Android的简单应用（三）——为你的程序添加监听器

平时在写程序时经常会遇到监听器，比如按钮的click监听器，按键监听器等等。而android中的监听器和java中的回调函数是同一个概念，都是在底层代码中定义一个接口来调用高层的代码。那么什么是回调函数呢？网上说的是“在WINDOWS中，程序员想让系统DLL调用自己编写的一个方法，于是利用DLL当中回调函数（CALLBACK）的接口来编写程序，使它调用，这个就称为回调。在调用接口时，需要严格的按照定义的参数和方法调用，并且需要处理函数的异步，否则会导致程序的崩溃。这样的解释似乎还是比较难懂，这里举个简单的例子，程序员A写了一段程序（程序a），其中预留有回调函数接口，并封装好了该程序。程序员B要让a调用自己的程序b中的一个方法，于是，他通过a中的接口回调自己b中的方法。”按照我的理解就是在程序A中先挖了一个坑，然后等着你在程序B中去填里面的内容。不填？那就是被坑咯~

一、定义回调函数

1. 建立一个类，其中包括一个接口的变量

   private CallBack callBack;//接口的变量

2. 建立一个接口的声明

   public interface CallBack{
      public void Out();//接口里面要实现的方法
   }

3. 在类中与接口绑定（或者说获得接口的引用）

   public void setCallBack(CallBack callBack){
           this.callBack = callBack;
       }
   
       public void Out(){
           callBack.Out();
           System.out.println("这是CallerClass的输出");
       }

4. 整个类

   public class CallerClass {
   
       private CallBack callBack;//接口的声明
   
       public CallerClass(){};
   
       public void setCallBack(CallBack callBack){
           this.callBack = callBack;
       }
   
       public void Out(){
           callBack.Out();
           System.out.println("这是CallerClass的输出");
       }
   
       public interface CallBack{
           public void Out();//接口里面要实现的方法
       }
   
   }

二、调用回调函数

我这里是在Activity中绑定接口，实际代码如下：

public class MainActivity extends Activity implements CallerClass.CallBack{  private CallerClass caller; ...}

继承了CallerClass中的CallBack接口，Out方法如下：

    @Override
    public void Out() {
        // TODO Auto-generated method stub
        System.out.println("这是Activity上的输出");
    }

最后在onCreate中调用CallerClass的Out()方法：

caller = new CallerClass();
caller.setCallBack(this);
caller.Out();

完整代码：

public class MainActivity extends Activity implements CallerClass.CallBack{

    private CallerClass caller;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        caller = new CallerClass();
        caller.setCallBack(this);
        caller.Out();

    }

    @Override
    public void Out() {
        // TODO Auto-generated method stub
        System.out.println("这是Activity上的输出");
    }

}

三、结果

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Perfect~

interface的作用感觉和C++中的指针差不多，它是一个地址的引用，但没有生成内存。所以我们能够预先为程序赋予要执行的方法。不止如此，interface在设计模式的作用更是强大，工厂模式，建造者模式，观察者模式无不用到interface。