Figure 1. The new activity wizard in Eclipse.
After completing the previous lesson, 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 MainActivity that
starts a new activity when the user clicks the Send button.
To respond to the button's on-click event, open the activity_main.xml
layout file and add the {@code android:onClick}
attribute to the {@link android.widget.Button <Button>} element:
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_send"
android:onClick="sendMessage" />
The {@code
android:onClick} attribute’s value, "sendMessage", is the name of a method in your
activity that the system calls when the user clicks the button.
Open the MainActivity class (located in the project's
src/ directory) and add the corresponding method:
/** Called when the user clicks the Send button */
public void sendMessage(View view) {
// Do something in response to button
}
This requires that you import the {@link android.view.View} class:
import android.view.View;
Tip: In Eclipse, press Ctrl + Shift + O to import missing classes (Cmd + Shift + O on Mac).
In order for the system to match this method to the method name given to {@code android:onClick}, the signature must be exactly as shown. Specifically, the method must:
Next, you’ll fill in this method to read the contents of the text field and deliver that text to another activity.
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.
Inside the {@code sendMessage()} method, create an {@link android.content.Intent} to start an activity called {@code DisplayMessageActivity}:
Intent intent = new Intent(this, DisplayMessageActivity.class);
The constructor used here takes two parameters:
The intent created in this lesson is what's considered an explicit intent, because the {@link android.content.Intent} specifies the exact app component to which the intent should be given. However, intents can also be implicit, 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 Interacting with Other Apps.
Note: 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.
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:
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);
Note:
You now need import statements for android.content.Intent
and android.widget.EditText. You'll define the EXTRA_MESSAGE
constant in a moment.
An {@link android.content.Intent} can carry a collection of various data types as key-value pairs called extras. The {@link android.content.Intent#putExtra putExtra()} method takes the key name in the first parameter and the value in the second parameter.
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:
public class MainActivity extends Activity {
public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
...
}
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.
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}.
With this new code, the complete {@code sendMessage()} method that's invoked by the Send button now looks like this:
/** 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);
}
Now you need to create the {@code DisplayMessageActivity} class in order for this to work.
Figure 1. The new activity wizard in Eclipse.
To create a new activity using Eclipse:
in the toolbar.Click Finish.
If you're using a different IDE or the command line tools, create a new file named
{@code DisplayMessageActivity.java} in the project's src/ directory, next to
the original {@code MainActivity.java} file.
Open the {@code DisplayMessageActivity.java} file. If you used Eclipse to create this activity:
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 lint errors.
The {@code DisplayMessageActivity} class should now look like this:
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);
}
}
If you used an IDE other than Eclipse, update your {@code DisplayMessageActivity} class with the above code.
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.
Note: 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.
If you used Eclipse, you can skip to the next section, because the template provides the title string for the new activity.
If you're using an IDE other than Eclipse, add the new activity's title to the {@code strings.xml} file:
<resources>
...
<string name="title_activity_display_message">My Message</string>
</resources>
All activities must be declared in your manifest file, AndroidManifest.xml, using an
{@code <activity>} element.
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:
<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>
The {@code android:parentActivityName} 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 Up navigation on Android 4.1 (API level 16) and higher. You can provide the same navigation behaviors for older versions of Android by using the Support Library and adding the {@code <meta-data>} element as shown here.
Note: 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 Adding Platforms and Packages 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 Android Dependencies). If you're not using Eclipse, you need to manually add the library to your project—follow the guide for setting up the Support Library then return here.
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.
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.
In the {@code DisplayMessageActivity} class’s {@link android.app.Activity#onCreate onCreate()} method, get the intent and extract the message delivered by {@code MainActivity}:
Intent intent = getIntent(); String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
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()}.
The complete {@link android.app.Activity#onCreate onCreate()} method for {@code DisplayMessageActivity} now looks like this:
@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);
}
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.
Figure 2. Both activities in the final app, running on Android 4.0.
That's it, you've built your first Android app!
To learn more, follow the link below to the next class.