The graphical user interface for an Android app is built using a hierarchy of
View objects are
usually UI widgets such as a button or text field and
ViewGroup objects are
invisible view containers that define how the child views are laid out, such as in a
grid or a vertical list.
Separating the UI layout into XML files is important for several reasons, but it's especially important on Android because it allows you to define alternative layouts for different screen sizes. For example, you can create two versions of a layout and tell the system to use one on "small" screens and the other on "large" screens. For more information, see the class about Supporting Different Hardware.
In this lesson, you'll create a layout in XML that includes a text input field and a button. In the following lesson, you'll respond when the button is pressed by sending the content of the text field to another activity.
Use a Linear Layout
main.xml file from the
directory (every new Android project includes this file by default).
Note: In Eclipse, when you open a layout file, you’re first shown the ADT Layout Editor. This is an editor that helps you build layouts using WYSIWYG tools. For this lesson, you’re going to work directly with the XML, so click the main.xml tab at the bottom of the screen to open the XML editor.
By default, the
main.xml file includes a layout with a
LinearLayout root view group and a
TextView child view.
You’re going to re-use the
LinearLayout in this lesson, but change its
contents and layout orientation.
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="horizontal" > </LinearLayout>
LinearLayout is a view group (a subclass of
ViewGroup) that lays out child views in either a vertical or horizontal orientation,
as specified by the
android:orientation attribute. Each child of a
LinearLayout appears on
the screen in the order in which it appears in the XML.
LinearLayout is the root view in the layout, it should fill
the entire screen area that's
available to the app by setting the width and height to
Note: Beginning with Android 2.2 (API level 8),
"fill_parent" has been renamed
"match_parent" to better reflect the
behavior. The reason is that if you set a view to
"fill_parent" it does not expand to
fill the remaining space after sibling views are considered, but instead expands to
match the size of the parent view no matter what—it will overlap any sibling
For more information about layout properties, see the XML Layout guide.
Add a Text Field
<EditText android:id="@+id/edit_message" android:layout_width="wrap_content" android:layout_height="wrap_content" android:hint="@string/edit_message" />
About resource objects
A resource object is simply a unique integer name that's associated with an app resource, such as a bitmap, layout file, or string.
Every resource has a
corresponding resource object defined in your project's
gen/R.java file. You can use the
object names in the
R class to refer to your resources, such as when you need to specify a
string value for the
attribute. You can also create arbitrary resource IDs that you associate with a view using the
which allows you to reference that view from other code.
The SDK tools generate the
R.java each time you compile your app. You should never
modify this file by hand.
About these attributes:
- This provides a unique identifier for the view, which you can use to reference the object
from your app code, such as to read and manipulate the object (you'll see this in the next
The at-symbol (
@) is required when you want to refer to a resource object from XML, followed by the resource type (
idin this case), then the resource name (
edit_message). (Other resources can use the same name as long as they are not the same resource type—for example, the string resource uses the same name.)
The plus-symbol (
+) is needed only when you're defining a resource ID for the first time. It tells the SDK tools that the resource ID needs to be created. Thus, when the app is compiled, the SDK tools use the ID value,
edit_message, to create a new identifier in your project's
gen/R.javafile that is now associated with the
EditTextelement. Once the resource ID is created, other references to the ID do not need the plus symbol. This is the only attribute that may need the plus-symbol. See the sidebox for more information about resource objects.
- Instead of using specific sizes for the width and height, the
"wrap_content"value specifies that the view should be only as big as needed to fit the contents of the view. If you were to instead use
"fill_parent", then the
EditTextelement would fill the screen, because it'd match the size of the parent
LinearLayout. For more information, see the XML Layouts guide.
- This is a default string to display when the text field is empty. Instead of using a hard-coded
string as the value, the
"@string/edit_message"value refers to a string resource defined in a separate file. Because this value refers to an existing resource, it does not need the plus-symbol. However, because you haven't defined the string resource yet, you’ll see a compiler error when you add the
"@string/edit_message"value. You'll fix this in the next section by defining the string resource.
Add String Resources
When you need to add text in the user interface, you should always specify each string of text in a resource file. String resources allow you to maintain a single location for all string values, which makes it easier to find and update text. Externalizing the strings also allows you to localize your app to different languages by providing alternative definitions for each string.
By default, your Android project includes a string resource file at
res/values/strings.xml. Open this file, delete the existing
string, and add one for the
"edit_message" string used by the
While you’re in this file, also add a string for the button you’ll soon add, called
The result for
strings.xml looks like this:
<?xml version="1.0" encoding="utf-8"?> <resources> <string name="app_name">My First App</string> <string name="edit_message">Enter a message</string> <string name="button_send">Send</string> </resources>
For more information about using string resources to localize your app for several languages, see the Supporting Various Devices class.
Add a Button
<Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/button_send" />
The height and width are set to
"wrap_content" so the button is only as big as
necessary to fit the button's text. This button doesn't need the
attribute, because it won't be referenced from the activity code.
Make the Input Box Fill in the Screen Width
This works fine for the button, but not as well for the text field, because the user might type
something longer and there's extra space left on the screen. So, it'd be nice to fill that width
using the text field.
LinearLayout enables such a design with the weight property, which
you can specify using the
The weight value allows you to specify the amount of remaining space each view should consume, relative to the amount consumed by sibling views, just like the ingredients in a drink recipe: "2 parts vodka, 1 part coffee liqueur" means two-thirds of the drink is vodka. For example, if you give one view a weight of 2 and another one a weight of 1, the sum is 3, so the first view gets 2/3 of the remaining space and the second view gets the rest. If you give a third view a weight of 1, then the first view now gets 1/2 the remaining space, while the remaining two each get 1/4.
The default weight for all views is 0, so if you specify any weight value
greater than 0 to only one view, then that view fills whatever space remains after each view is
given the space it requires. So, to fill the remaining space with the
EditText element, give it a weight of 1 and leave the button with no weight.
<EditText android:layout_weight="1" ... />
In order to improve the layout efficiency when you specify the weight, you should change the
width of the
EditText to be
zero (0dp). Setting the width to zero improves layout performance because using
"wrap_content" as the width requires the system to calculate a width that is
ultimately irrelevant because the weight value requires another width calculation to fill the
<EditText android:layout_weight="1" android:layout_width="0dp" ... />
shows the result when you assign all weight to the
Here’s how your complete layout file should now look:
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="horizontal"> <EditText android:id="@+id/edit_message" android:layout_weight="1" android:layout_width="0dp" android:layout_height="wrap_content" android:hint="@string/edit_message" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/button_send" /> </LinearLayout>
This layout is applied by the default
that the SDK tools generated when you created the project, so you can now run the app to see the
- In Eclipse, click Run from the toolbar.
- Or from a command line, change directories to the root of your Android project and
ant debug adb install bin/MyFirstApp-debug.apk
Continue to the next lesson to learn how you can respond to button presses, read content from the text field, start another activity, and more.