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Subsystems were introduced into Unreal Engine 4.22 and provide an elegant method for creating managers of any kind.
It minimizes the need to put a lot of "manager code" into extended classes of UGameInstance, AWorldSettings, AGameModeBase. It's common to end up with many various systems in a single class, which is messy and more difficult to maintain over time.
Instead, now we can use automatically instanced objects called subsystems. Engine creates an instance of this object together with its "parent" object, i.e. Game Instance, World. You don't need to store a pointer to it anywhere.
You can have as many subsystems for a given "parent" as you want, i.e Audio Manager and UI Manager as Game Instance subsystems, Weather Manager and Monster Spawn as World subsystems.
A Subsystem contains the lifecycle methods Initialize and Deinitialize, which will be invoked automatically and its lifetime will be determined by the engine class that it was inherited from.
Subsystem | Inherits from | Engine version
------------- | ---------------------- | --------------
Engine | UEngineSubsystem | 4.22
Editor | UEngineSubsystem | 4.22
Game Instance | UGameInstanceSubsystem | 4.22
Local Player | ULocalPlayerSubsystem | 4.22
World | UWorldSubsystem | 4.24
A Subsystem may be referenced from a Blueprint Graph in the same way a variable is referenced.