Smalltalk 语言 状态模式实战 自动售货机的状态管理

Smalltalkamuwap 发布于 6 天前 7 次阅读

Smalltalk【1】 语言状态模式【2】实战：自动售货机【3】的状态管理

状态模式是一种行为【4】设计模式【5】，它允许对象【6】在其内部状态改变时改变其行为。在软件设计中，状态模式特别适用于那些对象的行为依赖于其内部状态，并且状态改变会引起行为改变的场景。本文将使用Smalltalk语言，通过实现一个自动售货机（Vending Machine）的状态管理来展示状态模式的实战应用。

自动售货机背景

自动售货机是一种常见的商业设备，它允许用户通过投币或使用电子支付方式购买商品。自动售货机的核心功能包括：

- 存储商品
- 接收支付
- 发放商品
- 处理找零

在自动售货机的生命周期【7】中，其状态可能会从“空闲”变为“等待支付”、“等待商品选择”、“等待找零”等。每种状态都对应着不同的行为。

Smalltalk 语言简介

Smalltalk是一种面向对象的编程语言，它以其简洁的语法和强大的对象模型而闻名。在Smalltalk中，对象是所有事物的中心，每个对象都有自己的状态和行为。

状态模式实现

1. 定义状态接口【8】

我们需要定义一个状态接口，它将包含所有可能的状态方法【9】。

smalltalk
Class: VendingMachineState

Superclass: Object

Class Variables
classStateDictionary: Dictionary new

Instance Variables
vendingMachine: VendingMachine

Class Methods
+new: vm: VendingMachine
^ super new: vm

Instance Methods
dispenseProduct
handlePayment
handleProductSelection
handleReturnChange
handleNoProductSelected
handleNoPayment
handleNoChangeReturned

2. 实现具体状态类【10】

接下来，我们为每种状态实现具体的类。

smalltalk
Class: IdleState

Superclass: VendingMachineState

Instance Methods
dispenseProduct
"Implement the behavior for dispensing a product"
^ self error: 'Cannot dispense product in idle state.'

handlePayment
"Transition to PaymentState"
^ self vendingMachine setState: PaymentState

handleProductSelection
"Implement the behavior for handling product selection"
^ self error: 'Cannot select product in idle state.'

handleReturnChange
"Implement the behavior for handling return change"
^ self error: 'Cannot return change in idle state.'

handleNoProductSelected
"Implement the behavior for handling no product selected"
^ self error: 'No product selected in idle state.'

handleNoPayment
"Implement the behavior for handling no payment"
^ self error: 'No payment made in idle state.'

handleNoChangeReturned
"Implement the behavior for handling no change returned"
^ self error: 'No change returned in idle state.'

Class: PaymentState

Superclass: VendingMachineState

Instance Methods
dispenseProduct
"Implement the behavior for dispensing a product"
^ self error: 'Cannot dispense product in payment state.'

handlePayment
"Implement the behavior for handling payment"
^ self error: 'Payment already made in payment state.'

handleProductSelection
"Transition to ProductSelectionState"
^ self vendingMachine setState: ProductSelectionState

handleReturnChange
"Implement the behavior for handling return change"
^ self error: 'Cannot return change in payment state.'

handleNoProductSelected
"Implement the behavior for handling no product selected"
^ self error: 'No product selected in payment state.'

handleNoPayment
"Implement the behavior for handling no payment"
^ self vendingMachine setState: IdleState

handleNoChangeReturned
"Implement the behavior for handling no change returned"
^ self error: 'No change returned in payment state.'

Class: ProductSelectionState

Superclass: VendingMachineState

Instance Methods
dispenseProduct
"Implement the behavior for dispensing a product"
^ self error: 'Cannot dispense product in product selection state.'

handlePayment
"Implement the behavior for handling payment"
^ self error: 'Payment already made in product selection state.'

handleProductSelection
"Implement the behavior for handling product selection"
^ self error: 'Product already selected in product selection state.'

handleReturnChange
"Transition to ReturnChangeState"
^ self vendingMachine setState: ReturnChangeState

handleNoProductSelected
"Implement the behavior for handling no product selected"
^ self error: 'No product selected in product selection state.'

handleNoPayment
"Implement the behavior for handling no payment"
^ self error: 'No payment made in product selection state.'

handleNoChangeReturned
"Implement the behavior for handling no change returned"
^ self error: 'No change returned in product selection state.'

3. 实现自动售货机类

自动售货机类将维护当前状态，并允许状态之间的转换。

smalltalk
Class: VendingMachine

Superclass: Object

Instance Variables
state: VendingMachineState

Class Methods
+new
^ super new

Instance Methods
setState: newState
"Set the state of the vending machine"
self state: newState

dispenseProduct
"Call the dispenseProduct method of the current state"
^ self state dispenseProduct

handlePayment
"Call the handlePayment method of the current state"
^ self state handlePayment

handleProductSelection
"Call the handleProductSelection method of the current state"
^ self state handleProductSelection

handleReturnChange
"Call the handleReturnChange method of the current state"
^ self state handleReturnChange

handleNoProductSelected
"Call the handleNoProductSelected method of the current state"
^ self state handleNoProductSelected

handleNoPayment
"Call the handleNoPayment method of the current state"
^ self state handleNoPayment

handleNoChangeReturned
"Call the handleNoChangeReturned method of the current state"
^ self state handleNoChangeReturned

4. 测试【11】自动售货机

我们可以通过一系列的测试来验证自动售货机的状态管理是否正确。

smalltalk
vm := VendingMachine new.
vm setState: IdleState.
vm handlePayment.
vm handleProductSelection.
vm handlePayment.
vm dispenseProduct.
vm handleReturnChange.
vm handleNoChangeReturned.

总结

通过使用Smalltalk语言和状态模式，我们成功地实现了自动售货机的状态管理。状态模式使得自动售货机的代码更加模块化【12】和可维护，同时也提高了代码的可读性【13】。在实际应用中，状态模式可以用于处理复杂的业务逻辑，使得代码更加灵活和易于扩展【14】。

本文仅展示了状态模式在Smalltalk语言中的基本实现，实际应用中可能需要根据具体需求进行更复杂的扩展和优化【15】。