Understanding Activities

This blog begins by showing you to create an activity. To create an activity , you create a Java class that extends the Activity base class :

public class MainActivity extends AppcompatActivity
{
          @Override
          protected void onCreate(Bundle savedInstanceState)
          {
                    super.onCreate(savedInstanceState);
                    setContentView(R.laout.activity_main);
          }
}

Your activity class loads its user interface ( UI ) component using the XML file defined in your res/layout folder . In this example , you would load the UI from the main.xml file :

          setContentView(R.layout.activity_main);

Every activity you have in your application must be declared in your AndroidManifest.xml file , like this :

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android package="com.jfdimarzio.hellworld">
         
 <application
       android:allBackup="true"
       android:icon="@mipmap/ic_launcher"
       android:label="@string/app_name"
       android:supportRtl="true"
       android:theme="@style/AppTheme">
           <activity android:name=".MainActivity">
               <intent-filter>
                    <action android:name="android.intent.action.MAIN" />
                    <category android:name="android.intent.category.LAUNCHER" />
               </intent-filer>
            </activity>
 </application>

</maifest>


The Activity base class defines a series of events that govern the life cycle of an activity . Figure shows the lifecycle of an Activity.

The Activity class defines the following events:

• 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 ( either manually or by the system to conserve memory )
• onRestart( ) - Called when the activity has been stopped and is restarting again.

By default , the activity created for you contains the onCreate( ) event . Within this even handler is the code that helps to display the UI elements of your screen.

show the life cycle of an activity and the various is goes through - from when the activity is started until it ends.

The best way to understand the various stages of an activity is to create a new project , implement the various events , and then subject the activity to various user interactions.