What is a Subsystem? What should it be used for?

Development Programming & Scripting
2

Okay, poking through engine code has led to a couple discoveries:

  • Each object which can have a subsystem has a FSubsystemCollection associated with it. This FSubsystemCollection takes a template, so to use my GameInstance example, UGameInstance contains FSubsystemCollection<UGameInstanceSubsystem>.
  • When the GameInstance Init function gets called, the FSubsystemCollection looks at everything which inherits from UGameInstanceSubsystem (or whatever subsystem it’s using at the time). If USubsystem->ShouldCreateSubsystem returns true, then it’ll create the associated subsystem as a new object, parented to whatever the FSubsystemCollection says its parent should be ( UGameInstance, in our case). So in reality, all our subsystems get created as children of our main system.
  • The end result is that you can call UGameInstance->GetSubsystem with any valid child class of UGameInstanceSubsystem and it’ll return an instance of that class. It’s similar to having a component on an actor.
  • By itself, a subsystem doesn’t actually do anything. Nothing gets called on it, and it doesn’t override any functions. You have Initialize and Deinitialize to start and stop the subsystem, but otherwise it’s an open book as to what you do with it. It just makes code clearer for non-actor objects which could benefit from a component-like interface.
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