JavaScript 实现接口 （Interfaces In JavaScript）

接口是面向对象编程里的重要特性，遗憾的是JavaScript并没有提供对接口的支持！怎么实现接口呢？

在实际中，我们可以在注释中定义好接口，在实际的代码中予以实现

比如：

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/* interface Composite {     function add(child);     function remove(child);     function getChild(index); } interface FormItem {     function save(); } */ var CompositeForm = function(id, method, action) { // implements Composite, FormItem ... }; // Implement the Composite interface. CompositeForm.prototype.add = function(child) { ... }; CompositeForm.prototype.remove = function(child) { ... }; CompositeForm.prototype.getChild = function(index) { ... }; // Implement the FormItem interface. CompositeForm.prototype.save = function() { ... };

 

实现接口的程序员是否将这些接口都实现了呢？我们没办法保证！因为这里没有任何办法去检查是否都实现了

我们需要一个检查是否实现了接口的机制，可以这样：

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/* interface Composite {     function add(child);     function remove(child);     function getChild(index); } interface FormItem {     function save(); } */ var CompositeForm = function(id, method, action) {     this.implementsInterfaces = ['Composite', 'FormItem']; ... }; ... function addForm(formInstance) {     if(!implements(formInstance, 'Composite', 'FormItem')) {         throw new Error("Object does not implement a required interface.");     } ... } // The implements function, which checks to see if an object declares that it // implements the required interfaces. function implements(object) {     for(var i = 1; i < arguments.length; i++) { // Looping through all arguments     // after the first one.     var interfaceName = arguments[i];     var interfaceFound = false;     for(var j = 0; j < object.implementsInterfaces.length; j++) {         if(object.implementsInterfaces[j] == interfaceName) {         interfaceFound = true;         break;         }     }       if(!interfaceFound) {         return false; // An interface was not found.         }     }     return true; // All interfaces were found. }

这种方法让程序员在写的时候注明实现了哪些接口： this.implementsInterfaces = ['Composite', 'FormItem']; 在调用的时候使用implements方法来判断是否实现了，理论上可行，很有可能写上了实现了'Composite'接口，但是代码里却并没有add方法！因此，我们需要检验实现接口的类是否包含了接口里的方法！所以，接口必须从注释中解放出来：

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// Interfaces. var Composite = new Interface('Composite', ['add', 'remove', 'getChild']); var FormItem = new Interface('FormItem', ['save']); // CompositeForm class var CompositeForm = function(id, method, action) { // implements      Composite, FormItem ... }; ... function addForm(formInstance) {     Interface.ensureImplements(formInstance, Composite, FormItem);     // This function will throw an error if a required method is not implemented,     // halting execution of the function.     // All code beneath this line will be executed only if the checks pass. ... }

    定义接口Composite，FormItem，并且CompositeForm实现这两个接口，在使用的时候，用Interface.ensureImplements来检验formInstance是否实现了这两个接口中的所有方法。

来看看Interface的定义：两个参数，第一个参数是接口名称，第二个参数是接口包含的方法数组

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// Constructor. var Interface = function(name, methods) {     if(arguments.length != 2) {         throw new Error("Interface constructor called with " + arguments.length + "arguments, but expected exactly 2."); }     this.name = name;     this.methods = [];     for(var i = 0, len = methods.length; i < len; i++) {         if(typeof methods[i] !== 'string') {             throw new Error("Interface constructor expects method names to be " + "passed in as a string.");         }     this.methods.push(methods[i]);     } };

为Interface 添加建议接口是否实现的静态方法

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// Constructor. Interface.ensureImplements = function(object) {     if(arguments.length < 2) {         throw new Error("Function Interface.ensureImplements called with " + arguments.length + "arguments, but expected at least 2.");     }     for(var i = 1, len = arguments.length; i < len; i++) {         var interface = arguments[i];         if(interface.constructor !== Interface) {             throw new Error("Function Interface.ensureImplements expects arguments" + "two and above to be instances of Interface.");         }     for(var j = 0, methodsLen = interface.methods.length; j < methodsLen; j++) {         var method = interface.methods[j];         if(!object[method] || typeof object[method] !== 'function') {             throw new Error("Function Interface.ensureImplements: object " + "does not implement the " + interface.name + " interface. Method " + method + " was not found.");         }     }     } };

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