单例模式和工厂模式.md

单例模式

• 类只有一个实例
• 可以绑定在全局实例，也可以通过闭包来私有变量，还可以通过es6的symbol

  var Singleton = function(name) {
    this.name = name
    this.instance = null
  }

  Singleton.getInstance = function(name) {
    if(!this.instance) {
      this.instance = new Singleton(name)
    }
    return this.instance
  }

  var instance1 = Singleton.getInstance('a')
  var instance2 = Singleton.getInstance('b')
  console.log(instance1 === instance2)

  //使用new关键字实现单例
  var Singleton = (function(name) {
    let instance = null
    
    return function(name) {
        if(!instance) {
          this.name = name
          instance = this
        }
      return instance
    }
  })()
  
  var instance1 = new Singleton('a')
  var instance2 = new Singleton('b')
  console.log(instance1 === instance2)

  //惰性单例
  var Singleton = function(fn) {
    let instance;
    return function() {
      return instance || (instance = fn.apply(this, arguments))
    }
  }

  function createObj(name) {
    return {
      name: name
    }
  }

  var createSingleObj = Singleton(createObj)
  var instance1 = createSingleObj('a')
  var instance2 = createSingleObj('b')
  console.log(instance1 === instance2)

工厂模式

• 工厂模式用来创建对象，实用场景
  • 对象构建十分复杂
  • 需要依赖具体环境创建不同实例
  • 处理大量具有相同属性的小对象

  var Factory = (function () {
    let instance = function(name) {
      this.name = name
    }
    return function(name) {
      return new instance(name)
    }
  })()

  var f1 = new Factory('a')
  var f2 = new Factory('b')
  console.log(f1 === f2)