[UE4] Getting Multiplayer Player Pawn AI Navigation to work (C++)

Unreal Engine is an awesome piece of technology making it easy to do almost anything you might want.

When using the Top Down view however, there is a hurdle to get over when trying to get multiplayer to work. This is a C++ project solution to this problem based on a BluePrints solution.

The basic problem stems from the fact that

SimpleMoveToLocation was never intended to be used in a network environment. It’s simple after all 😉 Currently there’s no dedicated engine way of making player pawn follow a path. ” (from the same page)

To be able to get a working version of SimpleMoveToLocation, we need to do the following:

  • Create a proxy player class (BP_WarriorProxy is BP solution)
  • Set the proxy class as the default player controller class
  • Move the camera to the proxy (Since the actual player class is on the server, we can’t put a camera on it to display on the client)

The BP solution talks about four classes – our counterparts are as follows:

  • BP_WarriorProxy: ADemoPlayerProxy
  • BP_WarriorController: ADemoPlayerController (Auto-created when creating a c++ top down project)
  • BP_Warrior: ADemoCharacter (Auto-created when creating a C++ top down project)
  • BP_WarriorAI: AAIController – we have no reason to subclass it.

So, from a standard c++ top down project, the only class we need to add is the ADemoPlayerProxy – so go ahead and do that first.

The first thing we’ll do is rewire the ADemoGameMode class to initialise the proxy class instead of the default MyCharacter Blueprint.

 

ADemoGameMode::ADemoGameMode(const class FPostConstructInitializeProperties& PCIP) : Super(PCIP) 
{ 
    // use our custom PlayerController class 
    PlayerControllerClass = ADemoPlayerController::StaticClass(); 

    // set default pawn class to our Blueprinted character 
    /* static ConstructorHelpers::FClassFinder<apawn> PlayerPawnBPClass(TEXT("/Game/Blueprints/MyCharacter")); 
    if (PlayerPawnBPClass.Class != NULL) 
    { 
        DefaultPawnClass = PlayerPawnBPClass.Class; 
    }*/ 

    DefaultPawnClass = ADemoPlayerProxy::StaticClass(); } 

 

Our Player Proxy class declaration

/* This class will work as a proxy on the client - tracking the movements of the 
 * real Character on the server side and sending back controls. */ 
UCLASS() class Demo_API ADemoPlayerProxy : public APawn 
{ 
    GENERATED_UCLASS_BODY() 
    /** Top down camera */ 
    UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = Camera) TSubobjectPtr<class ucameracomponent=""> TopDownCameraComponent; 

    /** Camera boom positioning the camera above the character */ 
    UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = Camera) TSubobjectPtr<class uspringarmcomponent=""> CameraBoom; 

    // Needed so we can pick up the class in the constructor and spawn it elsewhere 
    TSubclassOf<aactor> CharacterClass; 

    // Pointer to the actual character. We replicate it so we know its location for the camera on the client 
    UPROPERTY(Replicated) ADemoCharacter* Character; 

    // The AI Controller we will use to auto-navigate the player 
    AAIController* PlayerAI; 

    // We spawn the real player character and other such elements here 
    virtual void BeginPlay() override; 

    // Use do keep this actor in sync with the real one 
    void Tick(float DeltaTime); 

    // Used by the controller to get moving to work 
    void MoveToLocation(const ADemoPlayerController* controller, const FVector&amp; vector); 
}; 

and the definition:


#include "Demo.h"
#include "DemoCharacter.h"
#include "AIController.h"
#include "DemoPlayerProxy.h"
#include "UnrealNetwork.h"


ADemoPlayerProxy::ADemoPlayerProxy(const class FPostConstructInitializeProperties&amp; PCIP)
: Super(PCIP)
{
	// Don't rotate character to camera direction
	bUseControllerRotationPitch = false;
	bUseControllerRotationYaw = false;
	bUseControllerRotationRoll = false;

	// It seems that without a RootComponent, we can't place the Actual Character easily
	TSubobjectPtr&lt;UCapsuleComponent&gt; TouchCapsule = PCIP.CreateDefaultSubobject<ucapsulecomponent>(this, TEXT("dummy"));
	TouchCapsule-&gt;InitCapsuleSize(1.0f, 1.0f);
	TouchCapsule-&gt;SetCollisionEnabled(ECollisionEnabled::NoCollision);
	TouchCapsule-&gt;SetCollisionResponseToAllChannels(ECR_Ignore);
	RootComponent = TouchCapsule;

	// Create a camera boom...
	CameraBoom = PCIP.CreateDefaultSubobject&lt;USpringArmComponent&gt;(this, TEXT("CameraBoom"));
	CameraBoom-&gt;AttachTo(RootComponent);
	CameraBoom-&gt;bAbsoluteRotation = true; // Don't want arm to rotate when character does
	CameraBoom-&gt;TargetArmLength = 800.f;
	CameraBoom-&gt;RelativeRotation = FRotator(-60.f, 0.f, 0.f);
	CameraBoom-&gt;bDoCollisionTest = false; // Don't want to pull camera in when it collides with level

	// Create a camera...
	TopDownCameraComponent = PCIP.CreateDefaultSubobject&lt;UCameraComponent&gt;(this, TEXT("TopDownCamera"));
	TopDownCameraComponent-&gt;AttachTo(CameraBoom, USpringArmComponent::SocketName);
	TopDownCameraComponent-&gt;bUseControllerViewRotation = false; // Camera does not rotate relative to arm

	if (Role == ROLE_Authority)
	{
		static ConstructorHelpers::FObjectFinder&lt;UClass&gt; PlayerPawnBPClass(TEXT("/Game/Blueprints/MyCharacter.MyCharacter_C"));
		CharacterClass = PlayerPawnBPClass.Object;
	}

}

void ADemoPlayerProxy::BeginPlay()
{
	Super::BeginPlay();
	if (Role == ROLE_Authority)
	{
		// Get current location of the Player Proxy
		FVector Location =  GetActorLocation();
		FRotator Rotation = GetActorRotation();

		FActorSpawnParameters SpawnParams;
		SpawnParams.Owner = this;
		SpawnParams.Instigator = Instigator;
		SpawnParams.bNoCollisionFail = true;

		// Spawn the actual player character at the same location as the Proxy
		Character = Cast&lt;ADemoCharacter&gt;(GetWorld()-&gt;SpawnActor(CharacterClass, &amp;Location, &amp;Rotation, SpawnParams));

		// We use the PlayerAI to control the Player Character for Navigation
		PlayerAI = GetWorld()-&gt;SpawnActor&lt;AAIController&gt;(GetActorLocation(), GetActorRotation());
		PlayerAI-&gt;Possess(Character);
	}

}

void ADemoPlayerProxy::Tick(float DeltaTime)
{

	Super::Tick(DeltaTime);
	if (Character)
	{
		// Keep the Proxy in sync with the real character
		FTransform CharTransform = Character-&gt;GetTransform();
		FTransform MyTransform = GetTransform();

		FTransform Transform;
		Transform.LerpTranslationScale3D(CharTransform, MyTransform, ScalarRegister(0.5f));

		SetActorTransform(Transform);
		
	}
}

void ADemoPlayerProxy::MoveToLocation(const ADemoPlayerController* controller, const FVector&amp; DestLocation)
{
	/** Looks easy - doesn't it.
	 *  What this does is to engage the AI to pathfind.
	 *  The AI will then "route" the character correctly.
	 *  The Proxy (and with it the camera), on each tick, moves to the location of the real character
	 *
	 *  And thus, we get the illusion of moving with the Player Character
	 */
	PlayerAI-&gt;MoveToLocation(DestLocation);
}

void ADemoPlayerProxy::GetLifetimeReplicatedProps(TArray&lt; class FLifetimeProperty &gt; &amp; OutLifetimeProps) const
{

	Super::GetLifetimeReplicatedProps(OutLifetimeProps);

	// Replicate to Everyone
	DOREPLIFETIME(ADemoPlayerProxy, Character);
}</ucapsulecomponent>

We’ll now cover changes to the Player Controller. We’ll rewire it here to ask the proxy to move, which will in turn ask the AIController to find a path and move the real player character. 

This involves changing the SetMoveDestination method to call a server side method to move the character. When the character moves, the player Proxy will automatically mirror the movements.

In the header file, add the following function


    /** Navigate player to the given world location (Server Version) */
    UFUNCTION(reliable, server, WithValidation)
    void ServerSetNewMoveDestination(const FVector DestLocation);

Now let’s implement it (DemoPlayerController.cpp):


bool ADemoPlayerController::ServerSetNewMoveDestination_Validate(const FVector DestLocation)
{
	return true;
}

/* Actual implementation of the ServerSetMoveDestination method */
void ADemoPlayerController::ServerSetNewMoveDestination_Implementation(const FVector DestLocation)
{
	ADemoPlayerProxy* Pawn = Cast<ademoplayerproxy>(GetPawn());
	if (Pawn)
	{
		UNavigationSystem* const NaDemoys = GetWorld()-&gt;GetNavigationSystem();
		float const Distance = FVector::Dist(DestLocation, Pawn-&gt;GetActorLocation());

		// We need to issue move command only if far enough in order for walk animation to play correctly
		if (NaDemoys &amp;&amp; (Distance &gt; 120.0f))
		{
			//NaDemoys-&gt;SimpleMoveToLocation(this, DestLocation);
			Pawn-&gt;MoveToLocation(this, DestLocation);
		}
	}

}

And finally, the rewiring of the original method:

void ADemoPlayerController::SetNewMoveDestination(const FVector DestLocation)
{
	ServerSetNewMoveDestination(DestLocation);
}

 

Finally, in terms of the character class, the only change is really to remove the camera components that we moved to the Player Proxy which I shall leave to you :-p

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