Implement callback method

In this example, demonstrate how to implement callback method in Android. The main activity implements interface of callback method, and register itself as the interface to sub-class. When a button in main activity clicked, it call a method in sub-class, then call the callback method in main activity.

SubClass.java

package com.example.androidcallback;

public class SubClass {
 
 interface MyCallbackClass{
        void callbackReturn();
    }
 
 MyCallbackClass myCallbackClass;
 
 void registerCallback(MyCallbackClass callbackClass){
  myCallbackClass = callbackClass;
 }

 void doSomething(){
  //do something here
  
  //call callback method
  myCallbackClass.callbackReturn();
 }

}

MainActivity.java

package com.example.androidcallback;

import com.example.androidcallback.SubClass.MyCallbackClass;

import android.os.Bundle;
import android.app.Activity;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity implements MyCallbackClass{
 
 Button buttonCallSubClass;
 TextView textResult;
 SubClass mySubClass;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        buttonCallSubClass = (Button)findViewById(R.id.callsubclass);
        textResult = (TextView)findViewById(R.id.result);
        
        mySubClass = new SubClass();
        
        mySubClass.registerCallback(this);
        
        buttonCallSubClass.setOnClickListener(new OnClickListener(){

   @Override
   public void onClick(View arg0) {
    mySubClass.doSomething();
   }});
    }

 @Override
 public void callbackReturn() {
  textResult.setText("Callback function called");
 }
   
}

layout

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />
    <Button
        android:id="@+id/callsubclass"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Call sub-class" />
    <TextView
        android:id="@+id/result"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</LinearLayout>