Subsystems.h

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/** @noop License Comment
 *  @file
 *  @copyright
 *  This Source Code is subject to the terms of the Mozilla Public License, v2.0.
 *  If a copy of the MPL was not distributed with this file You can obtain one at
 *  https://mozilla.org/MPL/2.0/
 *  
 *  @author David Mórász
 *  @date 2025
 */
 
#pragma once
 
#include "CoreMinimal.h"
#include "Mcro/Concepts.h"
#include "Mcro/AssertMacros.h"
#include "Kismet/GameplayStatics.h"
 
namespace Mcro::Subsystems
{
    using namespace Mcro::Concepts;
    using namespace Mcro::TypeName;
 
    template<typename T>
    concept CSubsystem = CDerivedFrom<T, USubsystem>;
 
    template<typename T>
    concept CEngineSubsystem = CDerivedFrom<T, UEngineSubsystem>;
 
    template<typename T>
    concept CGameInstanceSubsystem = CDerivedFrom<T, UGameInstanceSubsystem>;
 
    template<typename T>
    concept CLocalPlayerSubsystem = CDerivedFrom<T, ULocalPlayerSubsystem>;
 
    template<typename T>
    concept CWorldSubsystem = CDerivedFrom<T, UWorldSubsystem>;
 
    enum class EGameInstanceFallback
    {
        UseGameViewport,
        UseFirstWorldContext,
    };
 
    namespace Subsystems
    {
        /**
         *  Helper for getting a subsystem, the internal boilerplate will be chosen based on the type of subsystem T
         *  inherits from. Required arguments may vary depending on the type of subsystem.
         *  
         *  @tparam T  UEngineSubsystem derivative
         *  @return    Subsystem if exists or nullptr
         */
        template<CEngineSubsystem T>
        T* Get()
        {
            return GEngine ? GEngine->GetEngineSubsystem<T>() : nullptr;
        }
 
        /**
         *  Helper for getting a subsystem, the internal boilerplate will be chosen based on the type of subsystem T
         *  inherits from. Required arguments may vary depending on the type of subsystem.
         *  
         *  @tparam T  UGameInstanceSubsystem derivative
         *  @return    Subsystem if exists or nullptr
         */
        template<CGameInstanceSubsystem T>
        T* Get(
            const UObject* worldContextObject = nullptr,
            EGameInstanceFallback fallback = EGameInstanceFallback::UseGameViewport,
            EGetWorldErrorMode errorMode = EGetWorldErrorMode::LogAndReturnNull
        )
        {
            if (!GEngine)
            {
                return nullptr;
            }
            if (worldContextObject == nullptr)
            {
                if (IsRunningDedicatedServer() || GEngine->GameViewport == nullptr)
                {
                    fallback = EGameInstanceFallback::UseFirstWorldContext;
                }
                switch (fallback)
                {
                default:
                case EGameInstanceFallback::UseGameViewport:
                    {
                        const FWorldContext* worldContext = GEngine->GameViewport
                            ? GEngine->GetWorldContextFromGameViewport(GEngine->GameViewport)
                            : nullptr;
                        const UGameInstance* owningGameInstance = worldContext ? worldContext->OwningGameInstance : nullptr;
                        return owningGameInstance ? owningGameInstance->GetSubsystem<T>() : nullptr;
                    }
                case EGameInstanceFallback::UseFirstWorldContext:
                    {
                        for (auto it = GEngine->GetWorldContexts().CreateConstIterator(); it; ++it)
                        {
                            if (T* result = it->OwningGameInstance ? it->OwningGameInstance->GetSubsystem<T>() : nullptr)
                            {
                                return result;
                            }
                        }
                        return nullptr;
                    }
                }
            }
            if (const UWorld* world = GEngine->GetWorldFromContextObject(worldContextObject, errorMode))
            {
                return UGameInstance::GetSubsystem<T>(world->GetGameInstance());
            }
            return nullptr;
        }
 
        /**
         *  Helper for getting a subsystem, the internal boilerplate will be chosen based on the type of subsystem T
         *  inherits from. Required arguments may vary depending on the type of subsystem.
         *  
         *  @tparam T  ULocalPlayerSubsystem derivative
         *  @return    Subsystem if exists or nullptr
         */
        template<CLocalPlayerSubsystem T>
        T* Get(const UObject* worldContext)
        {
            const UWorld* world = GEngine->GetWorldFromContextObject(worldContext, EGetWorldErrorMode::LogAndReturnNull);
            APlayerController* controller = world && !world->IsNetMode(NM_DedicatedServer)
                ? UGameplayStatics::GetPlayerController(worldContext, 0)
                : nullptr;
            return controller ? ULocalPlayer::GetSubsystem<T>(controller->GetLocalPlayer()) : nullptr;
        }
 
        /**
         *  Helper for getting a subsystem, the internal boilerplate will be chosen based on the type of subsystem T
         *  inherits from. Required arguments may vary depending on the type of subsystem.
         *  
         *  @tparam T  UWorldSubsystem derivative
         *  @return    Subsystem if exists or nullptr
         */
        template<CWorldSubsystem T>
        T* Get(const UObject* worldContext, EGetWorldErrorMode errorMode = EGetWorldErrorMode::LogAndReturnNull)
        {
            if (const UWorld* world = GEngine ? GEngine->GetWorldFromContextObject(worldContext, errorMode) : nullptr)
            {
                return world->GetSubsystem<T>();
            }
            return nullptr;
        }
 
        /**
         *  Helper for getting a subsystem, the internal boilerplate will be chosen based on the type of subsystem T
         *  inherits from. Required arguments may vary depending on the type of subsystem. This is a checked version so
         *  if any steps fail to produce the target subsystem the program may crash.
         *  
         *  @tparam T  type of the subsystem
         *  @return    Guaranteed valid reference to a subsystem
         */
        template<CSubsystem T, typename... Args>
        T& GetChecked(Args... args)
        {
            T* result = Get<T>(args...);
            
            ASSERT_CRASH(result,
                ->WithMessageF(TEXT("Couldn't find required subsystem %s"), *TTypeString<T>)
            )
            
            return *result;
        }
 
        /**
         *  Helper for getting a subsystem, the internal boilerplate will be chosen based on the type of subsystem T
         *  inherits from. Required arguments may vary depending on the type of subsystem. This is an ensured version so
         *  if any steps fail to produce the target subsystem an ensure may be hit.
         *  
         *  @tparam T  type of the subsystem
         *  @return    Subsystem if exists or nullptr
         */
        template<CSubsystem T, typename... Args>
        T* GetEnsured(Args... args)
        {
            T* result = Get<T>(args...);
            ensure(result);
            return result;
        }
 
        /**
         *  Helper for checking if some other subsystem should be created. Useful when you want to make a subsystem
         *  which should only be created if some other subsystem should be also created.
         *  
         *  @tparam T  type of the subsystem
         *  
         *  @param outer
         *  be sure to pass outer argument which type aligns with checked subsystem outer. This may fail for example
         *  if you pass game instance as outer to check for world subsystem existence as ShouldCreateSubsystem of that
         *  subsystem may expect the outer to be World.
         *  
         *  @return  True if the target subsystem should be created
         */
        template<CSubsystem T>
        bool ShouldCreateSubsystem(UObject* outer)
        {
            const USubsystem* cdo = T::StaticClass()->template GetDefaultObject<USubsystem>();
            return cdo->ShouldCreateSubsystem(outer);
        }
    }
}