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

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();
    }
}

Answer 2

I think you need to understand more about what is context in Android. BroadcastReceiver is different from Activity, layout inflation is not needed.

Answer 3

You can implement an Activity without a UI. In the manifest you can specify android:theme="@android:style/Theme.NoDisplay". Take a look at this

You can also implement a Service which does not have any UI so you do not need layout inflation. Service just runs in background and shows no views. Take a look at Android Training and API Guide to learn more about Services