: TimeInterval) {
        print(time)
    }
}

let gameScene = GameScene()
PlaygroundPage.current.liveView = gameScene

最后一点： self.view = primaryView 代码非常重要。我在完整的代码中做了一个注释，但我发现这种方式太难在互联网上找到了。我认为这与UIViewController 有关（我对UIKit 了解不多。），但如果您现在有那段代码，SceneKit 或UIKit 只会显示一个空白屏幕。如果您没有弄清楚这些是单独的问题，那么它们真的很难破译。