basics/firstapp/building-ui.jd

page.title=Building a Simple User Interface
parent.title=Building Your First App
parent.link=index.html

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previous.title=Running Your App
previous.link=running-app.html
next.title=Starting Another Activity
next.link=starting-activity.html

@jd:body


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<div id="tb-wrapper">
<div id="tb">

<h2>This lesson teaches you to</h2>

<ol>
  <li><a href="#LinearLayout">Use a Linear Layout</a></li>
  <li><a href="#TextInput">Add a Text Field</a></li>
  <li><a href="#Strings">Add String Resources</a></li>
  <li><a href="#Button">Add a Button</a></li>
  <li><a href="#Weight">Make the Input Box Fill in the Screen Width</a></li>
</ol>


<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}guide/topics/ui/declaring-layout.html">XML Layouts</a></li>
</ul>


</div>
</div>


<p>The graphical user interface for an Android app is built using a hierarchy of {@link
android.view.View} and {@link android.view.ViewGroup} objects. {@link android.view.View} objects are
usually UI widgets such as a button or text field and {@link android.view.ViewGroup} objects are
invisible view containers that define how the child views are laid out, such as in a
grid or a vertical list.</p>

<p>Android provides an XML vocabulary that corresponds to the subclasses of {@link
android.view.View} and {@link android.view.ViewGroup} so you can define your UI in XML with a
hierarchy of view elements.</p>


<div class="sidebox-wrapper">
<div class="sidebox">
  <h2>Alternative Layouts</h2>
  <p>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 <a
href="{@docRoot}training/basics/supporting-devices/index.html">Supporting Different
Hardware</a>.</p>
</div>
</div>

<img src="{@docRoot}images/viewgroup.png" alt="" width="440" />
<p class="img-caption"><strong>Figure 1.</strong> Illustration of how {@link
android.view.ViewGroup} objects form branches in the layout and contain {@link
android.view.View} objects.</p>

<p>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.</p>


<h2 id="LinearLayout">Use a Linear Layout</h2>

<p>Open the <code>main.xml</code> file from the <code>res/layout/</code>
directory (every new Android project includes this file by default).</p>

<p class="note"><strong>Note:</strong> In Eclipse, when you open a layout file, youre first shown
the ADT Layout Editor. This is an editor that helps you build layouts using WYSIWYG tools. For this
lesson, youre going to work directly with the XML, so click the <em>main.xml</em> tab at
the bottom of the screen to open the XML editor.</p>

<p>By default, the <code>main.xml</code> file includes a layout with a {@link
android.widget.LinearLayout} root view group and a {@link android.widget.TextView} child view.
Youre going to re-use the {@link android.widget.LinearLayout} in this lesson, but change its
contents and layout orientation.</p>

<p>First, delete the {@link android.widget.TextView} element and change the value
<a href="{@docRoot}reference/android/widget/LinearLayout.html#attr_android:orientation">{@code
android:orientation}</a> to be <code>"horizontal"</code>. The result looks like this:</p>

<pre>
&lt;?xml version="1.0" encoding="utf-8"?>
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="horizontal" >
&lt;/LinearLayout>
</pre>

<p>{@link android.widget.LinearLayout} is a view group (a subclass of {@link
android.view.ViewGroup}) that lays out child views in either a vertical or horizontal orientation,
as specified by the <a
href="{@docRoot}reference/android/widget/LinearLayout.html#attr_android:orientation">{@code
android:orientation}</a> attribute. Each child of a {@link android.widget.LinearLayout} appears on
the screen in the order in which it appears in the XML.</p>

<p>The other two attributes, <a
href="{@docRoot}reference/android/view/View.html#attr_android:layout_width">{@code
android:layout_width}</a> and <a
href="{@docRoot}reference/android/view/View.html#attr_android:layout_height">{@code
android:layout_height}</a>, are required for all views in order to specify their size.</p>

<p>Because the {@link android.widget.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
<code>"fill_parent"</code>.</p>

<p class="note"><strong>Note:</strong> Beginning with Android 2.2 (API level 8),
<code>"fill_parent"</code> has been renamed <code>"match_parent"</code> to better reflect the
behavior. The reason is that if you set a view to <code>"fill_parent"</code> it does not expand to
fill the remaining space after sibling views are considered, but instead expands to
<em>match</em> the size of the parent view no matter what&mdash;it will overlap any sibling
views.</p>

<p>For more information about layout properties, see the <a
href="{@docRoot}guide/topics/ui/declaring-layout.html">XML Layout</a> guide.</p>


<h2 id="TextInput">Add a Text Field</h2>

<p>To create a user-editable text field, add an {@link android.widget.EditText
&lt;EditText>} element inside the {@link android.widget.LinearLayout &lt;LinearLayout>}. The {@link
android.widget.EditText} class is a subclass of {@link android.view.View} that displays an editable
text field.</p>

<p>Like every {@link android.view.View} object, you must define certain XML attributes to specify
the {@link android.widget.EditText} object's properties. Heres how you should declare it
inside the {@link android.widget.LinearLayout &lt;LinearLayout>} element:</p>

<pre>
    &lt;EditText android:id="@+id/edit_message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="@string/edit_message" />
</pre>


<div class="sidebox-wrapper">
<div class="sidebox">
  <h3>About resource objects</h3>
  <p>A resource object is simply a unique integer name that's associated with an app resource,
such as a bitmap, layout file, or string.</p>
  <p>Every resource has a
corresponding resource object defined in your project's {@code gen/R.java} file. You can use the
object names in the {@code R} class to refer to your resources, such as when you need to specify a
string value for the <a
href="{@docRoot}reference/android/widget/TextView.html#attr_android:hint">{@code android:hint}</a>
attribute. You can also create arbitrary resource IDs that you associate with a view using the <a
href="{@docRoot}reference/android/view/View.html#attr_android:id">{@code android:id}</a> attribute,
which allows you to reference that view from other code.</p>
  <p>The SDK tools generate the {@code R.java} each time you compile your app. You should never
modify this file by hand.</p>
</div>
</div>

<p>About these attributes:</p>

<dl>
<dt><a href="{@docRoot}reference/android/view/View.html#attr_android:id">{@code android:id}</a></dt>
<dd>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
lesson).

<p>The at-symbol (<code>&#64;</code>) is required when you want to refer to a resource object from
XML, followed by the resource type ({@code id} in this case), then the resource name ({@code
edit_message}). (Other resources can use the same name as long as they are not the same
resource type&mdash;for example, the string resource uses the same name.)</p>

<p>The plus-symbol (<code>+</code>) 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, <code>edit_message</code>, to create a new identifier in
your project's {@code gen/R.java} file that is now associated with the {@link
android.widget.EditText} element. 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.</p></dd>

<dt><a
href="{@docRoot}reference/android/view/View.html#attr_android:layout_width">{@code
android:layout_width}</a> and <a
href="{@docRoot}reference/android/view/View.html#attr_android:layout_height">{@code
android:layout_height}</a></dt>
<dd>Instead of using specific sizes for the width and height, the <code>"wrap_content"</code> 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 <code>"fill_parent"</code>, then the {@link android.widget.EditText}
element would fill the screen, because it'd match the size of the parent {@link
android.widget.LinearLayout}. For more information, see the <a
href="{@docRoot}guide/topics/ui/declaring-layout.html">XML Layouts</a> guide.</dd>

<dt><a
href="{@docRoot}reference/android/widget/TextView.html#attr_android:hint">{@code
android:hint}</a></dt>
<dd>This is a default string to display when the text field is empty. Instead of using a hard-coded
string as the value, the {@code "@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, youll see a compiler
error when you add the {@code "@string/edit_message"} value. You'll fix this in the next section by
defining the string resource.</dd>
</dl>


<h2 id="Strings">Add String Resources</h2>

<p>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.</p>

<p>By default, your Android project includes a string resource file at
<code>res/values/strings.xml</code>. Open this file, delete the existing <code>"hello"</code>
string, and add one for the
<code>"edit_message"</code> string used by the {@link android.widget.EditText &lt;EditText>}
element.</p>

<p>While youre in this file, also add a string for the button youll soon add, called
<code>"button_send"</code>.</p>

<p>The result for <code>strings.xml</code> looks like this:</p>

<pre>
&lt;?xml version="1.0" encoding="utf-8"?>
&lt;resources>
    &lt;string name="app_name">My First App&lt;/string>
    &lt;string name="edit_message">Enter a message&lt;/string>
    &lt;string name="button_send">Send&lt;/string>
&lt;/resources>
</pre>

<p>For more information about using string resources to localize your app for several languages,
see the <a
href="{@docRoot}training/basics/supporting-devices/index.html">Supporting Various Devices</a>
class.</p>


<h2 id="Button">Add a Button</h2>

<p>Now add a {@link android.widget.Button &lt;Button>} to the layout, immediately following the
{@link android.widget.EditText &lt;EditText>} element:</p>

<pre>
    &lt;Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_send" />
</pre>

<p>The height and width are set to <code>"wrap_content"</code> so the button is only as big as
necessary to fit the button's text. This button doesn't need the
<a href="{@docRoot}reference/android/view/View.html#attr_android:id">{@code android:id}</a>
attribute, because it won't be referenced from the activity code.</p>


<h2 id="Weight">Make the Input Box Fill in the Screen Width</h2>

<p>The layout is currently designed so that both the {@link android.widget.EditText} and {@link
android.widget.Button} widgets are only as big as necessary to fit their content, as shown in
figure 2.</p>

<img src="{@docRoot}images/training/firstapp/edittext_wrap.png" />
<p class="img-caption"><strong>Figure 2.</strong> The {@link android.widget.EditText} and {@link
android.widget.Button} widgets have their widths set to
<code>"wrap_content"</code>.</p>

<p>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.
{@link android.widget.LinearLayout} enables such a design with the <em>weight</em> property, which
you can specify using the <a
href="{@docRoot}reference/android/widget/LinearLayout.LayoutParams.html#weight">{@code
android:layout_weight}</a> attribute.</p>

<p>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.</p>

<p>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 {@link
android.widget.EditText} element, give it a weight of 1 and leave the button with no weight.</p>

<pre>
    &lt;EditText
        android:layout_weight="1"
        ... />
</pre>

<p>In order to improve the layout efficiency when you specify the weight, you should change the
width of the {@link android.widget.EditText} to be
zero (0dp). Setting the width to zero improves layout performance because using
<code>"wrap_content"</code> 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
remaining space.</p>
<pre>
    &lt;EditText
        android:layout_weight="1"
        android:layout_width="0dp"
        ... />
</pre>

<p>Figure 3
shows the result when you assign all weight to the {@link android.widget.EditText} element.</p>

<img src="{@docRoot}images/training/firstapp/edittext_gravity.png" />
<p class="img-caption"><strong>Figure 3.</strong> The {@link android.widget.EditText} widget is
given all the layout weight, so fills the remaining space in the {@link
android.widget.LinearLayout}.</p>

<p>Heres how your complete layout file should now look:</p>

<pre>
&lt;?xml version="1.0" encoding="utf-8"?>
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="horizontal">
    &lt;EditText android:id="@+id/edit_message"
        android:layout_weight="1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:hint="@string/edit_message" />
    &lt;Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_send" />
&lt;/LinearLayout>
</pre>

<p>This layout is applied by the default {@link android.app.Activity} class
that the SDK tools generated when you created the project, so you can now run the app to see the
results:</p>

<ul>
  <li>In Eclipse, click <strong>Run</strong> from the toolbar.</li>
  <li>Or from a command line, change directories to the root of your Android project and
execute:
<pre>
ant debug
adb install bin/MyFirstApp-debug.apk
</pre></li>
</ul>

<p>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.</p>