Tile only application

Having seen the questions about activities with no GUI here, here and here. In addition, these answer are rather old, I don't know if they're still relevant.

I want to create an app for which the only user interaction is a quick tile.

I understood there can't be no activity at all, so I have a blank activity with no display using @android:style/Theme.NoDisplay

<activity
    android:name=".MainActivity"
    android:label="@string/app_name"
    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>

<service
    android:name=".AdaptativeSleepTile"
    android:icon="@drawable/ic_launcher_background">
</service>

But the app appears in my app list (with nothing happening when I click it, logically), which is not what is written in the comments of this answer.

I can't remove the line <category android:name="android.intent.category.LAUNCHER"/> otherwise I get the error message

Could not identify launch activity: Default Activity not found
Error while Launching activity

So what should I do to have a service for which the only user interaction is the quick tile? (Same question would also apply for no interaction at all, or only widget I guess)

Using Android Studio 4 and Sdk 29

Solution

I want to create an app for which the only user interaction is a quick tile.

That may or may not be practical. Since Android 3.1, apps are installed in a "stopped state", and it takes an explicit Intent to move them out of the stopped state and allow them to run. Normally, that comes from the user tapping on an icon in a launcher to run your MAIN/LAUNCHER activity.

It is possible that simply having a TileService available in the manifest is enough to get you listed in the notification shade, and the act of adding the tile will be enough to move your app out of the stopped state. However, I certainly cannot guarantee that, and it would not surprise me if this does not work.

Also, please bear in mind that you may need an activity for other reasons:

To display your terms of service
To display your privacy policy
To provide access to tech support
To allow for configuration of the app
And so on

But the app appears in my app list (with nothing happening when I click it, logically), which is not what is written in the comments of this answer.

If you mean the first comment, that is simply wrong, as is pointed out by other comments on that answer.

I can't remove the line otherwise I get the error message

I assume that you are getting that from Android Studio. You will need to change your run configuration to not try starting an activity.

Same question would also apply for no interaction at all

Fortunately, there is no solution for that. Malware authors think that totally invisible apps are wonderful, and Android takes steps to prevent such apps from being able to work. And, this is why I would not be surprised if you need an activity for your TileService.