This lesson teaches you to
- Respond to the Send Button
- Build an Intent
- Start the Second Activity
- Create the Second Activity
- Receive the Intent
- Display the Message
You should also read
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.
Respond to the Send Button
To respond to the button's on-click event, open the fragment_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 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 }
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 theView
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 intents 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 DisplayMessageActivity
:
Intent intent = new Intent(this, DisplayMessageActivity.class);
This requires that you import the Intent
class:
import android.content.Intent;
Tip: In Eclipse, press Ctrl + Shift + O to import missing classes (Cmd + Shift + O on Mac).
The constructor used here takes two parameters:
- A
Context
as its first parameter (this
is used because theActivity
class is a subclass ofContext
) - The
Class
of the app component to which the system should deliver theIntent
(in this case, the activity that should be started)
Sending an intent to other apps
The intent created in this lesson is what's considered an explicit intent, because the
Intent
specifies the exact app component to which the intent should be given. However, intents
can also be implicit, in which case the 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
Intent
parameters. For more information, see the class about Interacting with Other Apps.
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 it can carry a bundle of data to the
activity as well. Inside the sendMessage()
method,
use findViewById()
to get the
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 an import statement for android.widget.EditText
.
You'll define the EXTRA_MESSAGE
constant in a moment.
An Intent
can carry a collection of various data types as key-value
pairs called extras. The 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 EXTRA_MESSAGE
definition to the top of the MainActivity
class:
public class MainActivity extends ActionBarActivity { 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.
Start the Second Activity
To start an activity, 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 new code, the complete 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 DisplayMessageActivity
class in order for this to
work.
Create the Second Activity
To create a new activity using Eclipse:
- Click New in the toolbar.
- In the window that appears, open the Android folder and select Android Activity. Click Next.
- Select BlankActivity and click Next.
- Fill in the activity details:
- Project: MyFirstApp
- Activity Name: DisplayMessageActivity
- Layout Name: activity_display_message
- Fragment Layout Name: fragment_display_message
- Title: My Message
- Hierarchial Parent: com.example.myfirstapp.MainActivity
- Navigation Type: None
Click Finish.
If you're using a different IDE or the command line tools, create a new file named
DisplayMessageActivity.java
in the project's src/
directory, next to
the original MainActivity.java
file.
Open the DisplayMessageActivity.java
file. If you used Eclipse to create this
activity:
- The class
already includes an implementation of the required
onCreate()
method. You will update the implementation of this method later. - There's also an implementation of the
onCreateOptionsMenu()
method, but you won't need it for this app so you can remove it. - There's also an implementation of
onOptionsItemSelected()
which handles the behavior for the action bar's Up behavior. Keep this one the way it is. - There's also a
PlaceholderFragment
class that extendsFragment
. You will not need this class in the final version of this activity.
Fragments decompose application functionality and UI into reusable modules. For more information on fragments, see the Fragments API Guide. The final version of this activity does not use fragments.
Note: Your activity may look different if you did not use the latest version of the ADT plugin. Make sure you install the latest version of the ADT plugin to complete this tutorial.
The DisplayMessageActivity
class should now look like this:
public class DisplayMessageActivity extends ActionBarActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_display_message); if (savedInstanceState == null) { getSupportFragmentManager().beginTransaction() .add(R.id.container, new PlaceholderFragment()).commit(); } } @Override public boolean onOptionsItemSelected(MenuItem item) { // Handle action bar item clicks here. The action bar will // automatically handle clicks on the Home/Up button, so long // as you specify a parent activity in AndroidManifest.xml. int id = item.getItemId(); if (id == R.id.action_settings) { return true; } return super.onOptionsItemSelected(item); } /** * A placeholder fragment containing a simple view. */ public static class PlaceholderFragment extends Fragment { public PlaceholderFragment() { } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View rootView = inflater.inflate(R.layout.fragment_display_message, container, false); return rootView; } } }
If you used an IDE other than Eclipse, update your DisplayMessageActivity
class with the above code.
All subclasses of Activity
must implement the 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 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 activity_display_message
layout that's requested by
setContentView()
. That's OK because
you will update this method later and won't be using that layout.
Add the title string
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 strings.xml
file:
<resources> ... <string name="title_activity_display_message">My Message</string> </resources>
Add it to the manifest
All activities must be declared in your manifest file, AndroidManifest.xml
, using an
<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 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 <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.
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 retrieve the data contained
within it.
In the DisplayMessageActivity
class’s onCreate()
method, get the intent and extract the message delivered by MainActivity
:
Intent intent = getIntent(); String message = intent.getStringExtra(MainActivity.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(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.
That's it, you've built your first Android app!
To learn more, follow the link below to the next class.