Reactive Programming and State Management in Unreal Engine

Development Programming & Scripting C++
1

I am coming from an Angular/RxJs perspective and attempting to learn C++. I am getting to the point in my development journey where I need to understand and manage game and player state.

I found a library called RxCpp developed for reactive programming in C++, but the library seems inactive. There is even a wrapper for RxCpp and unreal engine, but that library is also inactive.

Is there anyone using reactive programming in Unreal Engine?

2

Have u started using rxcpp in UE4

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3

I am beginning to understand how UE handles event-driven design. The code below is probably bloated, but the goal is to create an NgRx-like environment.

Store = Player State
Action = public IncreaseCoinsAction()
Reducer = private IncreaseCoinsReducer()

Anytime a player collides with a coin, it dispatches the increaseCoinAction.

CoinGamePlayerState.h


#pragma once

#include "CoreMinimal.h"
#include "GameFramework/PlayerState.h"
#include "CoinGamePlayerState.generated.h"

DECLARE_DYNAMIC_MULTICAST_DELEGATE(FIncreaseCoinsDelegate);

UCLASS()
class COINCOLLECTOR_API ACoinGamePlayerState : public APlayerState
{
	GENERATED_BODY()

	// Create Coins property
	UPROPERTY(EditAnywhere)
	int32 Coins;

	// Create delegate for increase coins
	UPROPERTY(BlueprintAssignable)
	FIncreaseCoinsDelegate IncreaseCoinsDelegate;

	// Create function for broadcast increase coins
	UFUNCTION(BlueprintCallable)
	void IncreaseCoinsAction();

	// Create begin play function
	virtual void BeginPlay() override;

private:
	// Create reducer function for increase coins
	UFUNCTION()
	void IncreaseCoinsReducer();
};

CoinGamePlayerState.cpp

// Fill out your copyright notice in the Description page of Project Settings.

#include "CoinGamePlayerState.h"

// BeginPlay
void ACoinGamePlayerState::BeginPlay()
{
    Super::BeginPlay();

    // Set Coins to 0
    Coins = 0;

    // Bind IncreaseCoins function to delegate add dynamic
    IncreaseCoinsDelegate.AddDynamic(this, &ACoinGamePlayerState::IncreaseCoinsReducer);
}

// Create function for broadcast increase coins
void ACoinGamePlayerState::IncreaseCoinsAction()
{
    // Broadcast increase coins
    IncreaseCoinsDelegate.Broadcast();
}

// Bind increase coins action to increase coins
void ACoinGamePlayerState::IncreaseCoinsReducer()
{
    // Increase coins
    Coins++;

    // Print coins
    GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Green, FString::Printf(TEXT("Coins: %d"), Coins));
}
4

I have experience on RxJS and Angular 12.x too, however, RxCPP looks really awful to me, I believe not many people would use that on local machine gaming logic, unless for some network traffic applications. But C++ has lots of mature network libraries that can outperform everything else, whereas l believe lots of MMOs with heavy network traffic use their own C++ developed low-level protocols.

I think C++ is fast for linear tasks, and things in C++ tend to happen with low-level optimizations. ParallelFor() can do simple multi-threading tasks, and OpenMP can make things better.

I wish other people could write some insights into this since I’m also a newcomer to UE development.

5

As I’m learning, just had another thought about this.
I coded a fairly complex gaming logic for a casual game.
Then I dropped into the trap of ‘Timeline cannot be inside a loop’.
Took me 1 week to figure out that the UE C++ compiler would do something unknown for the timeline animation inside any loop, it would make multiple timelines just run once and it would be after the loop !
Then I missed the reactive programming.

In the end, I solved this by recursive calling, the UE wouldn’t optimize that.

6

I think there is no need for State Management in Unreal due to highly dynamic environment which doesn’t have single source of truth for changes happenning in the game state. It just goes against the engine design and must have been implemented in the core to work.

But what I really miss and what could complement engine a lot is handling events in reactive-programming way. There are race conditions happening, when something is replicated before other thing you need is initialized, but sometimes it’s not. In such cases it’s very painful and event operators could really help to tackle it :frowning:

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