同时访问0x14572f2a0，但修改需要独占访问

Question

我正在关注“Swift 5 中的设计模式”一书中的示例代码。该示例适用于状态模式部分。

我收到内存访问错误：“同时访问 0x14572f2a0，但修改需要独占访问。”当我尝试运行代码时。

我不确定发生了什么...提前致谢！

代码：

import Foundation

public final class ATM {
    
    fileprivate var state: ATMState = IdleState()
    
    public func enter(pin: String) {
        state = EnterPinState(context: self)
        state.validate(pin: pin)
    }
    
    public func withdraw(amount: Float) -> Bool {
        return state.withdraw(amount: amount)
    }
}

fileprivate protocol ATMState {
    func validate(pin: String)
    mutating func withdraw(amount: Float) -> Bool
    func transactionCompleted(success:Bool)
}

extension ATMState {
    func validate(pin:String) {
        print("not implemented")
    }
    mutating func withdraw(amount: Float) -> Bool {
        print("not implemented")
        return false
    }
    func transactionCompleted(success:Bool) {
        print("not implemented")
    }
}

fileprivate struct IdleState: ATMState {
    
}

fileprivate struct EnterPinState: ATMState {
    
    let context: ATM
    
    func validate(pin: String) {
        guard pin == "1234" else {
            print("wrong pin")
            context.state = TransactionCompleteState(context:context)
            context.state.transactionCompleted(success: false)
            return
        }
        print("pin ok")
        context.state = WithdrawState(context: context)
    }
}

fileprivate struct TransactionCompleteState: ATMState {
    let context: ATM
    
    func transactionCompleted(success: Bool) {
        var statusMessage = success ? "Transaction complete..." : "Transaction failed..."
        print(statusMessage)
        context.state = IdleState()
    }
}

fileprivate struct WithdrawState: ATMState {
    var context: ATM
    var availableFunds: Float = 1000
    
    init(context: ATM) {
        self.context = context
    }
    
    mutating func withdraw(amount: Float) -> Bool {
        print("Withdraw $\(amount)")
        guard amount > 0 && availableFunds >= amount else {
            print("invalid")
            context.state = TransactionCompleteState(context: context)
            context.state.transactionCompleted(success: false)
            return false
        }
        availableFunds -= amount
        context.state = TransactionCompleteState(context: context)
        context.state.transactionCompleted(success: true)
        return true
    }
}

let atm = ATM()
atm.enter(pin:"1234")
atm.withdraw(amount: 10)

Answer 1

这就是您在操场上测试代码所得到的——没有调试器！如果您在实际应用中尝试该代码，调试器会准确告诉您问题所在。

问题出在这里：

public func withdraw(amount: Float) -> Bool {
    return state.withdraw(amount: amount) // <-- *
}

withdraw 状态的withdraw 方法设置它的context 的state 属性，这是这个ATM 实例。所以对state.withdraw的调用意味着我们直接改变这个ATM的状态值，通过用不同的状态对象替换它，并且调用它的改变方法 withdraw，一步到位。你不能这样做，因为对结构的属性和对整个结构的引用的双重突变是不连贯的。您必须分阶段进行，如下所示：

public func withdraw(amount: Float) -> Bool {
    // fetch
    var theState = self.state
    // mutate
    return theState.withdraw(amount: amount)
}

另一种方法是使所有状态类型类而不是结构。与结构不同，类是可变的。然而，这需要更多的重写，因此留给读者作为练习。