定义

定义一系列的算法，把它们一个个封装起来，并且使它们可以相互替换。

实现

思路：创建表示各种策略的对象，和一个行为随着策略对象改变而改变的 context 对象。

一个简单的加减乘例子：

interface Compute<T> {
  computeF (num1: T, num2: T): T
}

// 创建策略对象
class ComputeAdd implements Compute<number> {
  public computeF (
    num1: number,
    num2: number
  ): number {
    return num1 + num2
  }
}

class ComputeSub implements Compute<number> {
  public computeF (
    num1: number,
    num2: number
  ): number {
    return num1 - num2
  }
}

class ComputeMul implements Compute<String> {
  public computeF (
    num1: String,
    num2: String
  ): String {
    return `${num1} + ${num2}`
  }
}

// 创建行为类
class Context {
  public compute: Compute<any>

  public constructor (compute: Compute<any>) {
    this.compute = compute
  }

  public excuteCompute (
    num1: number,
    num2: number
  ): number {
    return this.compute.computeF(num1, num2)
  }
}

let context1 = new Context(new ComputeAdd())
                   .excuteCompute(1, 2)
let context2 = new Context(new ComputeSub())
                   .excuteCompute(1, 2)
let content3 = new Context(new ComputeMul())
                   .excuteCompute(1, 2)

console.log(context1, context2, content3) // 3, -1, 1 + 2

复杂一点的例子（打怪）：

// 武器接口
interface Weapon {
  useWeapon (): void
}

// 技能接口
interface Skill {
  useSkill (): void
}

// 武器类
class Gun implements Weapon {
  public useWeapon () {
    console.log( 'Weapon: Gun')
  }
}

class Stick implements Weapon {
  public useWeapon () {
    console.log('Weapon: Stick')
  }
}

// 技能类
class Magic implements Skill {
  public useSkill () {
    console.log('Skill: Magic')
  }
}

class Kongfu implements Skill {
  public useSkill () {
    console.log('Skill: Chinese Kongfu')
  }
}

// 抽象类，用于给其他类继承
abstract class Way {
  // 武器
  public weapon: Weapon
  
  // 技能
  public skill: Skill

 // 设置武器
  public setWeapon (weapon: Weapon): void {
    this.weapon = weapon
  }
  
  // 设置技能
  public setSkill (skill: Skill): void {
    this.skill = skill
  }

  public getWeaponAndSkill (): void {
    this.weapon.useWeapon()
    this.skill.useSkill()
  }

  // 抽象方法
  public abstract saySome (): void
}

class SimpleWay extends Way {
  public constructor () {
    super()
    this.weapon = new Gun()
    this.skill = new Magic()
  }

  public saySome () {
    console.log('屠龙宝刀，点击就送')
  }
}

const way = new SimpleWay()
way.saySome()

console.log('=======')

way.getWeaponAndSkill()

console.log('=======')

way.setWeapon(new Stick)
way.setSkill(new Kongfu)
way.getWeaponAndSkill()