EdgeFromLoop doesn't seem to call didBegin?

I'm trying to write a game where stars are falling from the top of the iphone screen to the bottom. I want to remove my star sprites when they're out of the screen. I've tried to use physicsContactDelegate, and create a edge around my frame with edgeFromLoop(self.frame), but didBegin is never called.

Below is my gameScene.swift file

import SpriteKit
import GameplayKit
import UIKit

class GameScene: SKScene, SKPhysicsContactDelegate {

    var entities = [GKEntity]()
    var graphs = [String : GKGraph]()

    private var lastUpdateTime : TimeInterval = 0

    struct PhysicsCategory {
      static let none: UInt32 = 0
      static let all : UInt32 = UInt32.max
      static let star: UInt32 = 0b1       // 1
      static let edge: UInt32 = 0b10      // 2
    }

    private var edge: SKPhysicsBody!

    override func sceneDidLoad() {

        self.lastUpdateTime = 0

        edge = SKPhysicsBody(edgeLoopFrom: self.frame)
        edge.categoryBitMask = PhysicsCategory.edge
        edge.contactTestBitMask = PhysicsCategory.star
        edge.collisionBitMask = PhysicsCategory.star
    }


    override func didMove(to view: SKView) {

        physicsWorld.contactDelegate = self

        run(SKAction.repeatForever(
            SKAction.sequence([
                SKAction.run(addStar),
                SKAction.wait(forDuration: 1.0)
            ])
      ))
    }

    func didBegin(_ contact: SKPhysicsContact) {

      print("didBegin called")
      // 1
      var firstBody: SKPhysicsBody
      var secondBody: SKPhysicsBody
      if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
        firstBody = contact.bodyA
        secondBody = contact.bodyB
      } else {
        firstBody = contact.bodyB
        secondBody = contact.bodyA
      }

      if((firstBody.categoryBitMask & PhysicsCategory.star != 0) &&
          (secondBody.categoryBitMask & PhysicsCategory.edge != 0)){

          print("call destroyStar")
          if let star = firstBody.node as? SKSpriteNode {
              destroyStar(star: star)
          }
      }
    }

    func addStar(){

        let star = SKSpriteNode(imageNamed: "star")

        let scale = CGFloat.random(in: 0.1 ... 0.5)
        star.xScale = scale
        star.yScale = scale
        star.zPosition = 1.0

        star.physicsBody = SKPhysicsBody(rectangleOf: star.size)
        star.physicsBody?.linearDamping = 1.0
        star.physicsBody?.friction = 1.0

        star.physicsBody?.isDynamic = true // 2
        star.physicsBody?.categoryBitMask = PhysicsCategory.star // 3
        star.physicsBody?.contactTestBitMask = PhysicsCategory.edge // 4
        star.physicsBody?.collisionBitMask = PhysicsCategory.edge

        let actualX = CGFloat.random(in: (-1*size.width/2)+50 ... (size.width/2)-50)

        star.position = CGPoint(x: actualX, y: self.size.height)
        addChild(star)
    }

    override func update(_ currentTime: TimeInterval) {
        // Called before each frame is rendered

        // Initialize _lastUpdateTime if it has not already been
        if (self.lastUpdateTime == 0) {
            self.lastUpdateTime = currentTime
        }

        // Calculate time since last update
        let dt = currentTime - self.lastUpdateTime

        // Update entities
        for entity in self.entities {
            entity.update(deltaTime: dt)
        }

        self.lastUpdateTime = currentTime
    }

    func destroyStar(star: SKSpriteNode){

        print("star destroyed")
        star.removeFromParent()

    }
}

And this is my GameViewController

import UIKit
import SpriteKit
import GameplayKit

class GameViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Load 'GameScene.sks' as a GKScene. This provides gameplay related content
        // including entities and graphs.
        if let scene = GKScene(fileNamed: "GameScene") {

            // Get the SKScene from the loaded GKScene
            if let sceneNode = scene.rootNode as! GameScene? {

                // Copy gameplay related content over to the scene
                sceneNode.entities = scene.entities
                sceneNode.graphs = scene.graphs

                // Set the scale mode to scale to fit the window
                sceneNode.scaleMode = .aspectFit

                // Present the scene
                if let view = self.view as! SKView? {
                    view.presentScene(sceneNode)

                    view.ignoresSiblingOrder = true

                    view.showsFPS = true
                    view.showsNodeCount = true
                }
            }
        }
    }

    override var shouldAutorotate: Bool {
        return true
    }

    override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
        if UIDevice.current.userInterfaceIdiom == .phone {
            return .allButUpsideDown
        } else {
            return .all
        }
    }

    override var prefersStatusBarHidden: Bool {
        return true
    }
}

I'm a swift beginner, I know my question is probably really basic but I'd really appreciate any help! Thanks!

You need to make edge and SKNode() and then assign the physicsBody to edge.

try this.

let edge = SKNode()

override func didMove(to View: SKView) {
    edge.physicsBody = SKPhysicsBody(edgeLoopFrom: self.frame)
    edge.physicsBody.categoryBitMask = PhysicsCategory.edge
    edge.physicsBody.contactTestBitMask = PhysicsCategory.star
    edge.physicsBody.collisionBitMask = PhysicsCategory.star

    self.addChild(edge)
}