This can be registered to the FSlateApplication anytime.
Snippet:
PlayerInputProcessor = MakeShared<FPlayerInputProcessor>();
FSlateApplication::Get().RegisterInputPreProcessor(PlayerInputProcessor);It will be used all the time, even in editor, so watch out depending on where you register it.
Subclass IInputProcessor, the only mandatory function to overwrite is Tick.
The input processor is used in the various related functions in the slate application class.
For example FSlateApplication::ProcessMouseButtonUpEvent will call HandleMouseButtonUpEvent on all registered processors.
The first processor that returns true will break the loop and override the default slate app behavior for that event.
If its not overridden it will go to FSlateApplication::RoutePointerUpEvent, which in this example will call SViewport::OnMouseButtonUp and FSceneViewport::OnMouseButtonUp.