# 命令模式

> Command Pattern

## 知识点一：定义

1. 把一系列方法调用、请求或操作封装到单一对象，根据不同的请求对客户端进行参数化或传递可执行的方法调用。
2. 典型的Command模式需要有一个接口，接口中有一个统一的方法(excute/run)，这就是"将命令/请求封装为对象"。

## 知识点二：具体例子

![image](/files/-LRk4VrJ-qC-EG2saJD8)

***Client*** 客户端，执行run方法 1. 引用Receiver对象

***Receiver*** 接收者，Calculator对象 1. 知道怎样操作相关的命令 1. （有时候）维护一个执行命令历史

***Command*** 命令类，Command类 1. 负责根据怎样的动作，执行怎样的命令 command模式，最主要与抽象类思想有关，作为一个抽象类，定义一个接口，不一定为它所有成员函数提供实现。它只作为一个类基，派生出其他类（AddCommand、SubCommand等）。实现缺失功能的派生类称为具体命令类(Concrete Command)。

***Invoker*** 调用者，具体执行calculator.execute(new AddCommand(100)); 1. 询问执行请求

```javascript
function add(x, y) { return x + y; }
function sub(x, y) { return x - y; }
function mul(x, y) { return x * y; }
function div(x, y) { return x / y; }

//Command 
var Command = function (execute, undo, value) {
    this.execute = execute;
    this.undo = undo;
    this.value = value;
}

//Concrete Command  
var AddCommand = function (value) {
    return new Command(add, sub, value);
};

var SubCommand = function (value) {
    return new Command(sub, add, value);
};

var MulCommand = function (value) {
    return new Command(mul, div, value);
};

var DivCommand = function (value) {
    return new Command(div, mul, value);
};

// Receiver
var Calculator = function () {
    //执行结果
    var current = 0;
    //执行历史
    var commands = [];

    function action(command) {
        var name = command.execute.name;
        return name.charAt(0).toUpperCase() + name.slice(1);
    }

    return {
        execute: function (command) {
            current = command.execute(current, command.value);
            commands.push(command);
            log.add(action(command) + ": " + command.value);
        },

        undo: function () {
            var command = commands.pop();
            current = command.undo(current, command.value);
            log.add("Undo " + action(command) + ": " + command.value);
        },

        getCurrentValue: function () {
            return current;
        }
    }
}

// log helper

var log = (function () {
    var log = "";

    return {
        add: function (msg) { log += msg + "\n"; },
        show: function () { console.log(log); log = ""; }
    }
})();

//Client 
function run() {
    var calculator = new Calculator();

    // issue commands

    calculator.execute(new AddCommand(100));
    calculator.execute(new SubCommand(24));
    calculator.execute(new MulCommand(6));
    calculator.execute(new DivCommand(2));

    // reverse last two commands

    calculator.undo();
    calculator.undo();

    log.add("\nValue: " + calculator.getCurrentValue());
    log.show();
}

run();
```

## 知识点三：特点

设计思想：分离职责，包括执行命令、发布命令委托给不同的对象。使用一个简单的命令把具体命令（Concrete Command）和调用方法（receiver的execute方法）绑定在一起。

优点： 1. 具有相同接口的Command子对象（Concrete Command）可以根据需要轻松交互 1. 扩展/维护子对象（Concrete Command）变得非常容易，几乎不用改大量代码 1. 你可以创建一个具有队列系统的命令队列，例如上面的的命令具有撤销命令功能

缺点： 1. 必须要使用很多类目和对象才能完成目标，开发者必须要小心正确地开发这些类 1. 每一个独立的命令都是一个具体命令（Concrete Command）类，这样会增加大量的类去实现方法和维护


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