docs/html/training/basics/firstapp/building-ui.jd

page.title=Building a Simple User Interface
trainingnavtop=true

page.tags=ui
helpoutsWidget=true

@jd:body


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<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>In this lesson, you create a layout in XML that includes a text field and a
button. In the next lesson, your app responds when the button is pressed by sending the
content of the text field to another activity.</p>

<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>.
{@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>

<p>Layouts are subclasses of the {@link android.view.ViewGroup}. In this exercise, you'll work with
a {@link android.widget.LinearLayout}.</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" height="214" />
<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>


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

<ol>
  <li>In Android Studio's <b>Project</b> window, open <b>app > res >
    layout > activity_main.xml</b>.
    <p>This XML file defines the layout of your activity. It contains the
      default "Hello World" text view.</p>
  </li>
  <li>When you open a layout file, you’re first shown the design editor in the
    <a href="/studio/write/layout-editor.html">Layout Editor</a>. For this lesson,
    you work directly with the XML, so click the <b>Text</b> tab at the bottom
    of the window to switch to the text editor.
  </li>
  <li>Delete everything and insert the following XML:
    <pre>&lt;?xml version="1.0" encoding="utf-8"?&gt;
&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"&gt;
&lt;/LinearLayout&gt;
</pre>

  </li>

</ol>

<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>Two other 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>In the <code>activity_main.xml</code> file, within the
{@link android.widget.LinearLayout &lt;LinearLayout>} element, add the following
{@link android.widget.EditText &lt;EditText>} element:</p>

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

<p>Don't worry about the error that appears for
<code>&#64;string/edit_message</code>; you'll fix that soon.</p>

<p>Here is a description of the attributes in the
  {@link android.widget.EditText &lt;EditText>} you added:</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>

<div class="sidebox-wrapper">
<div class="sidebox">
  <h3>Resource Objects</h3>
  <p>A resource object is 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 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} file 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>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 R.java} file that refers to the {@link
android.widget.EditText} element. With the resource ID 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>By default, your Android project includes a string resource file at
<b>res > values > strings.xml</b>. Here, you'll add two new strings.</p>

<ol>
<li>From the <b>Project</b> window, open <b>res > values > strings.xml</b>.</li>
<li>Add two strings so that your file looks like this:
<pre>&lt;?xml version="1.0" encoding="utf-8"?>
&lt;resources>
    &lt;string name="app_name">My First App&lt;/string>
    <b>&lt;string name="edit_message">Enter a message&lt;/string>
    &lt;string name="button_send">Send&lt;/string></b>
&lt;/resources>
</pre>
</li>
</ol>

<p>For text in the user interface, always specify each string as
a resource. String resources allow you to manage all UI text in a single location,
which makes the text easier to find and update. Externalizing the strings also allows you to
localize your app to different languages by providing alternative definitions for each
string resource.</p>

<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>Go back to the <code>activity_main.xml</code> file and add a button after the
  {@link android.widget.EditText &lt;EditText>}. Your file should look like this:</p>
<pre>&lt;LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent"&gt;
        &lt;EditText android:id="@+id/edit_message"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:hint="@string/edit_message" /&gt;
        <b>&lt;Button
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="@string/button_send" /&gt;</b>
&lt;/LinearLayout&gt;
</pre>

<p class="note"><strong>Note:</strong> 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>

<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 figure 2 shows.
</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. 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 soda, 1 part syrup" means two-thirds of the drink is soda. 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.</p>

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

<p>In <code>activity_main.xml</code>, modify the
  {@link android.widget.EditText &lt;EditText>} so that the attributes look like
  this:</p>

<pre>
&lt;EditText android:id="@+id/edit_message"
    <b>android:layout_weight="1"
    android:layout_width="0dp"</b>
    android:layout_height="wrap_content"
    android:hint="@string/edit_message" /&gt;
</pre>

<p>Setting the width to zero (0dp) 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>

<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 it fills the remaining space in the {@link
android.widget.LinearLayout}.</p>

<p>Here’s how your complete <code>activity_main.xml</code>layout file should now look:</p>

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

<h2>Run Your App</h2>

<p>To see how the app now looks on your device or emulator,
  click <strong>Run</strong>
    <img src="{@docRoot}images/tools/as-run.png"
    style="vertical-align:baseline;margin:0; max-height:1em" /> in the
    toolbar.</p>

<p>To add app behaviors such as responding to a button and starting
another activity, continue to the <a href="starting-activity.html">next
lesson</a>.</p>