GKState：为什么 self.stateMachine == 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"No StateMachine"。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，您可以实际打印它：

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()

GKState：为什么 self.stateMachine == nil？

GKState: Why is self.stateMachine == nil?

swift

gameplay-kit