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.
1. Using Eclipse, create a new Android project and name it Activity101.
2. In the Activity101Activity.java file, add the following statements in bold:
package net.learn2develop.Activity101;
import android.app.Activity; import android.os.Bundle; import android.util.Log;
public class Activity101Activity extends Activity {
String tag = “Lifecycle”;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main);
Log.d(tag, “In the onCreate() event”);
}
public void onStart()
{
super.onStart();
Log.d(tag, “In the onStart() event”);
}
public void onRestart()
{
super.onRestart();
Log.d(tag, “In the onRestart() event”);
}
public void onResume()
{
super.onResume();
Log.d(tag, “In the onResume() event”);
}
public void onPause()
{
super.onPause();
Log.d(tag, “In the onPause() event”);
}
public void onStop()
{
super.onStop();
Log.d(tag, “In the onStop() event”);
}
public void onDestroy()
{
super.onDestroy();
Log.d(tag, “In the onDestroy() event”);
}
}
3. Press F11 to debug the application on the Android emulator.
4. When the activity is first loaded, you should see something very similar to the following in the LogCat window (click the Debug perspective; see also Figure 2-2):
11-16 06:25:59.396: D/Lifecycle(559): In the onCreate() event 11-16 06:25:59.396: D/Lifecycle(559): In the onStart() event 11-16 06:25:59.396: D/Lifecycle(559): In the onResume() event
FIGURE 2-2
5. If you click the Back button on the Android emulator, the following is printed:
11-16 06:29:26.665: D/Lifecycle(559): In the onPause() event 11-16 06:29:28.465: D/Lifecycle(559): In the onStop() event
11-16 06:29:28.465: D/Lifecycle(559): In the onDestroy() event
6. Click the Home button and hold it there. Click the Activities icon and observe the following:
11-16 06:31:08.905: D/Lifecycle(559): In the onCreate() event 11-16 06:31:08.905: D/Lifecycle(559): In the onStart() event 11-16 06:31:08.925: D/Lifecycle(559): In the onResume() event
7. 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 06:32:00.585: D/Lifecycle(559): In the onPause() event 11-16 06:32:05.015: D/Lifecycle(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 06:32:50.515: D/Lifecycle(559): In the onRestart() event 11-16 06:32:50.515: D/Lifecycle(559): In the onStart() event
11-16 06:32:50.515: D/Lifecycle(559): In the onResume() event
The onRestart() event is now fired, followed 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 know, as whatever state the activity is currently in will be lost; hence, you need to write additional code in youractivity to preserve its state when it is destroyed (Chapter 3 shows you how). At this point, note that the onPause() method is called in both scenarios — when an activity is sent to the background, as well as when it is killed whenthe user presses the 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 the foreground.
➤ Use the onDestroy() method to free up resources before your activity is destroyed.
Applying Styles and Themes to an Activity
By default, an activity occupies the entire screen. However, you can apply a dialog theme to
an activity so that it is displayed as a floating dialog. For example, you might want to customize your activity to display as a pop-up, warning users about some actions that they are going to perform. In this case, displaying theactivity as a dialog is a good way to get their attention.
To apply a dialog theme to an activity, simply modify the <Activity> element in the
AndroidManifest.xml file by adding the android:theme attribute:
<?xml version=“1.0“ encoding=“utf-8“?>
package=”net.learn2develop.Activity101” android:versionCode=”1” android:versionName=”1.0” >
<uses-sdk android:minSdkVersion=”14” />
<application android:icon=”@drawable/ic_launcher” android:label=”@string/app_name” android:theme=”@android:style/Theme.Dialog”>
<activity android:label=”@string/app_name” android:name=”.Activity101Activity” >
<intent-filter >
<action android:name=”android.intent.action.MAIN” />
<category android:name=”android.intent.category.LAUNCHER” />
</intent-filter>
</activity>
</application>
</manifest>
This will make the activity appear as a dialog, as shown in Figure 2-3.
FIGURE 2-3
Hiding the Activity Title
You can also hide the title of an activity if desired (such as when you just want to display a status update to the user). To do so, use the requestWindowFeature() method and pass it the Window
.FEATURE_NO_TITLE constant, like this:
import android.app.Activity; import android.os.Bundle; import android.util.Log; import android.view.Window;
public class Activity101Activity extends Activity {
String tag = “Lifecycle”;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//---hides the title bar---
requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.main); Log.d(tag, “In the onCreate() event”);
}
}
This will hide the title bar, as shown in Figure 2-4.
FIGURE 2-4
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