Android Activity

An Activity represents a single screen with a user interface, similar to a window or frame in Java. Android activities are subclasses of the ContextThemeWrapper class.

If you have programmed with C, C++, or Java, you might know that programs start with the main() function. Similarly, Android systems initialize their programs by invoking the onCreate() callback within an activity. There is a sequence of callback methods to start an activity, and a corresponding sequence to close an activity, as illustrated in the activity lifecycle diagram below:

Activity class defines the following callbacks. You don't need to implement all the callback methods. However, it's important to understand each one to ensure your app behaves as users expect.

Example

This example demonstrates the lifecycle of an Android application activity through simple steps. Follow the steps below to modify the Android application created in the Hello World example section.

Here is the modified content of the main activity file src/com.example.helloworld/MainActivity.java, which includes each basic lifecycle method. The Log.d() method is used to generate log messages:

package com.example.helloworld;

import android.os.Bundle;
import android.app.Activity;
import android.util.Log;

public class MainActivity extends Activity {
   String msg = "Android : ";

   /** Called when the activity is first created. */
   @Override
   public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      Log.d(msg, "The onCreate() event");
   }

   /** Called when the activity is about to become visible. */
   @Override
   protected void onStart() {
      super.onStart();
      Log.d(msg, "The onStart() event");
   }

   /** Called when the activity has become visible. */
   @Override
   protected void onResume() {
      super.onResume();
      Log.d(msg, "The onResume() event");
   }

   /** Called when another activity gains focus. */
   @Override
   protected void onPause() {
      super.onPause();
      Log.d(msg, "The onPause() event");
   }

   /** Called when the activity is no longer visible. */
   @Override
   protected void onStop() {
      super.onStop();
      Log.d(msg, "The onStop() event");
   }

   /** Called just before the activity is destroyed. */
   @Override
   public void onDestroy() {
      super.onDestroy();
      Log.d(msg, "The onDestroy() event");
   }
}

The Activity class loads all UI components from the XML file in the project's res/layout. The following statement loads UI components from res/layout/activity_main.xml:

setContentView(R.layout.activity_main);

An application can have one or multiple activities without any restrictions. Each activity defined for the application must be declared in AndroidManifest.xml. The main activity of the application needs to be declared in the manifest, and its intent filter tag must include the MAIN action and LAUNCHER category, as shown below:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.example.helloworld"
   android:versionCode="1"
   android:versionName="1.0" >

   &lt;uses-sdk
      android:minSdkVersion="8"
      android:targetSdkVersion="22" />

   <application
       android:icon="@drawable/ic_launcher"
       android:label="@string/app_name"
       android:theme="@style/AppTheme" >

       <activity
          android:name=".MainActivity"
          android:label="@string/title_activity_main" >

          <intent-filter>
             <action android:name="android.intent.action.MAIN" />
             <category android:name="android.intent.category.LAUNCHER"/>
          </intent-filter>

       </activity>

   </application>
</manifest>

If neither the MAIN action nor the LAUNCHER category is declared in the activity, the application's icon will not appear in the main screen's app list.

Let's run the modified "Hello World!" application. Assume you have created an AVD during the environment setup. To run the application from Eclipse, open the activity file in the project and click the run button on the toolbar.

07-19 15:00:43.405: D/Android :(866): The onCreate() event
07-19 15:00:43.405: D/Android :(866): The onStart() event
07-19 15:00:43.415: D/Android :(866): The onResume() event

Now, let's click the red button on the Android emulator.

07-19 15:01:10.995: D/Android :(866): The onPause() event
07-19 15:01:12.705: D/Android :(866): The onStop() event

Let's click the menu button on the Android emulator again.

07-19 15:01:13.995: D/Android :(866): The onStart() event
07-19 15:01:14.705: D/Android :(866): The onResume() event

Next, let's click the back button on the Android emulator.

07-19 15:33:15.687: D/Android :(992): The onPause() event
07-19 15:33:15.525: D/Android :(992): The onStop() event
07-19 15:33:15.525: D/Android :(992): The onDestroy() event