JavaScript介面實現代碼 （Interfaces In JavaScript）

在實際中，我們可以在注釋中定義好介面，在實際的代碼中予以實現
比如： 複製代碼 代碼如下: /*
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() {
...
};

實現介面的程式員是否將這些介面都實現了呢？我們沒辦法保證！因為這裡沒有任何辦法去檢查是否都實現了
我們需要一個檢查是否實現了介面的機制，可以這樣： 複製代碼 代碼如下: /*
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.
}

這種方法讓程式員在寫的時候註明實現了哪些介面： this.implementsInterfaces = ['Composite', 'FormItem']; 在調用的時候使用implements方法來判斷是否實現了，理論上可行，很有可能寫上了實現了'Composite'介面，但是代碼裡卻並沒有add方法！因此，我們需要檢驗實現介面的類是否包含了介面裡的方法！所以，介面必須從注釋中解放出來： 複製代碼 代碼如下: // 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.
...
}

定義介面Composite，FormItem，並且CompositeForm實現這兩個介面，在使用的時候，用Interface.ensureImplements來檢驗formInstance是否實現了這兩個介面中的所有方法。
來看看Interface的定義：兩個參數，第一個參數是介面名稱，第二個參數是介面包含的方法數組 複製代碼 代碼如下: // 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]);
}
};

為Interface 添加建議介面是否實現的靜態方法 複製代碼 代碼如下: // 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.");
}
}
}
};