http://www.linux.com/learn/docs/690708-android-programming-multiple-choice-lists
In our previous Android coding tutorials (
part 1,
part 2),
you set up your dev environment, built a basic app, and then improved
it by adding a menu and a second Activity. In this tutorial we're going
to look at a very handy part of the Android API: ListView, ListActivity,
and the associated methods which give you an easy way to show the user a
list and then act when they click a list item.
Creating a ListView
A very common pattern in an Android activity is showing a list of
items for the user to select from. The Android API provides the ListView
and ListActivity classes to help out with this. Carrying on with the
Countdown app from previous tutorials, we'll list a few sample countdown
times for the user to select from to set the timer.
If all you want is a List, ListActivity will set your View up for you automatically; no need to write any XML at all. So
onCreate()can be very simple:
public class CountdownActivity extends ListActivity {
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Integer[] values = new Integer[] { 5, 10, 15, 20, 25, 30, 45, 60 };
ArrayAdapter adapter =
new ArrayAdapter(this, android.R.layout.simple_list_item_1,
values);
setListAdapter(adapter);
}
}
CountdownActivity now extends ListActivity. ListActivity does a lot
of the preparation work for you, so to show a list, you just need to
create an array of values to show, hook it up to an ArrayAdapter, and
set the ArrayAdapter as the ListActivity's ListAdapter. The ArrayAdapter
has three parameters:
- The current context (
this);
- The layout resource defining how each array element should be displayed;
- The array itself (
values).
For the layout resource, we're use a standard Android resource,
android.R.layout.simple_list_item_1. But you could create your own, or use another of the
standard layout items (of which more later). You can also
take a look at the XML of the standard resources.
The problem with this layout is that it
only shows a list. We
want to be able to see the countdown and the start button as well. This
means setting up our own XML layout, rather than relying on ListActivity
to generate its own layout. Add this to your XML, below the TextView
and the Button:
It's important that the ListView should have the ID
@android:id/list. This is what enables the ListActivity to do its magic without you explicitly setting up the List.
Now go back to
CountdownActivity.onCreate(), and put your previous display and button setup code back in, after the ListView setup:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Integer[] values .... etc ...
[ ... ]
setListAdapter(adapter);
setContentView(R.layout.activity_main);
countdownDisplay = (TextView) findViewById(R.id.time_display_box);
Button startButton = (Button) findViewById(R.id.startbutton);
[ .... etc .... ]
}
Again, it's important that you set up the ListView first, before
setContentView(),
or it won't work properly. Recompile and run, and you'll see the list
appear below the text and button. What you won't see, though, is
anything happening when you click the list elements. The next section
will tackle that problem.
One final note: you can also set up an empty element in the layout,
which will display if and only if the ListView is empty. Add this to
your XML:
(You'll need to set up the string value in
res/values/strings.xml, too). Now replace the array declaration line in
CountdownActivity.onCreate()with this one:
Integer[] values = new Integer[] { };
Compile and run, and you'll see the empty text displayed, and no
list. Put the array declaration back how it was, compile and run again,
and the list shows, but no text. In our app this isn't particularly
useful, but if you were populating the array from elsewhere in your
code, it's a neat trick to have available.
Clicking on List elements
Now we have the List set up, we need to make it do something when you
click on a list element. Specifically, it should set the countdown
seconds to the new value, and change the display. Happily, Android has a
straightforward API for this, too:
ListView list = getListView();
list.setOnItemClickListener(new OnItemClickListener() {
public void onItemClick(AdapterView parent, View view, int position,
long id ) {
countdownSeconds = (Integer) getListAdapter().getItem(position);
countdownDisplay.setText(Long.toString(countdownSeconds));
}
});
This is all pretty self-explanatory! We grab the ListView, set its OnItemClickListener, and create an
onItemClick() method for the Listener. As you can see here,
onItemClick()has
access to the position in the List of the item you clicked on. So we
can grab the ListAdapter, get the item from that position, and then cast
the value to an Integer. Save and run, and you have a list of values to
set your timer.
Changing the List's appearance
Earlier, we mentioned the
other standard layouts available. If you switch
simple_list_item_1 to
simple_list_item_single_choice,
and rerun your code, you'll see that you get a selection indicator next
to your list items. However, when you click it, the countdown value
changes, but the selection indicator doesn't do anything. To make this
work, you need to change your ListView, too. Add this attribute in your
XML:
Run it again, and the selection indicator does its job. If you were
using a ListActivity without an XML file, you could do this with a line
of code in your app:
getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
Conclusion
There are lots of situations in apps where you might want to show the
user a list. ListView and ListActivity make that very easy, and as
shown just above, there are plenty of ways to improve the UI experience.
You could also look at providing a context menu (in these tutorials
we've only used the options menu so far) when the user long-clicks on a
list item. Or you could look at some form of back-end
data storage,
and allow the user to add and edit their own list items, so they have a
list of countdown times that they regularly use. As ever, keep playing
with the code and see where it takes you!
For more Android programming training resources, please visit the
Linux training website.
