Displaying Notifications
So far, you have been using the Toast class to display messages to the user. While the Toast class is a handy way to show users alerts, it is not persistent. It flashes on the screen for a few seconds and then disappears. If it contains important information, users may easily miss it if they are not looking at the screen. For messages that are important, you should use a more persistent method. In this case, you should use the NotificationManager to display a persistent message at the top of the device, commonly known as the status bar (sometimes also referred to as the notification bar).
Displaying notifications on the Status Bar
1 . Using Eclipse, create a new Android project and name it Notifications.
2 . Add a new class file named NotificationView.java to the src folder of the project.
In addition, add a new notification.xml file to the res/layout folder as well.
3 . Populate the notification.xml file as follows:
<?xml version=”1.0”encoding=”utf-8”?>
<LinearLayout
xmlns:android=”http://schemas.android.com/apk/res/android”
android:orientation=”vertical”
android:layout_width=”fill_parent”
android:layout_height=”fill_parent”>
<TextView
android:layout_width=”fill_parent”
android:layout_height=”wrap_content”
android:text=”Here are the details for the notification...” />
</LinearLayout>
4 . Populate the NotificationView.java file as follows:
package com.emergingandroidtech.Notifications;
import android.app.Activity;
import android.app.NotificationManager;
import android.os.Bundle;
public class NotificationView extends Activity
{
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.notification);
//---look up the notification manager service---
NotificationManager nm = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
//---cancel the notification that we started
nm.cancel(getIntent().getExtras().getInt(“notificationID”));
}
}
5 . Add the following statements in bold to the AndroidManifest.xml file:
<?xml version=”1.0”encoding=”utf-8”?>
<manifest
xmlns:android=”http://schemas.android.com/apk/res/android” package=”com.emergingandroidtech.Notifications”
android:versionCode=”1”
android:versionName=”1.0”>
<application
android:icon=”@drawable/icon”
android:label=”@string/app_name”>
<activity
android:name=”.MainActivity”
android:label=”@string/app_name”>
<intent-filter> <action android:name=”android.intent.action.MAIN”/>
<category android:name=”android.intent.category.LAUNCHER”/>
</intent-filter>
</activity>
<activity
android:name=”.NotificationView”
android:label=”Details of notification”>
<intent-filter>
<action android:name=”android.intent.action.MAIN” />
<category android:name=”android.intent.category.DEFAULT” />
</intent-filter>
</activity>
</application>
<uses-sdk
android:minSdkVersion=”9”/>
<uses-permission
android:name=”android.permission.VIBRATE”/>
</manifest>
6 . Add the following statements in bold to the main.xml file:
<?xml version=”1.0”encoding=”utf-8”?>
<LinearLayout
xmlns:android=”http://schemas.android.com/apk/res/android”
android:orientation=”vertical”
android:layout_width=”fill_parent”
android:layout_height=”fill_parent”>
<Button android:id=”@+id/btn_displaynotif”
android:layout_width=”fill_parent”
android:layout_height=”wrap_content”
android:text=”Display Notification” />
</LinearLayout>
7 . Finally, add the following statements in bold to the MainActivity.java file:
package com.emergingandroidtech.Notifications;
import android.app.Activity;
import android.os.Bundle;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Intent;
import android.view.View;
import android.widget.Button;
public class MainActivity extends Activity
{
int notificationID = 1;
/**Called when the activity is first created.*/
@Override
public void onCreate(BundlesavedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Button button = (Button) findViewById(R.id.btn_displaynotif);
button.setOnClickListener(new Button.OnClickListener()
{
public void onClick(View v)
{
displayNotification();
}
});
}
protected void displayNotification()
{
//---PendingIntent to launch activity if the user selects
// this notification---
Intent i = new Intent(this, NotificationView.class);
i.putExtra(“notificationID”, notificationID);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, i, 0);
NotificationManager nm = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
Notification notif = new Notification( R.drawable.icon, “Reminder: Meeting starts in 5 minutes”, System.currentTimeMillis());
CharSequence from = “System Alarm”;
CharSequence message = “Meeting with customer at 3pm...”;
notif.setLatestEventInfo(this, from, message, pendingIntent);
//---100ms delay, vibrate for 250ms, pause for 100 ms and
// then vibrate for 500ms---
notif.vibrate = new long[] { 100, 250, 100, 500};
nm.notify(notificationID, notif);
}
}
8 . Click the Display Notification button and a notification will appear on the status bar.
9 . Clicking and dragging the status bar down will reveal the notification .
10 . Clicking on the notification will reveal the NotificationView activity. This also causes the notification to be dismissed from the status bar.
How It Works
To display a notification, you first created an Intent object to point to the NotificationView class:
//---PendingIntent to launch activity if the user selects
//this notification---
Intent i=newIntent(this,NotificationView.class);
i.putExtra(“notificationID”,notificationID);
This intent will be used to launch another activity when the user selects a notification from the list of notifications. In this example, you added a key/value pair to the Intent object so that you can tag the notification ID, identifying the notification to the target activity. This ID will be used to dismiss the notifications later. You would also need to create a PendingIntent object. A PendingIntent object helps you to perform an action on your application’s behalf, often at a later time, regardless of whether your application is running or not. In this case, you initialized it as follows:
PendingIntent pendingIntent= PendingIntent.getActivity(this,0,i,0);
The getActivity() method retrieves a PendingIntent object and you set it using the following arguments:
➤context — Application context
➤ request code — Request code for the intent
➤ intent — The intent for launching the target activity
➤ flags — The flags in which the activity is to be launched
You then obtain an instance of the NotificationManager class and create an instance of the Notification class:
NotificationManager nm = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
Notification notif = new Notification( R.drawable.icon, “Reminder: Meeting starts in 5 minutes”, System.currentTimeMillis());
The Notification class enables you to specify the notification’s main information when the notification first appears on the status bar. The second argument to the Notification constructor sets the “ticker text” on the status bar.
Next, you set the details of the notification using the setLatestEventInfo() method:
CharSequence from = “System Alarm”;
CharSequence message = “Meeting with customer at 3pm...”;
notif.setLatestEventInfo(this, from, message, pendingIntent);
//---100ms delay, vibrate for 250ms, pause for 100 ms and
// then vibrate for 500ms---
notif.vibrate = new long[] { 100, 250, 100, 500};
The preceding also sets the notification to vibrate the phone. Finally, to display the notification you use the notify() method:
nm.notify(notificationID, notif);
When the user clicks on the notification, the NotificationView activity is launched. Here, you dismiss the notification by using the cancel() method of the NotificationManager object and passing it the ID of the notification (passed in via the Intent object):
//---look up the notification manager service---
NotificationManager nm = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
//---cancel the notification that we started
nm.cancel(getIntent().getExtras().getInt(“notificationID”));
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