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Merge "docs: add 101 Training class "Building Your First App"" into ics-mr1

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Scott Main
2012-04-09 10:13:58 -07:00
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docs/html/guide/topics/ui/declaring-layout.jd
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@@ -194,7 +194,7 @@ contains property types that define the size and position for each child view, a
appropriate for the view group. As you can see in figure 1, the parent
view group defines layout parameters for each child view (including the child view group).</p>
<img src="{@docRoot}images/layoutparams.png" alt="" height="300" align="center"/>
<img src="{@docRoot}images/layoutparams.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> Visualization of a view hierarchy with layout
parameters associated with each view.</p>

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can build it up using Android's set of predefined widgets and layouts, or with custom Views that you
create yourself.</p>
<img src="{@docRoot}images/viewgroup.png" alt="" width="312" height="211" align="center"/>
<img src="{@docRoot}images/viewgroup.png" alt="" />
<p>
In order to attach the view hierarchy tree to the screen for rendering, your Activity must call the

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page.title=Building a Simple User Interface
parent.title=Building Your First App
parent.link=index.html
trainingnavtop=true
previous.title=Running Your App
previous.link=running-app.html
next.title=Starting Another Activity
next.link=starting-activity.html
@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">Use a Linear Layout</a></li>
<li><a href="#TextInput">Add a Text Input Box</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/supporting-hardware/index.html">Supporting Various Hardware</a>.</p>
</div>
</div>
<img src="{@docRoot}images/viewgroup.png" alt="" />
<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, 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 <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.
You’re 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 Input Box</h2>
<p>To create a user-editable text box, 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 box.</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>
</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 assiciated with the {@link
android.widget.EditText} element. Once the resource ID is created, other references to the ID do not
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 box is empty. Instead of using a hard-coded
string as the value, the value given in this example refers to a string resource. When you add the
{@code
"@string/edit_message"} value, you’ll see a compiler error because there’s no matching string
resource by that name. 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 you’re in this file, also add a 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;/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.</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 box, 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 box.
{@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 liquer" 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>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"
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:id="@+id/button_send"
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>

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page.title=Creating an Android Project
parent.title=Building Your First App
parent.link=index.html
trainingnavtop=true
next.title=Running Your App
next.link=running-app.html
@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="#Eclipse">Create a Project with Eclipse</a></li>
<li><a href="#CommandLine">Create a Project with Command Line Tools</a></li>
</ol>
<h2>You should also read</h2>
<ul>
<li><a href="{@docRoot}sdk/installing.html">Installing the
SDK</a></li>
<li><a href="{@docRoot}guide/developing/projects/index.html">Managing Projects</a></li>
</ul>
</div>
</div>
<p>An Android project contains all the files that comprise the source code for your Android
app. The Android SDK tools make it easy to start a new Android project with a set of
default project directories and files.</p>
<p>This lesson
shows how to create a new project either using Eclipse (with the ADT plugin) or using the
SDK tools from a command line.</p>
<p class="note"><strong>Note:</strong> You should already have the Android SDK installed, and if
you're using Eclipse, you should have installed the <a
href="{@docRoot}sdk/eclipse-adt.html">ADT plugin</a> as well. If you have not installed
these, see <a href="{@docRoot}sdk/installing.html">Installing the Android SDK</a> and return here
when you've completed the installation.</p>
<h2 id="Eclipse">Create a Project with Eclipse</h2>
<div class="figure" style="width:416px">
<img src="{@docRoot}images/training/firstapp/adt-firstapp-setup.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> The new project wizard in Eclipse.</p>
</div>
<ol>
<li>In Eclipse, select <strong>File &gt; New &gt; Project</strong>.
The resulting dialog should have a folder labeled <em>Android</em>. (If you don’t see the
<em>Android</em> folder,
then you have not installed the ADT plugin&mdash;see <a
href="{@docRoot}sdk/eclipse-adt.html#installing">Installing the ADT Plugin</a>).</li>
<li>Open the <em>Android</em> folder, select <em>Android Project</em> and click
<strong>Next</strong>.</li>
<li>Enter a project name (such as "MyFirstApp") and click <strong>Next</strong>.</li>
<li>Select a build target. This is the platform version against which you will compile your app.
<p>We recommend that you select the latest version possible. You can still build your app to
support older versions, but setting the build target to the latest version allows you to
easily optimize your app for a great user experience on the latest Android-powered devices.</p>
<p>If you don't see any built targets listed, you need to install some using the Android SDK
Manager tool. See <a href="{@docRoot}sdk/installing.html#AddingComponents">step 4 in the
installing guide</a>.</p>
<p>Click <strong>Next</strong>.</p></li>
<li>Specify other app details, such as the:
<ul>
<li><em>Application Name</em>: The app name that appears to the user. Enter "My First
App".</li>
<li><em>Package Name</em>: The package namespace for your app (following the same
rules as packages in the Java programming language). Your package name
must be unique across all packages installed on the Android system. For this reason, it's important
that you use a standard domain-style package name that’s appropriate to your company or
publisher entity. For
your first app, you can use something like "com.example.myapp." However, you cannot publish your
app using the "com.example" namespace.</li>
<li><em>Create Activity</em>: This is the class name for the primary user activity in your
app (an activity represents a single screen in your app). Enter "MyFirstActivity".</li>
<li><em>Minimum SDK</em>: Select <em>4 (Android 1.6)</em>.
<p>Because this version is lower than the build target selected for the app, a warning
appears, but that's alright. You simply need to be sure that you don't use any APIs that require an
<a href="{@docRoot}guide/appendix/api-levels.html">API level</a> greater than the minimum SDK
version without first using some code to verify the device's system version (you'll see this in some
other classes).</p>
</li>
</ul>
<p>Click <strong>Finish</strong>.</p>
</li>
</ol>
<p>Your Android project is now set up with some default files and you’re ready to begin
building the app. Continue to the <a href="running-app.html">next lesson</a>.</p>
<h2 id="CommandLine">Create a Project with Command Line Tools</h2>
<p>If you're not using the Eclipse IDE with the ADT plugin, you can instead create your project
using the SDK tools in a command line:</p>
<ol>
<li>Change directories into the Android SDK’s <code>tools/</code> path.</li>
<li>Execute:
<pre class="no-pretty-print">android list targets</pre>
<p>This prints a list of the available Android platforms that you’ve downloaded for your SDK. Find
the platform against which you want to compile your app. Make a note of the target id. We
recommend that you select the highest version possible. You can still build your app to
support older versions, but setting the build target to the latest version allows you to optimize
your app for the latest devices.</p>
<p>If you don't see any targets listed, you need to
install some using the Android SDK
Manager tool. See <a href="{@docRoot}sdk/installing.html#AddingComponents">step 4 in the
installing guide</a>.</p></li>
<li>Execute:
<pre class="no-pretty-print">
android create project --target &lt;target-id> --name MyFirstApp \
--path &lt;path-to-workspace>/MyFirstApp --activity MyFirstActivity \
--package com.example.myapp
</pre>
<p>Replace <code>&lt;target-id></code> with an id from the list of targets (from the previous step)
and replace
<code>&lt;path-to-workspace></code> with the location in which you want to save your Android
projects.</p></li>
</ol>
<p>Your Android project is now set up with several default configurations and you’re ready to begin
building the app. Continue to the <a href="running-app.html">next lesson</a>.</p>
<p class="note"><strong>Tip:</strong> Add the <code>platform-tools/</code> as well as the
<code>tools/</code> directory to your <code>PATH</code> environment variable.</p>

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page.title=Building Your First App
trainingnavtop=true
startpage=true
next.title=Creating an Android Project
next.link=creating-project.html
@jd:body
<div id="tb-wrapper">
<div id="tb">
<h2>Dependencies and prerequisites</h2>
<ul>
<li>Android 1.6 or higher</li>
<li><a href="http://developer.android.com/sdk/index.html">Android SDK</a></li>
</ul>
</div>
</div>
<p>Welcome to Android application development!</p>
<p>This class teaches you how to build your first Android app. You’ll learn how to create an Android
project and run a debuggable version of the app. You'll also learn some fundamentals of Android app
design, including how to build a simple user interface and handle user input.</p>
<p>Before you start this class, be sure that you have your development environment set up. You need
to:</p>
<ol>
<li>Download the Android SDK Starter Package.</li>
<li>Install the ADT plugin for Eclipse (if you’ll use the Eclipse IDE).</li>
<li>Download the latest SDK tools and platforms using the SDK Manager.</li>
</ol>
<p>If you haven't already done this setup, read <a href="{@docRoot}sdk/installing.html">Installing
the SDK</a>. Once you've finished the setup, you're ready to begin this class.</p>
<p>This class uses a tutorial format that incrementally builds a small Android app in order to teach
you some fundamental concepts about Android development, so it's important that you follow each
step.</p>
<p><strong><a href="creating-project.html">Start the first lesson &rsaquo;</a></strong></p>
<h2>Lessons</h2>
<dl>
<dt><b><a href="creating-project.html">Creating an Android Project</a></b></dt>
<dd>Shows how to create a project for an Android app, which includes a set of default
app files.</dd>
<dt><b><a href="running-app.html">Running Your Application</a></b></dt>
<dd>Shows how to run your app on an Android-powered device or the Android
emulator.</dd>
<dt><b><a href="building-ui.html">Building a Simple User Interface</a></b></dt>
<dd>Shows how to create a new user interface using an XML file.</dd>
<dt><b><a href="starting-activity.html">Starting Another Activity</a></b></dt>
<dd>Shows how to respond to a button press, start another activity, send it some
data, then receive the data in the subsequent activity.</dd>
</dl>

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page.title=Running Your App
parent.title=Building Your First App
parent.link=index.html
trainingnavtop=true
previous.title=Creating a Project
previous.link=creating-project.html
next.title=Building a Simple User Interface
next.link=building-ui.html
@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="#RealDevice">Run on a Real Device</a></li>
<li><a href="#Emulator">Run on the Emulator</a></li>
</ol>
<h2>You should also read</h2>
<ul>
<li><a href="{@docRoot}guide/developing/device.html">Using Hardware Devices</a></li>
<li><a href="{@docRoot}guide/developing/devices/index.html">Managing Virtual Devices</a></li>
<li><a href="{@docRoot}guide/developing/projects/index.html">Managing Projects</a></li>
</ul>
</div>
</div>
<p>If you followed the <a href="{@docRoot}creating-project.html">previous lesson</a> to create an
Android project, it includes a default set of "Hello World" source files that allow you to
run the app right away.</p>
<p>How you run your app depends on two things: whether you have a real Android-powered device and
whether you’re using Eclipse. This lesson shows you how to install and run your app on a
real device and on the Android emulator, and in both cases with either Eclipse or the command line
tools.</p>
<p>Before you run your app, you should be aware of a few directories and files in the Android
project:</p>
<dl>
<dt><code>AndroidManifest.xml</code></dt>
<dd>This manifest file describes the fundamental characteristics of the app and defines each of
its components. You'll learn about various declarations in this file as you read more training
classes.</dd>
<dt><code>src/</code></dt>
<dd>Directory for your app's main source files. By default, it includes an {@link
android.app.Activity} class that runs when your app is launched using the app icon.</dd>
<dt><code>res/</code></dt>
<dd>Contains several sub-directories for app resources. Here are just a few:
<dl style="margin-top:1em">
<dt><code>drawable-hdpi/</code></dt>
<dd>Directory for drawable objects (such as bitmaps) that are designed for high-density
(hdpi) screens. Other drawable directories contain assets designed for other screen densities.</dd>
<dt><code>layout/</code></dt>
<dd>Directory for files that define your app's user interface.</dd>
<dt><code>values/</code></dt>
<dd>Directory for other various XML files that contain a collection of resources, such as
string and color definitions.</dd>
</dl>
</dd>
</dl>
<p>When you build and run the default Android project, the default {@link android.app.Activity}
class in the <code>src/</code> directory starts and loads a layout file from the
<code>layout/</code> directory, which includes a "Hello World" message. Not real exciting, but it's
important that you understand how to build and run your app before adding real functionality to
the app.</p>
<h2 id="RealDevice">Run on a Real Device</h2>
<p>Whether you’re using Eclipse or the command line, you need to:</p>
<ol>
<li>Plug in your Android-powered device to your machine with a USB cable.
If you’re developing on Windows, you might need to install the appropriate USB driver for your
device. For help installing drivers, see the <a href=”{@docRoot}sdk/oem-usb.html”>OEM USB
Drivers</a> document.</li>
<li>Ensure that <strong>USB debugging</strong> is enabled in the device Settings (open Settings
and navitage to <strong>Applications > Development</strong> on most devices, or select
<strong>Developer options</strong> on Android 4.0 and higher).</li>
</ol>
<p>To run the app from Eclipse, open one of your project's files and click
<strong>Run</strong> from the toolbar. Eclipse installs the app on your connected device and starts
it.</p>
<p>Or to run your app from a command line:</p>
<ol>
<li>Change directories to the root of your Android project and execute:
<pre class="no-pretty-print">ant debug</pre></li>
<li>Make sure the Android SDK <code>platform-tools/</code> directory is included in your
<code>PATH</code> environment variable, then execute:
<pre class="no-pretty-print">adb install bin/MyFirstApp-debug.apk</pre></li>
<li>On your device, locate <em>MyFirstActivity</em> and open it.</li>
</ol>
<p>To start adding stuff to the app, continue to the <a href="building-ui.html">next
lesson</a>.</p>
<h2 id="Emulator">Run on the Emulator</h2>
<p>Whether you’re using Eclipse or the command line, you need to first create an <a
href="{@docRoot}guide/developing/devices/index.html">Android Virtual
Device</a> (AVD). An AVD is a
device configuration for the Android emulator that allows you to model
different device configurations.</p>
<div class="figure" style="width:457px">
<img src="{@docRoot}images/screens_support/avds-config.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> The AVD Manager showing a few virtual
devices.</p>
</div>
<p>To create an AVD:</p>
<ol>
<li>Launch the Android Virtual Device Manager:
<ol type="a">
<li>In Eclipse, select <strong>Window > AVD Manager</strong>, or click the <em>AVD
Manager</em> icon in the Eclipse toolbar.</li>
<li>From the command line, change directories to <code>&lt;sdk>/tools/</code> and execute:
<pre class="no-pretty-print">./android avd</pre></li>
</ol>
</li>
<li>In the <em>Android Virtual Device Device Manager</em> panel, click <strong>New</strong>.</li>
<li>Fill in the details for the AVD.
Give it a name, a platform target, an SD card size, and a skin (HVGA is default).</li>
<li>Click <strong>Create AVD</strong>.</li>
<li>Select the new AVD from the <em>Android Virtual Device Manager</em> and click
<strong>Start</strong>.</li>
<li>After the emulator boots up, unlock the emulator screen.</li>
</ol>
<p>To run the app from Eclipse, open one of your project's files and click
<strong>Run</strong> from the toolbar. Eclipse installs the app on your AVD and starts it.</p>
<p>Or to run your app from the command line:</p>
<ol>
<li>Change directories to the root of your Android project and execute:
<pre class="no-pretty-print">ant debug</pre></li>
<li>Make sure the Android SDK <code>platform-tools/</code> directory is included in your
<code>PATH</code> environment
variable, then execute:
<pre class="no-pretty-print">adb install bin/MyFirstApp-debug.apk</pre></li>
<li>On the emulator, locate <em>MyFirstActivity</em> and open it.</li>
</ol>
<p>To start adding stuff to the app, continue to the <a href="building-ui.html">next
lesson</a>.</p>

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page.title=Starting Another Activity
parent.title=Building Your First App
parent.link=index.html
trainingnavtop=true
previous.title=Building a Simpler User Interface
previous.link=building-ui.html
@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="#RespondToButton">Respond to the Send Button</a></li>
<li><a href="#BuildIntent">Build an Intent</a></li>
<li><a href="#StartActivity">Start the Second Activity</a></li>
<li><a href="#CreateActivity">Create the Second Activity</a>
<ol>
<li><a href="#AddToManifest">Add it to the manifest</a></li>
</ol>
</li>
<li><a href="#ReceiveIntent">Receive the Intent</a></li>
<li><a href="#DisplayMessage">Display the Message</a></li>
</ol>
<h2>You should also read</h2>
<ul>
<li><a href="{@docRoot}sdk/installing.html">Installing the
SDK</a></li>
</ul>
</div>
</div>
<p>After completing the <a href="building-ui.html">previous lesson</a>, you have an app that
shows an activity (a single screen) with a text box and a button. In this lesson, you’ll add some
code to <code>MyFirstActivity</code> that
starts a new activity when the user selects the Send button.</p>
<h2 id="RespondToButton">Respond to the Send Button</h2>
<p>To respond to the button's on-click event, open the <code>main.xml</code> layout file and add the
<a
href="{@docRoot}reference/android/view/View.html#attr_android:onClick">{@code android:onClick}</a>
attribute to the {@link android.widget.Button &lt;Button>} element:</p>
<pre>
&lt;Button android:id="@+id/button_send"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_send"
android:onClick="sendMessage" />
</pre>
<p>The <a
href="{@docRoot}reference/android/view/View.html#attr_android:onClick">{@code
android:onClick}</a> attribute’s value, <code>sendMessage</code>, is the name of a method in your
activity that you want to call when the user selects the button.</p>
<p>Add the corresponding method inside the <code>MyFirstActivity</code> class:</p>
<pre>
/** Called when the user selects the Send button */
public void sendMessage(View view) {
// Do something in response to button
}
</pre>
<p class="note"><strong>Tip:</strong> In Eclipse, press Ctrl + Shift + O to import missing classes
(Cmd + Shift + O on Mac).</p>
<p>Note that, in order for the system to match this method to the method name given to <a
href="{@docRoot}reference/android/view/View.html#attr_android:onClick">{@code android:onClick}</a>,
the signature must be exactly as shown. Specifically, the method must:</p>
<ul>
<li>Be public</li>
<li>Have a void return value</li>
<li>Have a {@link android.view.View} as the only parameter (this will be the {@link
android.view.View} that was clicked)</li>
</ul>
<p>Next, you’ll fill in this method to read the contents of the text box and deliver that text to
another activity.</p>
<h2 id="BuildIntent">Build an Intent</h2>
<p>An {@link android.content.Intent} is an object that provides runtime binding between separate
components (such as two activities). The {@link android.content.Intent} represents an
app’s "intent to do something." You can use an {@link android.content.Intent} for a wide
variety of tasks, but most often they’re used to start another activity.</p>
<p>Inside the {@code sendMessage()} method, create an {@link android.content.Intent} to start
an activity called {@code DisplayMessageActvity}:</p>
<pre>
Intent intent = new Intent(this, DisplayMessageActivity.class);
</pre>
<p>The constructor used here takes two parameters:</p>
<ul>
<li>A {@link
android.content.Context} as its first parameter ({@code this} is used because the {@link
android.app.Activity} class is a subclass of {@link android.content.Context})
<li>The {@link java.lang.Class} of the app component to which the system should deliver
the {@link android.content.Intent} (in this case, the activity that should be started)
</ul>
<div class="sidebox-wrapper">
<div class="sidebox">
<h3>Sending an intent to other apps</h3>
<p>The intent created in this lesson is what's considered an <em>explicit intent</em>, because the
{@link android.content.Intent}
specifies the exact app component to which the intent should be given. However, intents
can also be <em>implicit</em>, in which case the {@link android.content.Intent} does not specify
the desired component, but allows any app installed on the device to respond to the intent
as long as it satisfies the meta-data specifications for the action that's specified in various
{@link android.content.Intent} parameters. For more informations, see the class about <a
href="{@docRoot}training/intents/index.html">Interacting with Other Apps</a>.</p>
</div>
</div>
<p class="note"><strong>Note:</strong> The reference to {@code DisplayMessageActivity}
will raise an error if you’re using an IDE such as Eclipse because the class doesn’t exist yet.
Ignore the error for now; you’ll create the class soon.</p>
<p>An intent not only allows you to start another activity, but can carry a bundle of data to the
activity as well. So, use {@link android.app.Activity#findViewById findViewById()} to get the
{@link android.widget.EditText} element and add its message to the intent:</p>
<pre>
Intent intent = new Intent(this, DisplayMessageActivity.class);
EditText editText = (EditText) findViewById(R.id.edit_message);
String message = editText.getText().toString();
intent.putExtra(EXTRA_MESSAGE, message);
</pre>
<p>An {@link android.content.Intent} can carry a collection of various data types as key-value
pairs called <em>extras</em>. The {@link android.content.Intent#putExtra putExtra()} method takes a
string as the key and the value in the second parameter.</p>
<p>In order for the next activity to query the extra data, you should define your keys using a
public constant. So add the {@code EXTRA_MESSAGE} definition to the top of the {@code
MyFirstActivity} class:</p>
<pre>
public class MyFirstActivity extends Activity {
public final static String EXTRA_MESSAGE = "com.example.myapp.MESSAGE";
...
}
</pre>
<p>It's generally a good practice to define keys for extras with your app's package name as a prefix
to ensure it's unique, in case your app interacts with other apps.</p>
<h2 id="StartActivity">Start the Second Activity</h2>
<p>To start an activity, you simply need to call {@link android.app.Activity#startActivity
startActivity()} and pass it your {@link android.content.Intent}.</p>
<p>The system receives this call and starts an instance of the {@link android.app.Activity}
specified by the {@link android.content.Intent}.</p>
<p>With this method included, the complete {@code sendMessage()} method that's invoked by the Send
button now looks like this:</p>
<pre>
/** Called when the user selects the Send button */
public void sendMessage(View view) {
Intent intent = new Intent(this, DisplayMessageActivity.class);
EditText editText = (EditText) findViewById(R.id.edit_message);
String message = editText.getText().toString();
intent.putExtra(EXTRA_MESSAGE, message);
startActivity(intent);
}
</pre>
<p>Now you need to create the {@code DisplayMessageActivity} class in order for this to
work.</p>
<h2 id="CreateActivity">Create the Second Activity</h2>
<p>In your project, create a new class file under the <code>src/&lt;package-name&gt;/</code>
directory called <code>DisplayMessageActivity.java</code>.</p>
<p class="note"><strong>Tip:</strong> In Eclipse, right-click the package name under the
<code>src/</code> directory and select <strong>New > Class</strong>.
Enter "DisplayMessageActivity" for the name and {@code android.app.Activity} for the superclass.</p>
<p>Inside the class, add the {@link android.app.Activity#onCreate onCreate()} callback method:</p>
<pre>
public class DisplayMessageActivity extends Activity {
&#64;Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
}
</pre>
<p>All subclasses of {@link android.app.Activity} must implement the {@link
android.app.Activity#onCreate onCreate()} method. The system calls this when creating a new
instance of the activity. It is where you must define the activity layout and where you should
initialize essential activity components.</p>
<h3 id="AddToManifest">Add it to the manifest</h3>
<p>You must declare all activities in your manifest file, <code>AndroidManifest.xml</code>, using an
<a
href="{@docRoot}guide/topics/manifest/activity-element.html">{@code &lt;activity>}</a> element.</p>
<p>Because {@code DisplayMessageActivity} is invoked using an explicit intent, it does not require
any intent filters (such as those you can see in the manifest for <code>MyFirstActivity</code>). So
the declaration for <code>DisplayMessageActivity</code> can be simply one line of code inside the <a
href="{@docRoot}guide/topics/manifest/application-element.html">{@code &lt;application>}</a>
element:</p>
<pre>
&lt;application ... >
&lt;activity android:name="com.example.myapp.DisplayMessageActivity" />
...
&lt;/application>
</pre>
<p>The app is now runnable because the {@link android.content.Intent} in the
first activity now resolves to the {@code DisplayMessageActivity} class. If you run the app now,
pressing the Send button starts the
second activity, but it doesn't show anything yet.</p>
<h2 id="ReceiveIntent">Receive the Intent</h2>
<p>Every {@link android.app.Activity} is invoked by an {@link android.content.Intent}, regardless of
how the user navigated there. You can get the {@link android.content.Intent} that started your
activity by calling {@link android.app.Activity#getIntent()} and the retrieve data contained
within it.</p>
<p>In the {@code DisplayMessageActivity} class’s {@link android.app.Activity#onCreate onCreate()}
method, get the intent and extract the message delivered by {@code MyFirstActivity}:</p>
<pre>
Intent intent = getIntent();
String message = intent.getStringExtra(MyFirstActivity.EXTRA_MESSAGE);
</pre>
<h2 id="DisplayMessage">Display the Message</h2>
<p>To show the message on the screen, create a {@link android.widget.TextView} widget and set the
text using {@link android.widget.TextView#setText setText()}. Then add the {@link
android.widget.TextView} as the root view of the activity’s layout by passing it to {@link
android.app.Activity#setContentView setContentView()}.</p>
<p>The complete {@link android.app.Activity#onCreate onCreate()} method for {@code
DisplayMessageActivity} now looks like this:</p>
<pre>
&#64;Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Get the message from the intent
Intent intent = getIntent();
String message = intent.getStringExtra(MyFirstActivity.EXTRA_MESSAGE);
// Create the text view
TextView textView = new TextView(this);
textView.setTextSize(40);
textView.setText(message);
setContentView(textView);
}
</pre>
<p>You can now run the app, type a message in the text box, press Send, and view the message on the
second activity.</p>
<img src="{@docRoot}images/training/firstapp/firstapp.png" />
<p class="img-caption"><strong>Figure 1.</strong> Both activities in the final app, running
on Android 4.0.
<p>That's it, you've built your first Android app!</p>
<p>To learn more about building Android apps, continue to follow the
basic training classes. The next class is <a
href="{@docRoot}training/activity-lifecycle/index.html">Managing the Activity Lifecycle</a>.</p>