Understanding the Life Cycle of a Activity

1. Using Android Studio , create a new Android project and name is Activity101.

2. In the Activity101Activity.java file , add the following highlighted statements.


Package com.example.activity101;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

public class MainActivity extends AppCompatActivity
{
       String tag="LifeCycle Step";
      @Override
       protected void onCreate(Bundle savedInstanceState)
       {
               super.onCreate(savedInstanceState);
               setContentView(R.layout.activity_main);
               Log.d(tag,msg:"In the onCreate () event");
       }
       public void onStart()
       {
                super.onStart();
                Log.d(tag,msg:"In the onStart () event");
       }
       public void onRestart()
       {
                super.onRestart();
                Log.d(tag,msg:"In the onRestart () event");
       }
       public void onPause()
       {
                super.onPause();
                Log.d(tag,msg:"In the onPause () event");
       }
       public void onStop()
       {
                super.onStop();
                Log.d(tag,msg:"In the onStop () event");
       }
       public void onDestroy()
       {
                super.onDestroy();
                Log.d(tag,msg:"In the onDestroy () event");
       }
}

3. Press Shift + F9 to debug the application , or select Run -> Debug . Then select one of your Android Virtual Devices from the pop-up window.

4. When the activity is first loaded , you should see something very similar to the following in the logcat console . If you do not see the logcat console , click Android Monitor at the bottom of the Android Studio window:

11-16 06:25:59.369: D/Lifecycle step(559) : In the onCreate() event
11-16 06:25:59.369: D/Lifecycle step(559) : In the onStart() event
11-16 06:25:59.369: D/Lifecycle step(559) : In the onResume() event

5. If you click the Back button on the Android emulator , you see the following :

11-16 06:29:26.665: D/Lifecycle step(559) : In the onPause() event
11-16 06:29:28.465: D/Lifecycle step(559) : In the onStop() event
11-16 06:29:28.465: D/Lifecycle step(559) : In the onDestroy() event

6. Click the Home button , click the Overview icon , select the Android101 application , and observe the following :

11-16 06:25:59.369: D/Lifecycle step(559) : In the onCreate() event
11-16 06:25:59.369: D/Lifecycle step(559) : In the onStart() event
11-16 06:25:59.369: D/Lifecycle step(559) : In the onResume() event

7. Click the Home button and then click the Phone button on the Android emulator so that the activity is pushed to the background . Observe the output in the logcat window:

11-16 06:29:26.665: D/Lifecycle step(559) : In the onPause() event
11-16 06:29:28.465: D/Lifecycle step(559) : In the onStop() event

8. Notice that the onDestroy() event is not called , indicating that the activity is still in memory . Exit the phone dialer by clicking the Back button . The activity is now visible again. Observe the output in the logcat window :

11-16 06:29:28.465: D/Lifecycle step(559) : In the onRestart() event
11-16 06:29:28.465: D/Lifecycle step(559) : In the onStart() event
11-16 06:29:28.465: D/Lifecycle step(559) : In the onResume() event

The onRestart () event is now fired , following by the onStart() and onResume () methods.

How It Works 

As you can see from this simple example , an activity is destroyed when you click the Back button. This is crucial to understand because whatever state the activity is currently in will be lost . This means you need to write additional code in your activity to preserve its state when the activity is destroyed . At this point , note that the onPause ( ) method is called in both scenarios :

• When an activity is send to the background
• When a user kills an activity by tapping Back button

When an activity is started , the onStart() and onResume () methods are always called , regardless of whether the activity is restored from the background or newly created . When an activity is created for the first time , the onCreate () method is called .

From the preceding example , you can derive the following guidelines:

• Use the onCreate() method to create and instantiate the objects that you will be using in your application.
• Use the onResume() method to start any services or code that needs to run while your activity is in the foreground.
• Use the onPause() method to stop any services or code that does not need to run when your activity is not in foreground.
• Use the onDestroy() method to free up resources before your activity is destroyed.

NOTE Even of an application has only one activity and the activity is killed , the application is still running in memory.