Beginning Android

The classic listbox widget in Android is known as ListView. Include one of these in your layout, invoke setAdapter() to supply your data and child views, and attach a listener via setOnItemSelectedListener() to find out when the selection has changed. With that, you have a fully-functioning listbox.

However, if your activity is dominated by a single list, you might well consider creating your activity as a subclass of ListActivity, rather than the regular Activity base class. If your main view is just the list, you do not even need to supply a layout — ListActivity will construct a full- screen list for you. If you do want to customize the layout, you can, so long as you identify your ListView as @android:id/list, so ListActivity knows which widget is the main list for the activity.

For example, here is a layout pulled from the Selection/List sample project. This sample along with all others in this chapter can be found in the Source Code area of http://apress.com.

<?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:id='@+id/selection'
  android:layout_width='fill_parent'
  android:layout_height='wrap_content'/>
 <ListView
  android:id='@android:id/list'
  android:layout_width='fill_parent'
  android:layout_height='fill_parent'
  android:drawSelectorOnTop='false'
 />
</LinearLayout>

It is just a list with a label on top to show the current selection.

The Java code to configure the list and connect the list with the label is:

public class ListViewDemo extends ListActivity {
 TextView selection;
 String[] items={'lorem', 'ipsum', 'dolor', 'sit', 'amet',
  'consectetuer', 'adipiscing', 'elit', 'morbi', 'vel',
  'ligula', 'vitae', 'arcu', 'aliquet', 'mollis',
  'etiam', 'vel', 'erat', 'placerat', 'ante',
  'porttitor', 'sodales', 'pellentesque', 'augue', 'purus'};
 @Override
 public void onCreate(Bundle icicle) {
  super.onCreate(icicle);
  setContentView(R.layout.main);
  setListAdapter(new ArrayAdapter<String> (this,
   android.R.layout.simple_list_item_1,
   items));
  selection=(TextView)findViewById(R.id.selection);
 }
 public void onListItemClick(ListView parent, View v, int position,
  long id) {
  selection.setText(items[position]);
 }
}

With ListActivity, you can set the list adapter via setListAdapter() — in this case, providing an ArrayAdapter wrapping an array of nonsense strings. To find out when the list selection changes, override onListItemClick() and take appropriate steps based on the supplied child view and position (in this case, updating the label with the text for that position).

The results are shown in Figure 8-1.

Figure 8-1. The ListViewDemo sample application

Spin Control

In Android, the Spinner is the equivalent of the drop-down selector you might find in other toolkits (e.g., JComboBox in Java/Swing). Pressing the center button on the D-pad pops up a selection dialog for the user to choose an item from. You basically get the ability to select from a list without taking up all the screen space of a ListView, at the cost of an extra click or screen tap to make a change.

As with ListView, you provide the adapter for data and child views via setAdapter () and hook in a listener object for selections via setOnItemSelectedListener().

If you want to tailor the view used when displaying the drop-down perspective, you need to configure the adapter, not the Spinner widget. Use the setDropDownViewResource() method to supply the resource ID of the view to use.

For example, culled from the Selection/Spinner sample project, here is an XML layout for a simple view with a Spinner:

<?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:id='@+id/selection'
  android:layout_width='fill_parent'
  android:layout_height='wrap_content'
 />
 <Spinner android:id='@+id/spinner'
  android:layout_width='fill_parent'
  android:layout_height='wrap_content'
  android:drawSelectorOnTop='true'
 />
</LinearLayout>