/**
 * pre 单例模式
 * 定义：保证一个类仅有一个实例，并提供一个访问它的全局访问点
 * 应用：单例模式是一种常用的模式，有一些对象我们往往只需要一个，
 * 比如线程池、全局缓存、浏览器中的 window 对象等。
 */
//--------------singleton-01-------------
/*写法1*/
var Singleton = function(name){
 this.name = name;
 this.instance = null;
};

Singleton.prototype.getName = function(){
 alert(this.name);
};

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

var a = Singleton.getInstance("amy");
var b = Singleton.getInstance("ben");
alert(a === b);

// ------------singleton-02----------------
/*写法2*/
var Singleton = function(name){
 this.name = name;
}
Singleton.prototype.getName = function(){
 return this.name;
}

Singleton.getInstance = (function(){
 var instance = null;
 return function(name){
  if(!instance){
   instance = new Singleton(name);
  }
  return instance;
 }
})();

var a = Singleton.getInstance("amy");
var b = Singleton.getInstance("ben");
alert(a === b);

// ------------singleton03-----------
/*写法3*/
var Singleton = (function(){
 var instance;
 return function(name){
  if(instance){
   return instance;
  }
  this.name = name;
  instance = this;
 }
})();
var a = new Singleton("amy");
var b = new Singleton("ben");
alert(a === b);

//---------------- 示例 ---------------
var getSingleton = function(fn) {
 var result;
 return function() {
  if(!result) {
   result = fn.apply(this, arguments);
  }
  return result;
 }
};

var getSingletonVip = (function() {
 var instance;
 return function(fn) {
  return instance || (instance = fn.apply(this, arguments));
 }
})();

var createLoginUser = function() {
 var div = document.createElement("div");
 div.innerHTML = '这是登录框';
 document.body.appendChild(div);
 return div;
};

var createInfoGrid = function() {
 var div = document.createElement("div");
 div.innerHTML = '这是列表信息框';
 document.body.appendChild(div);
 return div;
};
//--执行singleton1 
var createUserDiv = getSingleton(createLoginUser);
createUserDiv();
createUserDiv();

//--执行singleton2
getSingletonVip(createLoginUser);
getSingletonVip(createLoginUser);