js design patterns: what is the template method pattern? Introduction to js template method pattern

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What is the template method pattern?

Definition: Define the skeleton of an algorithm in an operation, while deferring some steps to subclasses. Template methods allow subclasses to redefine specific steps of an algorithm without changing the structure of the algorithm.

Main solution: Some methods are common, but this method is rewritten in every subclass.

When to use: There are some general methods.

How to solve: Abstract these general algorithms.

js template method pattern application example: 1. When building a house, the foundation, wiring, and water pipes are the same. Only in the later stages of construction are there differences such as adding closets and fences. 2. The 81 difficulties set by Bodhisattva in Journey to the West are a top-level logical framework.

Advantages of js template method pattern: 1. Encapsulate the constant parts and expand the variable parts. 2. Extract public code to facilitate maintenance. 3. Behavior is controlled by the parent class and implemented by the subclass.

Disadvantages of the js template method pattern: Each different implementation requires a subclass to implement, resulting in an increase in the number of classes and making the system larger.

js template method pattern usage scenarios: 1. There are multiple methods common to subclasses, and the logic is the same. 2. Important and complex methods can be considered as template methods.

Making tea and brewing coffee

Let’s compare the similarities and differences in the process of making tea and coffee

##1 Boil water Boil water2Steep teaBrew coffee3Pour into cupPour into the cup4Add lemonAdd sugar

Steps	MakeTea	Make coffee

It can be clearly seen that there are only subtle differences in steps 2 and 4, let’s start with the implementation ：

const Drinks = function() {}
Drinks.prototype.firstStep = function() {
  console.log('烧开水')
}
Drinks.prototype.secondStep = function() {}
Drinks.prototype.thirdStep = function() {
  console.log('倒入杯子')
}
Drinks.prototype.fourthStep = function() {}
Drinks.prototype.init = function() { // 模板方法模式核心：在父类上定义好执行算法
  this.firstStep()
  this.secondStep()
  this.thirdStep()
  this.fourthStep()
}
const Tea = function() {}
Tea.prototype = new Drinks
Tea.prototype.secondStep = function() {
  console.log('浸泡茶叶')
}
Tea.prototype.fourthStep = function() {
  console.log('加柠檬')
}
const Coffee = function() {}
Coffee.prototype = new Drinks
Coffee.prototype.secondStep = function() {
  console.log('冲泡咖啡')
}
Coffee.prototype.fourthStep = function() {
  console.log('加糖')
}
const tea = new Tea()
tea.init()
// 烧开水
// 浸泡茶叶
// 倒入杯子
// 加柠檬
const coffee = new Coffee()
coffee.init()
// 烧开水
// 冲泡咖啡
// 倒入杯子
// 加糖

Hook

What if the guest does not want to add condiments (sugar, lemon), then you can introduce a hook to achieve it, the implementation logic is as follows:

// ...

Drinks.prototype.ifNeedFlavour = function() { // 加上钩子
  return true
}

Drinks.prototype.init = function() { // 模板方法模式核心：在父类上定义好执行算法
  this.firstStep()
  this.secondStep()
  this.thirdStep()
  if (this.ifNeedFlavour()) { // 默认是 true，也就是要加调料
    this.fourthStep()
  }
}

// ...
const Coffee = function() {}

Coffee.prototype = new Drinks()
// ...

Coffee.prototype.ifNeedFlavour = function() {
  return window.confirm('是否需要佐料吗？') // 弹框选择是否佐料
}

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