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：

/*
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：

/*
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：

// 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

// 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

// 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.");
        }
    }
    }
};