什麽是策略模式？

先看代碼片段1。

// 代碼片段1
var bonus = new Bonus();
bonus.setSalary(10000);
bonus.setStrategy(new performanceS());
console.log('bounsS' ,bonus.getBonus());
// => 40000

bonus.setStrategy(new performanceA());
console.log('bounsA',bonus.getBonus());
// => 30000

bonus是一個對象，而對象自帶上下文。
這個對象在運行的不同階段，通過setStrategy

下面的代碼片段2是代碼片段1的定義。

// 代碼片段2
var performanceS = function () { };
performanceS.prototype.calculate = function (salary) {
    return salary * 4;
};

var performanceA = function () { };
performanceA.prototype.calculate = function (salary) {
    return salary * 3;
};

var performanceB = function () { };
performanceB.prototype.calculate = function (salary) {
    return salary * 2;
};


var Bonus = function () {
    this.salary = null;  
    this.strategy = null;
};

Bonus.prototype.setSalary = function (salary) {
    this.salary = salary; 
};

Bonus.prototype.setStrategy = function (strategy) {
    this.strategy = strategy;
};


Bonus.prototype.getBonus = function () { 
    return this.strategy.calculate(this.salary); 
};
 

改進的策略模式

同樣的bonus.getBonus()，卻輸出結果不同。
當業務變得復雜，這會導致代碼難以預測。
我們稍微改進一下，讓對象初始化時接收一個策略對象，並且設置策略屬性不可修改。

// 代碼片段3
var bonusFactory = function(strategy){
    this.salary = null;  
    Object.defineProperty(this, 'strategy',{
        value: strategy,
        writable: false,
        configurable:false,
    })
};

bonusFactory.prototype.setSalary = function (salary) {
    this.salary = salary; 
};

bonusFactory.prototype.getBonus = function () { 
    return this.strategy.calculate(this.salary); 
};

var bonusS = new bonusFactory(new performanceS());
bonusS.setSalary(10000);
bonusS.strategy = 11;
console.log('bonusS', bonusS.getBonus());


var bonusA = new bonusFactory(new performanceA());
bonusA.setSalary(10000);
console.log('bonusA', bonusA.getBonus());
 

策略模式的函數式寫法

這裏使用了名為ramda的函數式工具庫。

var R = require('ramda');

var salaryS = function(salary) {
    return salary * 4;
};

var salaryA = function(salary) {
    return salary * 3;
};

var salaryB = function(salary) {
    return salary * 2;
};

var getBonus = function(salary, strategy){
    return strategy(salary); 
};

var getBonusFacotry = R.curry(getBonus);
var getBonusS = getBonusFacotry(R.__, salaryS);
var getBonusA = getBonusFacotry(R.__, salaryA);
var getBonusB = getBonusFacotry(R.__, salaryB);

var getBouns1000 = getBonusFacotry(1000, R.__);

console.log('封裝獎金計算方式的策略');

console.log(getBonusS(1000));
console.log(getBonusA(1000));
console.log(getBonusB(1000));

console.log('封裝獎金數目的策略');

console.log(getBouns1000(salaryS));
console.log(getBouns1000(salaryA));
console.log(getBouns1000(salaryB));

可以看到函數式寫法更加靈活和簡潔。

JavaScript形而上的策略模式