A door knocker is a very old item of door furniture that's not particularly popular in today's world. The main purpose of a door knocker is to allow people outside of a house or building to make their presence known to those inside it. In short, it's a fancier way of just using the classic hand-knock.

It might seem a bit strange that an app is called Door Knocker, right? The name does, in fact, make a bit of sense, as Door Knocker is an app that simply allows you to check the availability of portals, xdg-desktop-portal (from version 1.0 to 1.18) portals to be more precise.

XDG Desktop Portal is a portal frontend service for Flatpak (and other desktop containment frameworks). It works by showing various D-Bus interfaces (also known as portals).

In short, portals are used to grant sandboxed applications (such as Flatpak apps) access to various elements outside the sandbox. Portals allow apps (sandboxed or not) to implement various features without having to worry about the type and nature of a desktop environment (GNOME, Plasma, etc.) or the windowing system (Wayland, X11, etc.)/

As mentioned before, the app allows you to check the availability of all portals provided by xdg-desktop-portal (versions 1.0 to 1.18 supported). Hence, it shouldn't come as much of a surprise that the graphical user interface is quite simplistic.

There are two main sections: Unavailable Portals and Available Portals. You can click each individual app to see if a portal implementation is found or is missing, but this is fairly obvious even without doing so (unavailable portals are highlighted with red, and available ones are highlighted with green).