The graphical user interface for an Android app is built using a hierarchy of
View objects are
usually UI widgets such as buttons or
text fields 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.
Declaring your UI layout in XML rather than runtime code is useful for several reasons, but it's especially important so you can create different 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 Devices.
In this lesson, you'll create a layout in XML that includes a text 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.
Create a Linear Layout
activity_main.xml file from the
Note: In Eclipse, when you open a layout file, you’re first shown the Graphical 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 activity_main.xml tab at the bottom of the screen to open the XML editor.
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_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
"match_parent". This value declares that the view should expand its width
or height to match the width or height of the parent view.
For more information about layout properties, see the 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.
For more information, read the guide to Providing Resources.
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 sign (
@) is required when you're referring to any resource object from XML. It is followed by the resource type (
idin this case), a slash, then the resource name (
The plus sign (
+) before the resource type is needed only when you're defining a resource ID for the first time. When you compile the app, the SDK tools use the ID name to create a new resource ID in your project's
gen/R.javafile that refers to the
EditTextelement. Once the resource ID is declared once this way, other references to the ID do not need the plus sign. Using the plus sign is necessary only when specifying a new resource ID and not needed for concrete resources such as strings or layouts. 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
"match_parent", then the
EditTextelement would fill the screen, because it would match the size of the parent
LinearLayout. For more information, see the 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 refers to a concrete resource (not just an identifier), it does not need the plus sign. However, because you haven't defined the string resource yet, you’ll see a compiler error at first. You'll fix this in the next section by defining the string.
Note: This string resource has the same name as the element ID:
edit_message. However, references to resources are always scoped by the resource type (such as
string), so using the same name does not cause collisions.
Add String Resources
When you need to add text in the user interface, you should always specify each string as a resource. String resources allow you to manage all UI text in a single location, 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 resource.
By default, your Android project includes a string resource file at
res/values/strings.xml. Add a new string named
"edit_message" and set the value to "Enter a message." (You can delete
the "hello_world" string.)
While you’re in this file, also add a "Send" 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> <string name="action_settings">Settings</string> <string name="title_activity_main">MainActivity</string> </resources>
For more information about using string resources to localize your app for other languages, see the Supporting Different 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. So, it would be nice to fill the unused screen width
with the text field. You can do this inside a
LinearLayout with the weight property, which
you can specify using the
The weight value is a number that specifies the amount of remaining space each view should consume, relative to the amount consumed by sibling views. This works kind of like the amount of 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 fills 2/3 of the remaining space and the second view fills the rest. If you add a third view and give it a weight of 1, then the first view (with weight of 2) 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 all views are
given the space they require. So, to fill the remaining space in your layout 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" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_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.