| 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/index.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 field and a button. In this lesson, you’ll add some |
| code to <code>MainActivity</code> that |
| starts a new activity when the user clicks 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 <Button>} element:</p> |
| |
| <pre> |
| <Button |
| 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 the system calls when the user clicks the button.</p> |
| |
| <p>Open the <code>MainActivity</code> class and add the corresponding method:</p> |
| |
| <pre> |
| /** Called when the user clicks 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>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 field 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 intents 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 DisplayMessageActivity}:</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 information, see the class about <a |
| href="{@docRoot}training/basics/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 it 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 text value 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 the |
| key name in the first parameter and the value in the second parameter.</p> |
| |
| <p>In order for the next activity to query the extra data, you should define your key using a |
| public constant. So add the {@code EXTRA_MESSAGE} definition to the top of the {@code |
| MainActivity} class:</p> |
| |
| <pre> |
| public class MainActivity extends Activity { |
| public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE"; |
| ... |
| } |
| </pre> |
| |
| <p>It's generally a good practice to define keys for intent extras using your app's package name |
| as a prefix. This ensures they are 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}. 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 new code, the complete {@code sendMessage()} method that's invoked by the Send |
| button now looks like this:</p> |
| |
| <pre> |
| /** Called when the user clicks 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> |
| |
| <div class="figure" style="width:400px"> |
| <img src="{@docRoot}images/training/firstapp/adt-new-activity.png" alt="" /> |
| <p class="img-caption"><strong>Figure 1.</strong> The new activity wizard in Eclipse.</p> |
| </div> |
| |
| <p>To create a new activity using Eclipse:</p> |
| |
| <ol> |
| <li>Click New <img src="{@docRoot}images/tools/eclipse-new.png" |
| style="vertical-align:baseline;margin:0" /> in the toolbar.</li> |
| <li>In the window that appears, open the <strong>Android</strong> folder |
| and select <strong>Android Activity</strong>. Click <strong>Next</strong>.</li> |
| <li>Select <strong>BlankActivity</strong> and click <strong>Next</strong>.</li> |
| <li>Fill in the activity details: |
| <ul> |
| <li><em>Project</em>: MyFirstApp</li> |
| <li><em>Activity Name</em>: DisplayMessageActivity</li> |
| <li><em>Layout Name</em>: activity_display_message</li> |
| <li><em>Navigation Type</em>: None</li> |
| <li><em>Hierarchial Parent</em>: com.example.myfirstapp.MainActivity</li> |
| <li><em>Title</em>: My Message</li> |
| </ul> |
| <p>Click <strong>Finish</strong>.</p> |
| </li> |
| </ol> |
| |
| <p>If you're using a different IDE or the command line tools, create a new file named |
| {@code DisplayMessageActivity.java} in the project's <code>src/</code> directory, next to |
| the original {@code MainActivity.java} file.</p> |
| |
| <p>Open the {@code DisplayMessageActivity.java} file. If you used Eclipse to create it, the class |
| already includes an implementation of the required {@link android.app.Activity#onCreate onCreate()} |
| method. There's also an implemtation of the {@link android.app.Activity#onCreateOptionsMenu |
| onCreateOptionsMenu()} method, but |
| you won't need it for this app so you can remove it. The class should look like this:</p> |
| |
| <pre> |
| public class DisplayMessageActivity extends Activity { |
| @Override |
| public void onCreate(Bundle savedInstanceState) { |
| super.onCreate(savedInstanceState); |
| setContentView(R.layout.activity_display_message); |
| } |
| } |
| </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 |
| perform initial setup for the 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 <activity>}</a> element.</p> |
| |
| <p>When you use the Eclipse tools to create the activity, it creates a default entry. It should |
| look like this:</p> |
| |
| <pre> |
| <application ... > |
| ... |
| <activity |
| android:name=".DisplayMessageActivity" |
| android:label="@string/title_activity_display_message" > |
| <meta-data |
| android:name="android.support.PARENT_ACTIVITY" |
| android:value="com.example.myfirstapp.MainActivity" /> |
| </activity> |
| </application> |
| </pre> |
| |
| <p>The <a href="{@docRoot}guide/topics/manifest/meta-data-element.html">{@code |
| <meta-data>}</a> element declares the name of this activity's parent activity |
| within the app's logical hierarchy. The Android <a |
| href="{@docRoot}tools/extras/support-library.html">Support Library</a> uses this information |
| to implement default navigation behaviors, such as <a |
| href="{@docRoot}design/patterns/navigation.html">Up navigation</a>.</p> |
| |
| <p class="note"><strong>Note:</strong> During <a |
| href="{@docRoot}sdk/installing/adding-packages.html">installation</a>, you should have downloaded |
| the latest Support Library. Eclipse automatically includes this library in your app project (you |
| can see the library's JAR file listed under <em>Android Dependencies</em>). If you're not using |
| Eclipse, you may need to manually add the library to your project—follow this guide for <a |
| href="{@docRoot}tools/extras/support-library.html#SettingUp">setting up the Support Library</a>.</p> |
| |
| <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, |
| clicking 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 retrieve the 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 MainActivity}:</p> |
| |
| <pre> |
| Intent intent = getIntent(); |
| String message = intent.getStringExtra(MainActivity.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> |
| @Override |
| public void onCreate(Bundle savedInstanceState) { |
| super.onCreate(savedInstanceState); |
| |
| // Get the message from the intent |
| Intent intent = getIntent(); |
| String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE); |
| |
| // Create the text view |
| TextView textView = new TextView(this); |
| textView.setTextSize(40); |
| textView.setText(message); |
| |
| // Set the text view as the activity layout |
| setContentView(textView); |
| } |
| </pre> |
| |
| <p>You can now run the app. When it opens, type a message in the text field, click Send, |
| and the message appears on the second activity.</p> |
| |
| <img src="{@docRoot}images/training/firstapp/firstapp.png" /> |
| <p class="img-caption"><strong>Figure 2.</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/basics/activity-lifecycle/index.html">Managing the Activity |
| Lifecycle</a>.</p> |
| |
| |
| |
| |