How do I set my .icon file as an app icon?

Where does that file extension (.icon) come from? I have never heard of it. The macOS system traditionally uses 'icns' as an icon file format. But not really any more...

@barnabaskordas

From Add your Icon Composer file to an Xcode project:

If you create your Icon Composer file outside of Xcode, you can add it to your Xcode project anytime to view your icon in Simulator and on real devices.

Just drag the Icon Composer file from Finder to the Project navigator, and Xcode provides feedback on where to drop it in a target folder. Alternatively, choose Add Files from the Add button at the bottom of the Project navigator and select your Icon Composer file in the dialog that appears.

In the project editor, select the target and the General tab. Under App Icons and Launch Screen, ensure that the name in the App Icon text field matches the name of the Icon Composer file without the extension. You can have multiple Icon Composer files in your project but only one that matches the name in the App Icon text field.

For an example, see the Configuring your app to use alternate app icons sample code.

@Tomato

For more information, see Creating your app icon using Icon Composer.