Android: Activity Life Cycle

• Download demo life cycle

Introduction

An activity is a single, focused thing that the user can do. Almost all activities interact with the user, so the Activity class takes care of creating a window for you in which you can place your UI with setContentView(View).

Background

Activity is a window that contains the user interface of your application. As there are various states of activity like Running, Paused, Stopped and Killed.

Activity base class contains events that govern the life cycle of an activity.

• onCreate(): Called when the activity is first created
• onStart(): Called when the activity becomes visible to the user
• onResume(): Called when the activity starts interacting with the user
• onPause(): Called when the current activity is being paused and the previous activity is being resumed
• onStop(): Called when the activity is no longer visible to the user
• onDestroy(): Called before the activity is destroyed by the system
• onRestart(): Called when the activity has been stopped and is restarting again

Using the Code

To create an activity, your class need to extends Activity base class. An Application has more than one activity.

Create a project with the following details:

• ProjectName: LifeCycle
• PackageName: sat.tuts4mobile.lifecycle
• ActivityName: LifeCycleActivity

In the LifeCycleActivity.java file, add the following code:

package sat.tuts4mobile.lifecycle; 
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;
public class LifeCycleActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main); 
        Toast.makeText(LifeCycleActivity.this,"ON CREATE", Toast.LENGTH_SHORT).show(); 
    } 
    @Override
    protected void onStart() {
       // TODO Auto-generated method stub
       super.onStart(); 
        Toast.makeText(LifeCycleActivity.this,"ON START", Toast.LENGTH_SHORT).show();
    } 
    @Override
    protected void onResume() {
       // TODO Auto-generated method stub
       super.onResume(); 
        Toast.makeText(LifeCycleActivity.this,"ON RESUME", Toast.LENGTH_SHORT).show(); 
    } 
    @Override
    protected void onPause() {
       // TODO Auto-generated method stub
       super.onPause(); 
        Toast.makeText(LifeCycleActivity.this,"ON PAUSE", Toast.LENGTH_SHORT).show(); 
    } 
    @Override
    protected void onRestart() {
       // TODO Auto-generated method stub
       super.onRestart(); 
        Toast.makeText(LifeCycleActivity.this,"ON RESTART", Toast.LENGTH_SHORT).show(); 
    } 
    @Override
    protected void onStop() {
       // TODO Auto-generated method stub
       super.onStop(); 
        Toast.makeText(LifeCycleActivity.this,"ON STOP", Toast.LENGTH_SHORT).show(); 
    } 
    @Override
    protected void onDestroy() {
       // TODO Auto-generated method stub
       super.onDestroy(); 
        Toast.makeText(LifeCycleActivity.this,"ON DESTROY", Toast.LENGTH_SHORT).show();
    } 
}  

1. When you run the following code, you see when the application starts there are three toast messages that come after one another. First is ON CREATE , second is ON START and third is ON RESUME.
2. When you press the home button from the emulator, ON PAUSE and ON STOP methods call.
3. And when you open the application again ON RESTART, ON START and ON RESUME methods call.
4. And at last, when you press the back button from emulator, ON PAUSE , ON STOP and ON DESTROY method calls.

Points of Interest

You can also follow my blog at http://tuts4mobile.blogspot.in for mobile application related articles.