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Diffstat (limited to 'docs/html/training/basics/firstapp/starting-activity.jd')
| -rw-r--r-- | docs/html/training/basics/firstapp/starting-activity.jd | 137 |
1 files changed, 91 insertions, 46 deletions
diff --git a/docs/html/training/basics/firstapp/starting-activity.jd b/docs/html/training/basics/firstapp/starting-activity.jd index 37bc871..cbd063a 100644 --- a/docs/html/training/basics/firstapp/starting-activity.jd +++ b/docs/html/training/basics/firstapp/starting-activity.jd @@ -43,8 +43,8 @@ SDK</a></li> <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>MyFirstActivity</code> that -starts a new activity when the user selects the Send button.</p> +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> @@ -64,13 +64,13 @@ attribute to the {@link android.widget.Button <Button>} element:</p> <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> +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>Add the corresponding method inside the <code>MyFirstActivity</code> class:</p> +<p>Open the <code>MainActivity</code> class and add the corresponding method:</p> <pre> -/** Called when the user selects the Send button */ +/** Called when the user clicks the Send button */ public void sendMessage(View view) { // Do something in response to button } @@ -79,7 +79,7 @@ public void sendMessage(View view) { <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 +<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> @@ -99,11 +99,11 @@ another activity.</p> <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 +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 DisplayMessageActvity}:</p> +an activity called {@code DisplayMessageActivity}:</p> <pre> Intent intent = new Intent(this, DisplayMessageActivity.class); @@ -127,7 +127,7 @@ specifies the exact app component to which the intent should be given. However, 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 +{@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> @@ -136,9 +136,9 @@ href="{@docRoot}training/basics/intents/index.html">Interacting with Other Apps< 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 +<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 message to the intent:</p> +{@link android.widget.EditText} element and add its text value to the intent:</p> <pre> Intent intent = new Intent(this, DisplayMessageActivity.class); @@ -148,37 +148,36 @@ 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> +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 keys using a +<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 -MyFirstActivity} class:</p> +MainActivity} class:</p> <pre> -public class MyFirstActivity extends Activity { - public final static String EXTRA_MESSAGE = "com.example.myapp.MESSAGE"; +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 extras with your app's package name as a prefix -to ensure it's unique, in case your app interacts with other apps.</p> +<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}.</p> - -<p>The system receives this call and starts an instance of the {@link android.app.Activity} +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 method included, the complete {@code sendMessage()} method that's invoked by the Send +<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 selects the Send button */ +/** 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); @@ -195,20 +194,48 @@ work.</p> <h2 id="CreateActivity">Create the Second Activity</h2> -<p>In your project, create a new class file under the <code>src/<package-name>/</code> -directory called <code>DisplayMessageActivity.java</code>.</p> +<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 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>To create a new activity using Eclipse:</p> -<p>Inside the class, add the {@link android.app.Activity#onCreate onCreate()} callback method:</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> @@ -216,7 +243,7 @@ public class DisplayMessageActivity extends Activity { <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> +perform initial setup for the activity components.</p> @@ -226,22 +253,39 @@ initialize essential activity components.</p> <a href="{@docRoot}guide/topics/manifest/activity-element.html">{@code <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 <application>}</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="com.example.myapp.DisplayMessageActivity" /> ... + <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, -pressing the Send button starts the +clicking the Send button starts the second activity, but it doesn't show anything yet.</p> @@ -249,15 +293,15 @@ second activity, but it doesn't show anything yet.</p> <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 +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 MyFirstActivity}:</p> +method, get the intent and extract the message delivered by {@code MainActivity}:</p> <pre> Intent intent = getIntent(); -String message = intent.getStringExtra(MyFirstActivity.EXTRA_MESSAGE); +String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE); </pre> @@ -279,22 +323,23 @@ public void onCreate(Bundle savedInstanceState) { // Get the message from the intent Intent intent = getIntent(); - String message = intent.getStringExtra(MyFirstActivity.EXTRA_MESSAGE); + 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, type a message in the text field, press Send, and view the message on -the second activity.</p> +<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 1.</strong> Both activities in the final app, running +<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> |