5.7 实现高级JavaScript技术(三)


createInheritance函数有两个参数,父对象和子对象。这个函数只是迭代处理父对象的所有成员(成员就是属性或函数),如果某个成员在子对象中不存在,则复制到子对象。

  使用createInheritance函数相当简单:首先创建子对象的一个实例,然后使用createInheritance函数,为它传递子对象以及父对象的一个实例,如下:

  var child = new Child();
  createInheritance(new Parent(), child);

  父对象中有而子对象中没有的所有属性和方法将复制到子对象。

  5.7.4 汇合

  前面已经了解到,JavaScript中也可以实现私有属性,而且JavaScript也能像C++和Java一样支持基于类的继承方法。为了展示这些是怎样实现的,下面说明如何转换前面使用Vehicle、SportsCar和CementTruck对象的示例,从而使用信息隐藏和继承的新模式。代码清单5-5列出了新的对象定义。

  代码清单5-5 classicalInheritance.js

  function Vehicle() {
  var wheelCount = 4;
  var curbWeightInPounds = 4000;
  this.getWheelCount = function() {
  return wheelCount;
  }
  this.setWheelCount = function(count) {
  wheelCount = count;
  }
  this.getCurbWeightInPounds = function() {
  return curbWeightInPounds;
  }
  this.setCurbWeightInPounds = function(weight) {
  curbWeightInPounds = weight;
  }
  this.refuel = function() {
  return "Refueling Vehicle with regular 87 octane gasoline";
  }
  this.mainTasks = function() {
  return "Driving to work, school, and the grocery store";
  }
  }
  function SportsCar() {
  this.refuel = function() {
  return "Refueling SportsCar with premium 94 octane gasoline";
  }
  this.mainTasks = function() {
  return "Spirited driving, looking good, driving to the beach";
  }
  }
  function CementTruck() {
  this.refuel = function() {
  return "Refueling CementTruck with diesel fuel";
  }
  this.mainTasks = function() {
  return "Arrive at construction site, extend boom, deliver cement";
  }
  }

 需要注意,SportsCar和CementTruck对象没有定义自己的wheelCount和curbWeightInPounds属性,也没有相关的存取函数,因为这些属性和函数会从Vehicle对象继承。

  与前面一样,需要一个简单的HTML页面来测试这些新对象。代码清单5-6列出了测试这些新对象的HTML页面。要特别注意createInheritance函数,看看如何使用这个函数在Vehicle和SportsCar对象之间以及Vehicle和CementTruck对象之间创建继承关系。还要注意describe函数有所修改,以试图直接访问wheelCount和curbWeightInPounds属性。这样做会返回一个undefined值。

  代码清单5-6 classicalInheritance.html

  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
  <html xmlns="http://www.w3.org/1999/xhtml">
  <head>
  <title>Classical Inheritance in JavaScript</title>
  <script type="text/javascript" src="classicalInheritance.js"></script>
  <script type="text/javaScript">
  function createInheritance(parent, child) {
  var property;
  for(property in parent) {
  if(!child[property]) {
  child[property] = parent[property];
  }
  }
  }
  function describe(vehicle) {
  var description = "";
  description = description + "Number of wheels (via property): "
  + vehicle.wheelCount;
  description = description + "
Number of wheels (via accessor): "
  + vehicle.getWheelCount();
  description = description + "
Curb Weight (via property): "
  + vehicle.curbWeightInPounds;
  description = description + "
Curb Weight (via accessor): "
  + vehicle.getCurbWeightInPounds();
  description = description + "
Refueling Method: " + vehicle.refuel();
  description = description + "
Main Tasks: " + vehicle.mainTasks();
  alert(description);
  }
  function createVehicle() {
  var vehicle = new Vehicle();
  describe(vehicle);
  }
  function createSportsCar() {
  var sportsCar = new SportsCar();
  createInheritance(new Vehicle(), sportsCar);
  sportsCar.setCurbWeightInPounds(3000);
  describe(sportsCar);
  }
  function createCementTruck() {
  var cementTruck = new CementTruck();
  createInheritance(new Vehicle(), cementTruck);
  cementTruck.setWheelCount(10);
  cementTruck.setCurbWeightInPounds(10000);
  describe(cementTruck);
  }
  </script>
  </head>
  <body>
  <h1>Examples of Classical Inheritance in JavaScript</h1>
  <br/><br/>
  <button onclick="createVehicle();">Create an instance of Vehicle</button>
  <br/><br/>
  <button onclick="createSportsCar();">Create an instance of SportsCar</button>
  <br/><br/>
  <button onclick="createCementTruck();">Create an instance of CementTruck</button>
  </body>
  </html>


  分别点击页面上的各个按钮会得到图5-17所示的结果。正如所料,试图直接访问私有属性就会返回undefined。

                          

 图5-17 创建Vehicle、SportsCar和CementTruck对象,并使用describe函数描述它们的结果。私有属性不能直接访问,见警告框中的undefined值

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