用JavaScript实现单例模式的最简单/最干净的方法是什么?
当前回答
function Once() {
return this.constructor.instance || (this.constructor.instance = this);
}
function Application(name) {
let app = Once.call(this);
app.name = name;
return app;
}
如果你在上课:
class Once {
constructor() {
return this.constructor.instance || (this.constructor.instance = this);
}
}
class Application extends Once {
constructor(name) {
super();
this.name = name;
}
}
测试:
console.log(new Once() === new Once());
let app1 = new Application('Foobar');
let app2 = new Application('Barfoo');
console.log(app1 === app2);
console.log(app1.name); // Barfoo
其他回答
最明确的答案应该是Addy Osmani所著的《学习JavaScript设计模式》一书。
var mySingleton = (function () { // Instance stores a reference to the singleton var instance; function init() { // Singleton // Private methods and variables function privateMethod(){ console.log( "I am private" ); } var privateVariable = "I'm also private"; var privateRandomNumber = Math.random(); return { // Public methods and variables publicMethod: function () { console.log( "The public can see me!" ); }, publicProperty: "I am also public", getRandomNumber: function() { return privateRandomNumber; } }; }; return { // Get the singleton instance if one exists // or create one if it doesn't getInstance: function () { if ( !instance ) { instance = init(); } return instance; } }; })();
您可以在每次新的执行中返回相同的实例-
function Singleton() {
// lazy
if (Singleton.prototype.myInstance == undefined) {
Singleton.prototype.myInstance = { description: "I am the instance"};
}
return Singleton.prototype.myInstance;
}
a = new Singleton();
b = new Singleton();
console.log(a); // { description: "I am the instance"};
console.log(b); // { description: "I am the instance"};
console.log(a==b); // true
JavaScript中的单例是使用模块模式和闭包实现的。
下面的代码是不言自明的
// Singleton example.
var singleton = (function() {
var instance;
function init() {
var privateVar1 = "this is a private variable";
var privateVar2 = "another var";
function pubMethod() {
// Accessing private variables from inside.
console.log(this.privateVar1);
console.log(this.privateVar2);
console.log("inside of a public method");
};
}
function getInstance() {
if (!instance) {
instance = init();
}
return instance;
};
return {
getInstance: getInstance
}
})();
var obj1 = singleton.getInstance();
var obj2 = singleton.getInstance();
console.log(obj1 === obj2); // Check for type and value.
下面是我实现单例模式的演练片段。这是我在面试过程中想到的,我觉得我应该在某个地方捕捉到这一点。
/*************************************************
* SINGLETON PATTERN IMPLEMENTATION *
*************************************************/
// Since there aren't any classes in JavaScript, every object
// is technically a singleton if you don't inherit from it
// or copy from it.
var single = {};
// Singleton Implementations
//
// Declaring as a global object...you are being judged!
var Logger = function() {
// global_log is/will be defined in the GLOBAL scope here
if(typeof global_log === 'undefined'){
global_log = this;
}
return global_log;
};
// The below 'fix' solves the GLOABL variable problem, but
// the log_instance is publicly available and thus can be
// tampered with.
function Logger() {
if(typeof Logger.log_instance === 'undefined') {
Logger.log_instance = this;
}
return Logger.log_instance;
};
// The correct way to do it to give it a closure!
function logFactory() {
var log_instance; // Private instance
var _initLog = function() { // Private init method
log_instance = 'initialized';
console.log("logger initialized!")
}
return {
getLog : function(){ // The 'privileged' method
if(typeof log_instance === 'undefined') {
_initLog();
}
return log_instance;
}
};
}
/***** TEST CODE ************************************************
// Using the Logger singleton
var logger = logFactory(); // Did I just give LogFactory a closure?
// Create an instance of the logger
var a = logger.getLog();
// Do some work
// Get another instance of the logger
var b = logger.getLog();
// Check if the two logger instances are same
console.log(a === b); // true
*******************************************************************/
同样可以在我的主旨页面上找到。
我认为最简单的方法是声明一个简单的对象文字:
var myInstance = {
method1: function () {
// ...
},
method2: function () {
// ...
}
};
如果你想在你的单例实例上有私有成员,你可以这样做:
var myInstance = (function() {
var privateVar = '';
function privateMethod () {
// ...
}
return { // public interface
publicMethod1: function () {
// All private members are accessible here
},
publicMethod2: function () {
}
};
})();
这被称为模块模式,它基本上允许您通过使用闭包来封装对象上的私有成员。
如果你想阻止对单例对象的修改,你可以使用ES5 object .freeze方法冻结它。
这将使对象不可变,防止对其结构和值进行任何修改。
如果你正在使用ES6,你可以很容易地使用ES模块来表示一个单例,你甚至可以通过在模块范围内声明变量来保持私有状态:
// my-singleton.js
const somePrivateState = []
function privateFn () {
// ...
}
export default {
method1() {
// ...
},
method2() {
// ...
}
}
然后你可以简单地导入单例对象来使用它:
import myInstance from './my-singleton.js'
// ...
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