| 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></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>activity_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 (located in the project's |
| <code>src/</code> directory) 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>This requires that you import the {@link android.view.View} class:</p> |
| <pre> |
| import android.view.View; |
| </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. Inside the {@code sendMessage()} method, |
| 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 class="note"><strong>Note:</strong> |
| You now need import statements for <code>android.content.Intent</code> |
| and <code>android.widget.EditText</code>. You'll define the <code>EXTRA_MESSAGE</code> |
| constant in a moment.</p> |
| |
| <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 the key |
| for your intent's extra 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, 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 <strong>New</strong> <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><strong>Project</strong>: MyFirstApp</li> |
| <li><strong>Activity Name</strong>: DisplayMessageActivity</li> |
| <li><strong>Layout Name</strong>: activity_display_message</li> |
| <li><strong>Title</strong>: My Message</li> |
| <li><strong>Hierarchial Parent</strong>: com.example.myfirstapp.MainActivity</li> |
| <li><strong>Navigation Type</strong>: None</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 this |
| activity:</p> |
| <ul> |
| <li>The class |
| already includes an implementation of the required {@link android.app.Activity#onCreate onCreate()} |
| method.</li> |
| <li>There's also an implementation of the {@link android.app.Activity#onCreateOptionsMenu |
| onCreateOptionsMenu()} method, but |
| you won't need it for this app so you can remove it.</li> |
| <li>There's also an implementation of {@link android.app.Activity#onOptionsItemSelected |
| onOptionsItemSelected()} which handles the behavior for the action bar's <em>Up</em> behavior. |
| Keep this one the way it is.</li> |
| </ul> |
| |
| <p>Because the {@link android.app.ActionBar} APIs are available only on {@link |
| android.os.Build.VERSION_CODES#HONEYCOMB} (API level 11) and higher, you must add a condition |
| around the {@link android.app.Activity#getActionBar()} method to check the current platform version. |
| Additionally, you must add the {@code @SuppressLint("NewApi")} tag to the |
| {@link android.app.Activity#onCreate onCreate()} method to avoid <a |
| href="{@docRoot}tools/help/lint.html">lint</a> errors.</p> |
| |
| <p>The {@code DisplayMessageActivity} class should now look like this:</p> |
| |
| <pre> |
| public class DisplayMessageActivity extends Activity { |
| |
| @SuppressLint("NewApi") |
| @Override |
| protected void onCreate(Bundle savedInstanceState) { |
| super.onCreate(savedInstanceState); |
| setContentView(R.layout.activity_display_message); |
| |
| // Make sure we're running on Honeycomb or higher to use ActionBar APIs |
| if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) { |
| // Show the Up button in the action bar. |
| getActionBar().setDisplayHomeAsUpEnabled(true); |
| } |
| } |
| |
| @Override |
| public boolean onOptionsItemSelected(MenuItem item) { |
| switch (item.getItemId()) { |
| case android.R.id.home: |
| NavUtils.navigateUpFromSameTask(this); |
| return true; |
| } |
| return super.onOptionsItemSelected(item); |
| } |
| } |
| </pre> |
| |
| <p>If you used an IDE other than Eclipse, update your {@code DisplayMessageActivity} |
| class with the above code.</p> |
| |
| <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. This method is where you must define the activity layout |
| with the {@link android.app.Activity#setContentView setContentView()} method |
| and is where you should |
| perform initial setup for the activity components.</p> |
| |
| <p class="note"><strong>Note:</strong> If you are using an IDE other than Eclipse, your project |
| does not contain the {@code activity_display_message} layout that's requested by |
| {@link android.app.Activity#setContentView setContentView()}. That's OK because |
| you will update this method later and won't be using that layout.</p> |
| |
| |
| <h3 id="AddTitle">Add the title string</h3> |
| |
| <p>If you used Eclipse, you can skip to the <a href="#AddToManifest">next section</a>, |
| because the template provides |
| the title string for the new activity.</p> |
| |
| <p>If you're using an IDE other than Eclipse, |
| add the new activity's title to the {@code strings.xml} file:</p> |
| <pre> |
| <resources> |
| ... |
| <string name="title_activity_display_message">My Message</string> |
| </resources> |
| </pre> |
| |
| |
| |
| <h3 id="AddToManifest">Add it to the manifest</h3> |
| |
| <p>All activities must be declared 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. If you're |
| using a different IDE, you need to add the manifest entry yourself. It should |
| look like this:</p> |
| |
| <pre> |
| <application ... > |
| ... |
| <activity |
| android:name="com.example.myfirstapp.DisplayMessageActivity" |
| android:label="@string/title_activity_display_message" |
| android:parentActivityName="com.example.myfirstapp.MainActivity" > |
| <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/activity-element.html#parent">{@code |
| android:parentActivityName}</a> attribute declares the name of this activity's parent activity |
| within the app's logical hierarchy. The system uses this value |
| to implement default navigation behaviors, such as <a |
| href="{@docRoot}design/patterns/navigation.html">Up navigation</a> on |
| Android 4.1 (API level 16) and higher. You can provide the same navigation behaviors for |
| older versions of Android by using the |
| <a href="{@docRoot}tools/support-library/index.html">Support Library</a> and adding |
| the <a href="{@docRoot}guide/topics/manifest/meta-data-element.html">{@code |
| <meta-data>}</a> element as shown here.</p> |
| |
| <p class="note"><strong>Note:</strong> Your Android SDK should already include |
| the latest Android Support Library. It's included with the ADT Bundle but if you're using |
| a different IDE, you should have installed it during the |
| <a href="{@docRoot}sdk/installing/adding-packages.html">Adding Platforms and Packages</a> step. |
| When using the templates in Eclipse, the Support Library is automatically added to your app project |
| (you can see the library's JAR file listed under <em>Android Dependencies</em>). If you're not using |
| Eclipse, you need to manually add the library to your project—follow the guide for <a |
| href="{@docRoot}tools/support-library/setup.html">setting up the Support Library</a> |
| then return here.</p> |
| |
| <p>If you're developing with Eclipse, you can run the app now, but not much happens. |
| Clicking the Send button starts the second activity but it uses |
| a default "Hello world" layout provided by the template. You'll soon update the |
| activity to instead display a custom text view, so if you're using a different IDE, |
| don't worry that the app won't yet compile.</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> |
| |
| |
| |
| |