开启另外一个Activity

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 MyFirstActivity that starts a new activity when the user selects the Send button.

Respond to the Send Button

To respond to the button's on-click event, open the main.xml layout file and add the android:onClick attribute to the <Button> element:

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage" />

The android:onClick attribute’s value, sendMessage, is the name of a method in your activity that you want to call when the user selects the button.

Add the corresponding method inside the MyFirstActivity class:

/** Called when the user selects the Send button */
public void sendMessage(View view) {
    // Do something in response to button
}

Tip: In Eclipse, press Ctrl + Shift + O to import missing classes (Cmd + Shift + O on Mac).

Note that, in order for the system to match this method to the method name given to android:onClick, the signature must be exactly as shown. Specifically, the method must:

• Be public
• Have a void return value
• Have a View as the only parameter (this will be the View that was clicked)

Next, you’ll fill in this method to read the contents of the text field and deliver that text to another activity.

Build an Intent

An Intent is an object that provides runtime binding between separate components (such as two activities). The Intent represents an app’s "intent to do something." You can use an Intent for a wide variety of tasks, but most often they’re used to start another activity.

Inside the sendMessage() method, create an Intent to start an activity called DisplayMessageActvity:

Intent intent = new Intent(this, DisplayMessageActivity.class);

The constructor used here takes two parameters:

• A Context as its first parameter (this is used because the Activity class is a subclass of Context)
• The Class of the app component to which the system should deliver the Intent (in this case, the activity that should be started)

Note: The reference to 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 can carry a bundle of data to the activity as well. So, use findViewById() to get the EditText element and add its message 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);

An Intent can carry a collection of various data types as key-value pairs called extras. The putExtra() method takes a string as the key and the value in the second parameter.

In order for the next activity to query the extra data, you should define your keys using a public constant. So add the EXTRA_MESSAGE definition to the top of the MyFirstActivity class:

public class MyFirstActivity extends Activity {
    public final static String EXTRA_MESSAGE = "com.example.myapp.MESSAGE";
    ...
}

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.

Start the Second Activity

To start an activity, you simply need to call startActivity() and pass it your Intent.

The system receives this call and starts an instance of the Activity specified by the Intent.

With this method included, the complete sendMessage() method that's invoked by the Send button now looks like this:

/** 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);
}

Now you need to create the DisplayMessageActivity class in order for this to work.

Create the Second Activity

In your project, create a new class file under the src/<package-name>/ directory called DisplayMessageActivity.java.

Tip: In Eclipse, right-click the package name under the src/ directory and select New > Class. Enter "DisplayMessageActivity" for the name and android.app.Activity for the superclass.

Inside the class, add the onCreate() callback method:

public class DisplayMessageActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }
}

All subclasses of Activity must implement the 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.

Add it to the manifest

You must declare all activities in your manifest file, AndroidManifest.xml, using an <activity> element.

Because DisplayMessageActivity is invoked using an explicit intent, it does not require any intent filters (such as those you can see in the manifest for MyFirstActivity). So the declaration for DisplayMessageActivity can be simply one line of code inside the <application> element:

<application ... >
    <activity android:name="com.example.myapp.DisplayMessageActivity" />
    ...
</application>

The app is now runnable because the Intent in the first activity now resolves to the DisplayMessageActivity class. If you run the app now, pressing the Send button starts the second activity, but it doesn't show anything yet.

Receive the Intent

Every Activity is invoked by an Intent, regardless of how the user navigated there. You can get the Intent that started your activity by calling getIntent() and the retrieve data contained within it.

In the DisplayMessageActivity class’s onCreate() method, get the intent and extract the message delivered by MyFirstActivity:

Intent intent = getIntent();
String message = intent.getStringExtra(MyFirstActivity.EXTRA_MESSAGE);

Display the Message

To show the message on the screen, create a TextView widget and set the text using setText(). Then add the TextView as the root view of the activity’s layout by passing it to setContentView().

The complete onCreate() method for 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(MyFirstActivity.EXTRA_MESSAGE);

    // Create the text view
    TextView textView = new TextView(this);
    textView.setTextSize(40);
    textView.setText(message);

    setContentView(textView);
}

You can now run the app, type a message in the text field, press Send, and view the message on the second activity.

Figure 1. Both activities in the final app, running on Android 4.0.

That's it, you've built your first Android app!

To learn more about building Android apps, continue to follow the basic training classes. The next class is Managing the Activity Lifecycle.