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
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@jd:body


<!-- This is the training bar -->
<div id="tb-wrapper">
<div id="tb">

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

<ol>
  <li><a href="#LinearLayout">Create 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">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 href="{@docRoot}guide/topics/ui/controls/button.html">buttons</a> or
<a href="{@docRoot}guide/topics/ui/controls/text.html">text fields</a> 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 using
a hierarchy of UI elements.</p>


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

<img src="{@docRoot}images/viewgroup.png" alt="" width="400" />
<p class="img-caption"><strong>Figure 1.</strong> Illustration of how {@link
android.view.ViewGroup} objects form branches in the layout and contain other {@link
android.view.View} objects.</p>

<p>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.</p>


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

<p>Open the <code>activity_main.xml</code> file from the <code>res/layout/</code>
directory.</p>

<p class="note"><strong>Note:</strong> 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 <em>activity_main.xml</em> tab at
the bottom of the screen to open the XML editor.</p>

<p>The BlankActivity template you used to start this project creates the
<code>activity_main.xml</code> file with a {@link
android.widget.RelativeLayout} root view and a {@link android.widget.TextView} child view.</p>

<p>First, delete the {@link android.widget.TextView &lt;TextView>} element and change the {@link
  android.widget.RelativeLayout &lt;RelativeLayout>} element to {@link
  android.widget.LinearLayout &lt;LinearLayout>}. Then add the
<a href="{@docRoot}reference/android/widget/LinearLayout.html#attr_android:orientation">{@code
android:orientation}</a> attribute and set it to <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"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_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>"match_parent"</code>. This value declares that the view should expand its width
or height to <em>match</em> the width or height of the parent view.</p>

<p>For more information about layout properties, see the <a
href="{@docRoot}guide/topics/ui/declaring-layout.html">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>}.</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. Here’s 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>
  <p>For more information, read the guide to <a
href="{@docRoot}guide/topics/resources/providing-resources.html">Providing Resources</a>.</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 sign (<code>&#64;</code>) is required when you're referring to any resource object from
XML. It is followed by the resource type ({@code id} in this case), a slash, then the resource name
({@code edit_message}).</p>

<p>The plus sign (<code>+</code>) 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 {@code gen/R.java} file that refers to the {@link
android.widget.EditText} element. 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.</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>"match_parent"</code>, then the {@link android.widget.EditText}
element would fill the screen, because it would match the size of the parent {@link
android.widget.LinearLayout}. For more information, see the <a
href="{@docRoot}guide/topics/ui/declaring-layout.html">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 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.
<p class="note"><strong>Note:</strong> This string resource has the same name as the element ID:
{@code edit_message}. However, references
to resources are always scoped by the resource type (such as {@code id} or {@code string}), so using
the same name does not cause collisions.</p>
</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 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.</p>

<p>By default, your Android project includes a string resource file at
<code>res/values/strings.xml</code>. Open this file and delete the {@code &lt;string>} element
named <code>"hello_world"</code>. Then add a new one named
<code>"edit_message"</code> and set the value to "Enter a message."</p>

<p>While you’re in this file, also add a "Send" string for the button you’ll 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;string name="menu_settings">Settings&lt;/string>
    &lt;string name="title_activity_main">MainActivity&lt;/string>
&lt;/resources>
</pre>

<p>For more information about using string resources to localize your app for other languages,
see the <a
href="{@docRoot}training/basics/supporting-devices/index.html">Supporting Different 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. So, it would be nice to fill the unused screen width
with the text field. You can do this inside a
{@link android.widget.LinearLayout} 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 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.</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 all views are
given the space they require. So, to fill the remaining space in your layout 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>Here’s 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"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_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 Run <img src="{@docRoot}images/tools/eclipse-run.png"
                                 style="vertical-align:baseline;margin:0" /> 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>