Implement callback function with interface

This example demonstrate how to implement callback function with interface.

Create a MyAsyncTask class extends AsyncTask. Inside MyAsyncTask, interface DoSomething declared. MyAsyncTask class handle the background timing only, know knowing about the actual jobs. The actual jobs are in the class implement DoSomething.

package com.example.androidcallback;

import android.os.AsyncTask;
import android.os.SystemClock;

public class MyAsyncTask extends AsyncTask<Void, Void, Void> {

 interface DoSomething {
  void doInBackground(int progress);
  void doPostExecute();
 }

 DoSomething myDoSomethingCallBack;
 int myMax;
 
 MyAsyncTask(DoSomething callback, int max){
  myDoSomethingCallBack = callback;
  myMax = max;
 }

 @Override
 protected Void doInBackground(Void... params) {
  for (int i = 0; i <= myMax; i++) {
   SystemClock.sleep(100);
   myDoSomethingCallBack.doInBackground(i);
  }
  return null;
 }

 @Override
 protected void onPostExecute(Void result) {
  super.onPostExecute(result);
  myDoSomethingCallBack.doPostExecute();
 }

}

MainActivity.java, implements DoSomething. Itself (this) will be passed to MyAsyncTask constructor, to implement callback function.

package com.example.androidcallback;

import com.example.androidcallback.MyAsyncTask.DoSomething;

import android.os.Bundle;
import android.widget.ProgressBar;
import android.widget.Toast;
import android.app.Activity;

public class MainActivity extends Activity implements DoSomething{
 
 ProgressBar myProgressBar;
 MyAsyncTask myAsyncTask;
 int myProgress;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  
  myProgressBar = (ProgressBar)findViewById(R.id.myprogressbar);
  
  myProgress = 0;
  myAsyncTask = new MyAsyncTask(this, 100);
  myAsyncTask.execute();
 }

 @Override
 public void doInBackground(int i) {
  myProgressBar.setProgress(i);
 }

 @Override
 public void doPostExecute() {
  Toast.makeText(MainActivity.this, 
    "Finish", Toast.LENGTH_LONG).show();
 }

}

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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:autoLink="web"
        android:text="http://android-er.blogspot.com/"
        android:textStyle="bold" />
    
    <ProgressBar 
        android:id="@+id/myprogressbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        style="?android:attr/progressBarStyleHorizontal"
        android:max="100"
        android:progress="0" />

</LinearLayout>

Download the files.