是否可以在 JavaScript 中模拟抽象基类?最优雅的方法是什么?
说,我想做类似的事情: -
var cat = new Animal('cat');
var dog = new Animal('dog');
cat.say();
dog.say();
它应该输出:'bark','meow'
是否可以在 JavaScript 中模拟抽象基类?最优雅的方法是什么?
说,我想做类似的事情: -
var cat = new Animal('cat');
var dog = new Animal('dog');
cat.say();
dog.say();
它应该输出:'bark','meow'
创建抽象类的一种简单方法是:
/**
@constructor
@abstract
*/
var Animal = function() {
if (this.constructor === Animal) {
throw new Error("Can't instantiate abstract class!");
}
// Animal initialization...
};
/**
@abstract
*/
Animal.prototype.say = function() {
throw new Error("Abstract method!");
}
“Animal
类”和say
方法是抽象的。
创建实例会引发错误:
new Animal(); // throws
这就是您从中“继承”的方式:
var Cat = function() {
Animal.apply(this, arguments);
// Cat initialization...
};
Cat.prototype = Object.create(Animal.prototype);
Cat.prototype.constructor = Cat;
Cat.prototype.say = function() {
console.log('meow');
}
Dog
看起来很像。
这就是你的场景的结果:
var cat = new Cat();
var dog = new Dog();
cat.say();
dog.say();
在这里摆弄(查看控制台输出)。
根据 ES6,你可以使用 JavaScript 类和继承来完成你所需要的。
在 ECMAScript 2015 中引入的 JavaScript 类主要是 JavaScript 现有的基于原型的继承的语法糖。
参考:https ://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes
首先,我们定义我们的抽象类。这个类不能被实例化,但可以扩展。我们还可以定义必须在扩展该类的所有类中实现的函数。
/**
* Abstract Class Animal.
*
* @class Animal
*/
class Animal {
constructor() {
if (this.constructor == Animal) {
throw new Error("Abstract classes can't be instantiated.");
}
}
say() {
throw new Error("Method 'say()' must be implemented.");
}
eat() {
console.log("eating");
}
}
之后,我们可以创建具体的类。这些类将继承抽象类的所有功能和行为。
/**
* Dog.
*
* @class Dog
* @extends {Animal}
*/
class Dog extends Animal {
say() {
console.log("bark");
}
}
/**
* Cat.
*
* @class Cat
* @extends {Animal}
*/
class Cat extends Animal {
say() {
console.log("meow");
}
}
/**
* Horse.
*
* @class Horse
* @extends {Animal}
*/
class Horse extends Animal {}
而结果...
// RESULTS
new Dog().eat(); // eating
new Cat().eat(); // eating
new Horse().eat(); // eating
new Dog().say(); // bark
new Cat().say(); // meow
new Horse().say(); // Error: Method say() must be implemented.
new Animal(); // Error: Abstract classes can't be instantiated.
你的意思是这样的:
function Animal() {
//Initialization for all Animals
}
//Function and properties shared by all instances of Animal
Animal.prototype.init=function(name){
this.name=name;
}
Animal.prototype.say=function(){
alert(this.name + " who is a " + this.type + " says " + this.whattosay);
}
Animal.prototype.type="unknown";
function Cat(name) {
this.init(name);
//Make a cat somewhat unique
var s="";
for (var i=Math.ceil(Math.random()*7); i>=0; --i) s+="e";
this.whattosay="Me" + s +"ow";
}
//Function and properties shared by all instances of Cat
Cat.prototype=new Animal();
Cat.prototype.type="cat";
Cat.prototype.whattosay="meow";
function Dog() {
//Call init with same arguments as Dog was called with
this.init.apply(this,arguments);
}
Dog.prototype=new Animal();
Dog.prototype.type="Dog";
Dog.prototype.whattosay="bark";
//Override say.
Dog.prototype.say = function() {
this.openMouth();
//Call the original with the exact same arguments
Animal.prototype.say.apply(this,arguments);
//or with other arguments
//Animal.prototype.say.call(this,"some","other","arguments");
this.closeMouth();
}
Dog.prototype.openMouth=function() {
//Code
}
Dog.prototype.closeMouth=function() {
//Code
}
var dog = new Dog("Fido");
var cat1 = new Cat("Dash");
var cat2 = new Cat("Dot");
dog.say(); // Fido the Dog says bark
cat1.say(); //Dash the Cat says M[e]+ow
cat2.say(); //Dot the Cat says M[e]+ow
alert(cat instanceof Cat) // True
alert(cat instanceof Dog) // False
alert(cat instanceof Animal) // True
您可能想查看 Dean Edwards 的基类:http ://dean.edwards.name/weblog/2006/03/base/
或者,Douglas Crockford 在 JavaScript 中的经典继承上有这个示例/文章:http ://www.crockford.com/javascript/inheritance.html
是否可以在 JavaScript 中模拟抽象基类?
当然。有大约一千种方法可以在 JavaScript 中实现类/实例系统。这是一个:
// Classes magic. Define a new class with var C= Object.subclass(isabstract),
// add class members to C.prototype,
// provide optional C.prototype._init() method to initialise from constructor args,
// call base class methods using Base.prototype.call(this, ...).
//
Function.prototype.subclass= function(isabstract) {
if (isabstract) {
var c= new Function(
'if (arguments[0]!==Function.prototype.subclass.FLAG) throw(\'Abstract class may not be constructed\'); '
);
} else {
var c= new Function(
'if (!(this instanceof arguments.callee)) throw(\'Constructor called without "new"\'); '+
'if (arguments[0]!==Function.prototype.subclass.FLAG && this._init) this._init.apply(this, arguments); '
);
}
if (this!==Object)
c.prototype= new this(Function.prototype.subclass.FLAG);
return c;
}
Function.prototype.subclass.FLAG= new Object();
var cat = new Animal('cat');
当然,这并不是真正的抽象基类。你的意思是这样的:
var Animal= Object.subclass(true); // is abstract
Animal.prototype.say= function() {
window.alert(this._noise);
};
// concrete classes
var Cat= Animal.subclass();
Cat.prototype._noise= 'meow';
var Dog= Animal.subclass();
Dog.prototype._noise= 'bark';
// usage
var mycat= new Cat();
mycat.say(); // meow!
var mygiraffe= new Animal(); // error!
Animal = function () { throw "abstract class!" }
Animal.prototype.name = "This animal";
Animal.prototype.sound = "...";
Animal.prototype.say = function() {
console.log( this.name + " says: " + this.sound );
}
Cat = function () {
this.name = "Cat";
this.sound = "meow";
}
Dog = function() {
this.name = "Dog";
this.sound = "woof";
}
Cat.prototype = Object.create(Animal.prototype);
Dog.prototype = Object.create(Animal.prototype);
new Cat().say(); //Cat says: meow
new Dog().say(); //Dog says: woof
new Animal().say(); //Uncaught abstract class!
您可能想要强制执行的另一件事是确保您的抽象类没有被实例化。您可以通过定义一个充当 FLAG 设置为抽象类构造函数的函数来做到这一点。然后,这将尝试构造 FLAG,该 FLAG 将调用包含要抛出的异常的构造函数。下面的例子:
(function(){
var FLAG_ABSTRACT = function(__class){
throw "Error: Trying to instantiate an abstract class:"+__class
}
var Class = function (){
Class.prototype.constructor = new FLAG_ABSTRACT("Class");
}
//will throw exception
var foo = new Class();
})()
问题很老了,但我创建了一些可能的解决方案,如何创建抽象“类”并阻止创建该类型的对象。
//our Abstract class
var Animal=function(){
this.name="Animal";
this.fullname=this.name;
//check if we have abstract paramater in prototype
if (Object.getPrototypeOf(this).hasOwnProperty("abstract")){
throw new Error("Can't instantiate abstract class!");
}
};
//very important - Animal prototype has property abstract
Animal.prototype.abstract=true;
Animal.prototype.hello=function(){
console.log("Hello from "+this.name);
};
Animal.prototype.fullHello=function(){
console.log("Hello from "+this.fullname);
};
//first inheritans
var Cat=function(){
Animal.call(this);//run constructor of animal
this.name="Cat";
this.fullname=this.fullname+" - "+this.name;
};
Cat.prototype=Object.create(Animal.prototype);
//second inheritans
var Tiger=function(){
Cat.call(this);//run constructor of animal
this.name="Tiger";
this.fullname=this.fullname+" - "+this.name;
};
Tiger.prototype=Object.create(Cat.prototype);
//cat can be used
console.log("WE CREATE CAT:");
var cat=new Cat();
cat.hello();
cat.fullHello();
//tiger can be used
console.log("WE CREATE TIGER:");
var tiger=new Tiger();
tiger.hello();
tiger.fullHello();
console.log("WE CREATE ANIMAL ( IT IS ABSTRACT ):");
//animal is abstract, cannot be used - see error in console
var animal=new Animal();
animal=animal.fullHello();
如您所见,最后一个对象给了我们错误,这是因为原型中的 Animal 具有 property abstract
。为了确保它是 Animal 而不是Animal.prototype
原型链中的东西,我这样做:
Object.getPrototypeOf(this).hasOwnProperty("abstract")
所以我检查我最近的原型对象是否有abstract
属性,只有直接从Animal
原型创建的对象才会有这个条件为真。函数hasOwnProperty
只检查当前对象的属性而不是他的原型,所以这让我们 100% 确定属性是在此处声明的,而不是在原型链中。
从 Object 继承的每个对象都继承了hasOwnProperty方法。该方法可用于判断一个对象是否具有指定属性作为该对象的直接属性;与 in 运算符不同,此方法不检查对象的原型链。更多关于它:
在我的提议中,我们不必像@Jordão 当前的最佳答案一样constructor
每次都进行更改。Object.create
解决方案还可以在层次结构中创建许多抽象类,我们只需要abstract
在原型中创建属性。
您可以使用对象原型创建抽象类,一个简单的示例如下:
var SampleInterface = {
addItem : function(item){}
}
您可以更改或不更改上述方法,这取决于您实施它。如需详细观察,您可能需要访问此处。
function Animal(type) {
if (type == "cat") {
this.__proto__ = Cat.prototype;
} else if (type == "dog") {
this.__proto__ = Dog.prototype;
} else if (type == "fish") {
this.__proto__ = Fish.prototype;
}
}
Animal.prototype.say = function() {
alert("This animal can't speak!");
}
function Cat() {
// init cat
}
Cat.prototype = new Animal();
Cat.prototype.say = function() {
alert("Meow!");
}
function Dog() {
// init dog
}
Dog.prototype = new Animal();
Dog.prototype.say = function() {
alert("Bark!");
}
function Fish() {
// init fish
}
Fish.prototype = new Animal();
var newAnimal = new Animal("dog");
newAnimal.say();
由于__proto__
它不是标准变量,因此不能保证有效,但至少在 Firefox 和 Safari 中有效。
如果你不明白它是如何工作的,请阅读原型链。
在这种情况下,我们可以使用Factory
设计模式。Javascript 用于prototype
继承父级的成员。
定义父类构造函数。
var Animal = function() {
this.type = 'animal';
return this;
}
Animal.prototype.tired = function() {
console.log('sleeping: zzzZZZ ~');
}
然后创建子类。
// These are the child classes
Animal.cat = function() {
this.type = 'cat';
this.says = function() {
console.log('says: meow');
}
}
然后定义子类构造函数。
// Define the child class constructor -- Factory Design Pattern.
Animal.born = function(type) {
// Inherit all members and methods from parent class,
// and also keep its own members.
Animal[type].prototype = new Animal();
// Square bracket notation can deal with variable object.
creature = new Animal[type]();
return creature;
}
测试一下。
var timmy = Animal.born('cat');
console.log(timmy.type) // cat
timmy.says(); // meow
timmy.tired(); // zzzZZZ~
这是完整示例编码的Codepen 链接。
我认为所有这些答案特别是前两个(由some和jordão)用传统的原型基础 JavaScript 概念清楚地回答了这个问题。
Creational Patterns
现在,由于您希望动物类构造函数根据传递给构造的参数来运行,我认为这与例如Factory Pattern的基本行为非常相似。
在这里,我做了一个小方法让它以这种方式工作。
var Animal = function(type) {
this.type=type;
if(type=='dog')
{
return new Dog();
}
else if(type=="cat")
{
return new Cat();
}
};
Animal.prototype.whoAreYou=function()
{
console.log("I am a "+this.type);
}
Animal.prototype.say = function(){
console.log("Not implemented");
};
var Cat =function () {
Animal.call(this);
this.type="cat";
};
Cat.prototype=Object.create(Animal.prototype);
Cat.prototype.constructor = Cat;
Cat.prototype.say=function()
{
console.log("meow");
}
var Dog =function () {
Animal.call(this);
this.type="dog";
};
Dog.prototype=Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;
Dog.prototype.say=function()
{
console.log("bark");
}
var animal=new Animal();
var dog = new Animal('dog');
var cat=new Animal('cat');
animal.whoAreYou(); //I am a undefined
animal.say(); //Not implemented
dog.whoAreYou(); //I am a dog
dog.say(); //bark
cat.whoAreYou(); //I am a cat
cat.say(); //meow
//Your Abstract class Animal
function Animal(type) {
this.say = type.say;
}
function catClass() {
this.say = function () {
console.log("I am a cat!")
}
}
function dogClass() {
this.say = function () {
console.log("I am a dog!")
}
}
var cat = new Animal(new catClass());
var dog = new Animal(new dogClass());
cat.say(); //I am a cat!
dog.say(); //I am a dog!
/****************************************/
/* version 1 */
/****************************************/
var Animal = function(params) {
this.say = function()
{
console.log(params);
}
};
var Cat = function() {
Animal.call(this, "moes");
};
var Dog = function() {
Animal.call(this, "vewa");
};
var cat = new Cat();
var dog = new Dog();
cat.say();
dog.say();
/****************************************/
/* version 2 */
/****************************************/
var Cat = function(params) {
this.say = function()
{
console.log(params);
}
};
var Dog = function(params) {
this.say = function()
{
console.log(params);
}
};
var Animal = function(type) {
var obj;
var factory = function()
{
switch(type)
{
case "cat":
obj = new Cat("bark");
break;
case "dog":
obj = new Dog("meow");
break;
}
}
var init = function()
{
factory();
return obj;
}
return init();
};
var cat = new Animal('cat');
var dog = new Animal('dog');
cat.say();
dog.say();
"use strict";
function Abstract (...arg){
// create abstract constructor
if( this.constructor.name === 'Object' || this.constructor === Abstract ) throw { ErrorType : "can't call abstract class with new !" , }
// ceate abstract method
Object.defineProperty( this , 'config' , {
value : function(){
console.log('config parent')
}
});
// or other
return this ;
};
class Home extends Abstract{
name = '';
constructor(...arg){
super(...arg) ;
}
config(){
// this method not working
console.log('config child')
}
}
let y = new Home( "home" , 'dasd');
y.config();
如果您想确保您的基类及其成员是严格抽象的,这里有一个基类可以为您执行此操作:
class AbstractBase{
constructor(){}
checkConstructor(c){
if(this.constructor!=c) return;
throw new Error(`Abstract class ${this.constructor.name} cannot be instantiated`);
}
throwAbstract(){
throw new Error(`${this.constructor.name} must implement abstract member`);}
}
class FooBase extends AbstractBase{
constructor(){
super();
this.checkConstructor(FooBase)}
doStuff(){this.throwAbstract();}
doOtherStuff(){this.throwAbstract();}
}
class FooBar extends FooBase{
constructor(){
super();}
doOtherStuff(){/*some code here*/;}
}
var fooBase = new FooBase(); //<- Error: Abstract class FooBase cannot be instantiated
var fooBar = new FooBar(); //<- OK
fooBar.doStuff(); //<- Error: FooBar must implement abstract member
fooBar.doOtherStuff(); //<- OK
严格模式使得无法在 throwAbstract 方法中记录调用者,但错误应该发生在显示堆栈跟踪的调试环境中。