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
这个知识是基于我正在学习Java,虽然Java和Javascript是不同的,但单例的概念和Java如何做到这一点是一样的。在我看来,JS的类样式本身是干净的,而不是var初始化。
class Singleton {
// use hashtag which entails that the variable can only be accessed from self scope
static #instance = null;
static getInstance() {
if (this.#instance === null) this.#instance = new Singleton();
return this.#instance;
}
// some class property
hello = 'world';
// or initialize the variable in the constructor, depend on your preference
constructor() {
// this.hello = 'world';
}
/* you can also add parameters on the constructor & getInstance
* e.g.
* static getInstance(param1, param2) {...new Singleton(param1, param2)}
* constructor(param1, param2) {...}
*/
}
// this is the same code for java and normal way for singleton for class
// just use static so you can get instance
// testing the singleton
var s1,s2;
s1 = Singleton.getInstance();
s2 = Singleton.getInstance();
// you cannot access the property, immediately
if (Singleton.hello === undefined) console.log('getInstance so you can access this');
console.log(s1.hello);
// result: "world"
console.log(s2.hello);
// result: "world"
// set the value of Singleton object
s2.hello = "hi";
console.log(s1.hello);
// result: "hi"
console.log(s2.hello);
// result: "hi"
// this is just an evidence which means that they are the same even in property level
if (s1 === s2) console.log("S1 & S2 is the same object");
// result: "S1 & S2 is the same object"
// don't use something like `var s1 = new Singleton();`
// this will defeat your purpose of just (1 object), one instance of class
其他回答
简短的回答:
由于JavaScript的非阻塞特性,JavaScript中的单例在使用时非常难看。全局变量将在整个应用程序中为您提供一个实例,而不需要所有这些回调,模块模式将内部隐藏在接口后面。参见Christian C. Salvadó的回答。
但是,既然你想要一个单胞胎……
var singleton = function(initializer) {
var state = 'initial';
var instance;
var queue = [];
var instanceReady = function(createdInstance) {
state = 'ready';
instance = createdInstance;
while (callback = queue.shift()) {
callback(instance);
}
};
return function(callback) {
if (state === 'initial') {
state = 'waiting';
queue.push(callback);
initializer(instanceReady);
} else if (state === 'waiting') {
queue.push(callback);
} else {
callback(instance);
}
};
};
用法:
var singletonInitializer = function(instanceReady) {
var preparedObject = {property: 'value'};
// Calling instanceReady notifies the singleton that the instance is ready to use
instanceReady(preparedObject);
}
var s = singleton(singletonInitializer);
// Get the instance and use it
s(function(instance) {
instance.doSomething();
});
解释:
单例在整个应用程序中提供了不止一个实例:它们的初始化延迟到第一次使用。当您处理初始化开销很大的对象时,这确实是一个大问题。昂贵通常意味着I/O,在JavaScript中I/O总是意味着回调。
不要相信那些给出instance = singleton.getInstance()这样接口的答案,它们都没有抓住重点。
如果它们没有在实例就绪时运行回调,那么当初始化器执行I/O时,它们将不起作用。
是的,回调看起来总是比函数调用更丑,函数调用会立即返回一个对象实例。但是同样:当你执行I/O时,回调是必须的。如果你不想做任何I/O,那么在程序启动时实例化就足够便宜了。
例1,简单的初始化器:
var simpleInitializer = function(instanceReady) {
console.log("Initializer started");
instanceReady({property: "initial value"});
}
var simple = singleton(simpleInitializer);
console.log("Tests started. Singleton instance should not be initalized yet.");
simple(function(inst) {
console.log("Access 1");
console.log("Current property value: " + inst.property);
console.log("Let's reassign this property");
inst.property = "new value";
});
simple(function(inst) {
console.log("Access 2");
console.log("Current property value: " + inst.property);
});
例2,初始化I/O:
在这个例子中,setTimeout模拟了一些昂贵的I/O操作。这说明了为什么JavaScript中的单例真的需要回调。
var heavyInitializer = function(instanceReady) {
console.log("Initializer started");
var onTimeout = function() {
console.log("Initializer did his heavy work");
instanceReady({property: "initial value"});
};
setTimeout(onTimeout, 500);
};
var heavy = singleton(heavyInitializer);
console.log("In this example we will be trying");
console.log("to access singleton twice before it finishes initialization.");
heavy(function(inst) {
console.log("Access 1");
console.log("Current property value: " + inst.property);
console.log("Let's reassign this property");
inst.property = "new value";
});
heavy(function(inst) {
console.log("Access 2. You can see callbacks order is preserved.");
console.log("Current property value: " + inst.property);
});
console.log("We made it to the end of the file. Instance is not ready yet.");
这就是我如何使用ES6特性实现单例模式。是的,我知道这看起来不像面向对象的方法,但我发现这个方法很容易实现,而且是一种干净的实现方式。
const Singleton = (() => {
var _instance = !_instance && new Object('Object created....');
return () => _instance;
})();
//************************************************************************
var instance1 = Singleton();
var instance2 = Singleton();
console.log(instance1 === instance2); // true
let MySingleton = (function () {
var _instance
function init() {
if(!_instance) {
_instance = { $knew: 1 }
}
return _instance
}
let publicAPIs = {
getInstance: function() {
return init()
}
}
// this prevents customize the MySingleton, like MySingleton.x = 1
Object.freeze(publicAPIs)
// this prevents customize the MySingleton.getInstance(), like MySingleton.getInstance().x = 1
Object.freeze(publicAPIs.getInstance())
return publicAPIs
})();
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; } }; })();
简单地使用类表达式:
const singleton = new (class {
hello() { return 'world'; }
})();
console.log(singleton.hello()); //=> world
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