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命令

命令在 TypeScript 中的实现

命令是一种行为设计模式 它可将请求或简单操作转换为一个对象

此类转换让你能够延迟进行或远程执行请求 还可将其放入队列中

在 TypeScript 中使用模式

复杂度

流行度

使用示例 命令模式在 TypeScript 代码中很常见 大部分情况下 它被用于代替包含行为的参数化 UI 元素的回调函数 此外还被用于对任务进行排序和记录操作历史记录等

识别方法 命令模式可以通过抽象或接口类型 发送者 中的行为方法来识别 该类型调用另一个不同的抽象或接口类型 接收者 实现中的方法 该实现则是在创建时由命令模式的实现封装 命令类通常仅限于一些特殊行为

概念示例

本例说明了命令设计模式的结构并重点回答了下面的问题

  • 它由哪些类组成
  • 这些类扮演了哪些角色
  • 模式中的各个元素会以何种方式相互关联

index.ts: 概念示例

/**
 * The Command interface declares a method for executing a command.
 */
interface Command {
    execute(): void;
}

/**
 * Some commands can implement simple operations on their own.
 */
class SimpleCommand implements Command {
    private payload: string;

    constructor(payload: string) {
        this.payload = payload;
    }

    public execute(): void {
        console.log(`SimpleCommand: See, I can do simple things like printing (${this.payload})`);
    }
}

/**
 * However, some commands can delegate more complex operations to other objects,
 * called "receivers."
 */
class ComplexCommand implements Command {
    private receiver: Receiver;

    /**
     * Context data, required for launching the receiver's methods.
     */
    private a: string;

    private b: string;

    /**
     * Complex commands can accept one or several receiver objects along with
     * any context data via the constructor.
     */
    constructor(receiver: Receiver, a: string, b: string) {
        this.receiver = receiver;
        this.a = a;
        this.b = b;
    }

    /**
     * Commands can delegate to any methods of a receiver.
     */
    public execute(): void {
        console.log('ComplexCommand: Complex stuff should be done by a receiver object.');
        this.receiver.doSomething(this.a);
        this.receiver.doSomethingElse(this.b);
    }
}

/**
 * The Receiver classes contain some important business logic. They know how to
 * perform all kinds of operations, associated with carrying out a request. In
 * fact, any class may serve as a Receiver.
 */
class Receiver {
    public doSomething(a: string): void {
        console.log(`Receiver: Working on (${a}.)`);
    }

    public doSomethingElse(b: string): void {
        console.log(`Receiver: Also working on (${b}.)`);
    }
}

/**
 * The Invoker is associated with one or several commands. It sends a request to
 * the command.
 */
class Invoker {
    private onStart: Command;

    private onFinish: Command;

    /**
     * Initialize commands.
     */
    public setOnStart(command: Command): void {
        this.onStart = command;
    }

    public setOnFinish(command: Command): void {
        this.onFinish = command;
    }

    /**
     * The Invoker does not depend on concrete command or receiver classes. The
     * Invoker passes a request to a receiver indirectly, by executing a
     * command.
     */
    public doSomethingImportant(): void {
        console.log('Invoker: Does anybody want something done before I begin?');
        if (this.isCommand(this.onStart)) {
            this.onStart.execute();
        }

        console.log('Invoker: ...doing something really important...');

        console.log('Invoker: Does anybody want something done after I finish?');
        if (this.isCommand(this.onFinish)) {
            this.onFinish.execute();
        }
    }

    private isCommand(object): object is Command {
        return object.execute !== undefined;
    }
}

/**
 * The client code can parameterize an invoker with any commands.
 */
const invoker = new Invoker();
invoker.setOnStart(new SimpleCommand('Say Hi!'));
const receiver = new Receiver();
invoker.setOnFinish(new ComplexCommand(receiver, 'Send email', 'Save report'));

invoker.doSomethingImportant();

Output.txt: 执行结果

Invoker: Does anybody want something done before I begin?
SimpleCommand: See, I can do simple things like printing (Say Hi!)
Invoker: ...doing something really important...
Invoker: Does anybody want something done after I finish?
ComplexCommand: Complex stuff should be done by a receiver object.
Receiver: Working on (Send email.)
Receiver: Also working on (Save report.)

命令在其他编程语言中的实现

命令在 Java 中的实现 命令在 C# 中的实现 命令在 PHP 中的实现 命令在 Python 中的实现 命令在 Ruby 中的实现 命令在 Swift 中的实现