How to Launch an Activity Without a Ui

How to launch an Activity without a UI?

You need to add the Intent flag,

intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

Or

call "finish();" after firing the intent.

Must every activity have a layout?

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();
    }
}

Start activity without showing it

I found the solution:

• Activity A starts activity B with extra parameter (intent.putExtra("something", true))
• Activity B:

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (getIntent().hasExtra("something") && getIntent().getBooleanExtra("something", false) {
            //show activity B
        setContentView(R.layout.activity_B);
    } else {
            //don't show activity B, start activity C
        startActivityForResult(activity_C, ACTIVITY_NOT_INITIALIZED); //start activity C
    }
}

//when came back from activity C, check if activity B was already initialized

protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (requestCode == ACTIVITY_NOT_INITIALIZED) {
        //show activity B
        setContentView(R.layout.activity_B);
  }
}

How to start android activity without showing main activity in the back

I solved my problem like this :

1- I added to the main activity attributes in the manifest file :

android:launchMode="singleTask"

I tested it does not makes loading time when re-launching activity longer than usually because this attribute does not erase the app cache and keeps the intent instance in the memory.

2- Add this to main activity class :

@Override
void onPause()
    {
    super.onPause();
    finish();
    }

Thank you all for your help.

Android Activity with no GUI

Your best bet would seem to be using a BroadcastReceiver. You can create a new BroadcastReceiver that listens for the Intent to trigger your notification and start your service like this:

public class MyIntentReceiver extends BroadcastReceiver {    
  @Override 
  public void onReceive(Context _context, Intent _intent) {
    if (_intent.getAction().equals(MY_INTENT)) {
      // TODO Broadcast a notification
      _context.startService(new Intent(_context, MyService.class));
    }
  }    
}

And you can register this IntentReceiver directly in the application Manifest without needing to include it within an Activity:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.domain.myapplication">
  <application android:icon="@drawable/icon" android:label="@string/app_name">
    <service android:enabled="true" android:name="MyService"></service>
    <receiver android:enabled="true" android:name="MyIntentReceiver">
      <intent-filter>
        <action android:name="MY_INTENT" />
      </intent-filter>
    </receiver>
  </application>
</manifest> 

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