Smalltalk【1】 语言中策略模式【2】的实际应用
策略模式是一种行为设计模式，它定义了一系列算法，并将每一个算法封装【3】起来，使它们可以互相替换。策略模式让算法的变化独立于使用算法的客户。本文将探讨在Smalltalk语言中如何实现策略模式，并通过一个实际案例来展示其应用。
Smalltalk 简介
Smalltalk是一种面向对象的编程语言，它由Alan Kay等人于1970年代初期设计。Smalltalk以其简洁、直观和易于学习而闻名。在Smalltalk中，对象是核心，所有的操作都是通过对象来完成的。
策略模式概述
策略模式的主要目的是将算法的封装与使用算法的客户解耦【4】。在策略模式中，通常包含以下角色：
- Context【5】（环境类）：使用算法的类，它维持一个策略对象的引用。
- Strategy【6】（策略接口）：定义所有支持的算法的公共接口。
- ConcreteStrategy【7】（具体策略类）：实现算法接口的具体类。
Smalltalk 中策略模式的实现
以下是一个简单的Smalltalk示例，展示了如何实现策略模式。
1. 定义策略接口
smalltalk
Strategy := class {
initialize
|name|
name: 'Strategy'
doSomething: anObject
"Strategy method to be implemented by concrete strategies."
"Default implementation is to do nothing."
end
}
2. 实现具体策略
smalltalk
ConcreteStrategyA := class inherits Strategy {
initialize
super initialize
name: 'ConcreteStrategyA'
doSomething: anObject
"Implementation of ConcreteStrategyA"
anObject << "ConcreteStrategyA is doing something with: " << anObject.
end
}
ConcreteStrategyB := class inherits Strategy {
initialize
super initialize
name: 'ConcreteStrategyB'
doSomething: anObject
"Implementation of ConcreteStrategyB"
anObject << "ConcreteStrategyB is doing something with: " << anObject.
end
}
3. 定义环境类
smalltalk
Context := class {
initialize
|strategy|
strategy: nil.
initializeStrategy: aStrategy
strategy: aStrategy.
doSomething
"Context uses a strategy to perform an operation."
strategy doSomething: self.
end
}
4. 使用策略模式
smalltalk
context := Context new.
context initializeStrategy: ConcreteStrategyA new.
context doSomething.
"Output: ConcreteStrategyA is doing something with: Context"
context initializeStrategy: ConcreteStrategyB new.
context doSomething.
"Output: ConcreteStrategyB is doing something with: Context"
实际应用案例
以下是一个在Smalltalk中应用策略模式的实际案例：不同类型的排序算法。
1. 定义排序策略【8】接口
smalltalk
SortStrategy := class {
initialize
|name|
name: 'SortStrategy'
sort: anArray
"Sort method to be implemented by concrete strategies."
"Default implementation is to return the array unchanged."
anArray.
end
}
2. 实现具体排序策略
smalltalk
BubbleSortStrategy := class inherits SortStrategy {
initialize
super initialize
name: 'BubbleSortStrategy'
sort: anArray
"Implementation of Bubble Sort"
(1 to: anArray size - 1) do: [ |i|
(anArray size - i) do: [ |j|
(j > 1 and: [anArray at: j - 1 > anArray at: j]) ifTrue: [
anArray swapAt: j - 1 with: j.
].
].
].
anArray.
end
}
SelectionSortStrategy := class inherits SortStrategy {
initialize
super initialize
name: 'SelectionSortStrategy'
sort: anArray
"Implementation of Selection Sort"
(1 to: anArray size - 1) do: [ |i|
|minIndex|
minIndex: i.
(i + 1 to: anArray size) do: [ |j|
(anArray at: j < anArray at: minIndex) ifTrue: [minIndex: j].
].
anArray swapAt: i with: minIndex.
].
anArray.
end
}
3. 使用排序策略
smalltalk
array := [5, 3, 8, 4, 1].
sortContext := Context new.
sortContext initializeStrategy: BubbleSortStrategy new.
sortedArray := sortContext sort: array.
"Output: sortedArray: [1, 3, 4, 5, 8]"
sortContext initializeStrategy: SelectionSortStrategy new.
sortedArray := sortContext sort: array.
"Output: sortedArray: [1, 3, 4, 5, 8]"
总结
在Smalltalk语言中，策略模式可以有效地将算法的封装与使用算法的客户解耦。通过定义策略接口和具体策略类，我们可以轻松地替换和扩展算法。本文通过两个案例展示了策略模式在Smalltalk中的实际应用，希望能为读者提供一些启示。
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