Networking and Multiplayer Setup

Unreal Engine, a powerful tool for game development, also provides comprehensive support for building multiplayer games. This tutorial will guide you through the basics of setting up a networked game with Unreal Engine using the engine's in-built networking features.

To create a multiplayer game, the first thing you need to understand is about Client-Server model. In this model, one machine acts as the server (the authority in terms of game state) while the others are the clients, which send and receive data from the server.

Setting up Player Character

First, we'll set up the player character for our multiplayer game. Let's assume we are creating a game like a space war, so our characters will be spaceship pilots named Buzz and Woody.
The basic player character can be created with the following code:

class ASpaceWarCharacter : public ACharacter 
{
    GENERATED_BODY()

public:
    ASpaceWarCharacter();
    void MoveForward(float Val);
    void MoveRight(float Val);
    virtual void SetupPlayerInputComponent(class UInputComponent* InputComponent) override;
};

In the ASpaceWarCharacter::SetupPlayerInputComponent method, you map the movement inputs to their corresponding methods:

void ASpaceWarCharacter::SetupPlayerInputComponent(UInputComponent* PlayerInputComponent)
{
    // Set up gameplay key bindings
    PlayerInputComponent->BindAxis("MoveForward", this, &ASpaceWarCharacter::MoveForward);
    PlayerInputComponent->BindAxis("MoveRight", this, &ASpaceWarCharacter::MoveRight);
}

ASpaceWarCharacter::MoveForwardandASpaceWarCharacter::MoveRight` methods change the character's location:

void ASpaceWarCharacter::MoveForward(float Value)
{
    if (Value != 0.0f)
    {
        // add movement in that direction
        AddMovementInput(GetActorForwardVector(), Value);
    }
}

void ASpaceWarCharacter::MoveRight(float Value)
{
    if (Value != 0.0f)
    {
        // add movement in that direction
        AddMovementInput(GetActorRightVector(), Value);
    }
}

Server and Clients

In multiplayer games, the server has the highest authority and maintains the game state. Clients are updated based on the server's game state. In Unreal Engine, only the server can make changes to the game state.

Creating a server in Unreal Engine isn't as complicated as you might think. When launching your game from the editor, go to the "Play" dropdown menu, check "Run Dedicated Server" and specify the number of clients you wish to connect to the server.

Remember that the server doesn't have a graphical representation, it's a standalone process running in the background.

Multiplayer games require replicating certain information across the network. In Unreal Engine, there are specifically two types of replication: variable replication and function (RPC) replication.

Replicating Variables

Replication of variables ensures that all clients have up-to-date information about the game state. Use the UPROPERTY(Replicated) macro to mark a variable for replication. The server is the authority on the value of this variable, and clients will be updated with any changes.

For instance, let's assume that a spaceship has a parameter named "Fuel", and other players can steal fuel from any spaceship. This variable needs to be replicated:

UCLASS()
class AMySpaceShip : public APawn
{
    GENERATED_BODY()

public:
    AMySpaceShip();

    UPROPERTY(Replicated)
    float Fuel;
...
};

To manage this variable correctly and ensure it's updated across all clients, we need to implement a function GetLifetimeReplicatedProps in the spaceship's class:

void AMySpaceShip::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const
{
    Super::GetLifetimeReplicatedProps(OutLifetimeProps);

    DOREPLIFETIME(AMySpaceShip, Fuel);
}

Replicating Functions

In Unreal Engine, you can replicate functions using Remote Procedure Calls (RPCs). This ensures that a particular function invoked on a client or server is also executed on all relevant machines connected to the network. The various types of RPCs in Unreal Engine include Server, Client, and Multicast.

Let's illustrate RPCs using an example. Assume that Buzz's spaceship can shoot lasers. So we will create a function for shooting, which needs to be executed on all machines for syncing.

Our server function marked with UFUNCTION(Server, Reliable, WithValidation) looks like this:

UFUNCTION(Server, Reliable, WithValidation)
void ServerShootLaser();
bool ServerShootLaser_Validate();
void ServerShootLaser_Implementation();

Also, the implementation could reduce the spaceship's fuel, instantiate a laser object, and then launch it:

void AMySpaceShip::ServerShootLaser_Implementation()
{
    if (Fuel > 0)
    {
        // ... instantiate laser, update fuel and launch it
    }
}

This tutorial provided you a simple glance into Unreal Engine's networking and multiplayer setup capabilities in creating a basic multiplayer gameplay functionality maintaining a shared state between connected clients. As the complexity of the game increases, further concepts like lag compensation, handling disconnected clients, etc., come into play. However, getting comfortable with these fundamental principles first will set you on the right path to mastering multiplayer game development with Unreal Engine.