android - Must every activity have a layout?

Question

I'm just getting started with Android and was reading up BroadcastReceiver. Since the MainActivity was being used only to get the alarm time in seconds, it got me thinking whether layout XML files are must for every activity in Android. I mean, is it possible to have an app that when launched, shows no view, but successfully sets up a receiver?

Answer 1

The answer is yes it's possible. Activities don't have to have a UI. It's mentioned in the documentation, e.g.:

An activity is a single, focused thing that the user can do. Almost all activities interact with the user [...]

(see http://developer.android.com/reference/android/app/Activity.html)

Related SO question: https://stackoverflow.com/a/12817384/534471

To e.g. display a Toast from an Activity without layout you would define the activity in your manifest like so:

<activity
    android:name=".MainActivity"
    android:theme="@android:style/Theme.NoDisplay">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

The code would look like this:

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Toast.makeText(this, "I'm alive", Toast.LENGTH_LONG).show();
        finish();
    }
}