swift - GKState : Why is self. 状态机 == nil？

Question

我的 Playground 中有以下代码:

import GameplayKit
class TestClass {
    var sm: GKStateMachine

    init() {
        sm = GKStateMachine(states: [MyState()])
        sm.enter(MyState.self)
    }
}

class MyState: GKState {

    override init() {
        super.init()
    }
    override func didEnter(from previousState: GKState?) {
        printStateM()
    }
    func printStateM() {
        if (self.stateMachine == nil) {
            print("StateMachine")
        } else {
            print("No StateMachine")
        }
    }
}

var t = TestClass()

输出为“无状态机”。我想知道为什么MyState的StateMachine属性是nil？

Answer 1

因为你打错了:

import GameplayKit
class TestClass {
    var sm: GKStateMachine

    init() {
        sm = GKStateMachine(states: [MyState()])
        sm.enter(MyState.self)
    }
}

class MyState: GKState {

    override init() {
        super.init()
    }
    override func didEnter(from previousState: GKState?) {
        printStateM()
    }
    func printStateM() {
        if (self.stateMachine != nil) { // ⚠️
            print("StateMachine")
        } else {
            print("No StateMachine")
        }
    }
}

var t = TestClass()

更好的是，您实际上可以打印它:

import GameplayKit
class TestClass {
    var sm: GKStateMachine

    init() {
        sm = GKStateMachine(states: [MyState()])
        sm.enter(MyState.self)
    }
}

class MyState: GKState {

    override init() {
        super.init()
    }

    override func didEnter(from previousState: GKState?) {
        printStateM()
    }

    func printStateM() {
        if let stateMachine = self.stateMachine {
            print("StateMachine: \(stateMachine)")
        } else {
            print("No StateMachine")
        }
    }
}

var t = TestClass()