在Swift 3.0中检测两个对象的碰撞？

import SpriteKit

class GameScene: SKScene, SKPhysicsContactDelegate {
    var object1: SKSpriteNode!
    var object2: SKSpriteNode!
    
    override func didMove(to view: SKView) {
        // 创建两个对象
        object1 = SKSpriteNode(color: .red, size: CGSize(width: 50, height: 50))
        object1.position = CGPoint(x: 100, y: 100)
        addChild(object1)
        
        object2 = SKSpriteNode(color: .blue, size: CGSize(width: 50, height: 50))
        object2.position = CGPoint(x: 200, y: 200)
        addChild(object2)
        
        // 设置物理属性
        object1.physicsBody = SKPhysicsBody(rectangleOf: object1.size)
        object1.physicsBody?.isDynamic = true
        object1.physicsBody?.categoryBitMask = 1
        object1.physicsBody?.contactTestBitMask = 2
        
        object2.physicsBody = SKPhysicsBody(rectangleOf: object2.size)
        object2.physicsBody?.isDynamic = true
        object2.physicsBody?.categoryBitMask = 2
        object2.physicsBody?.contactTestBitMask = 1
        
        // 设置代理
        physicsWorld.contactDelegate = self
    }
    
    func didBegin(_ contact: SKPhysicsContact) {
        if contact.bodyA.categoryBitMask == 1 && contact.bodyB.categoryBitMask == 2 {
            // object1和object2发生碰撞
            print("Collision detected!")
        }
    }
}