How to keep Widgets on screen when using ServerTravel

Has anyone figured out where in source code widgets are removed when changing levels by calling ServerTravel, to remove that line/those lines in Unreal Engine 5.3.X / 5.4.X?

I tried looking around the code for ServerTravel but I couldn’t find the removal code.

Has anyone figured out

It was written by employees of Epic Games, this is not rocket science, you just have to follow some breadcrumbs.

What happens when you switch to a new world is the following (so you can trace the steps):

The current World gets a URL to the new world in UWorld::SeamlessTravel, where it uses a handler instance to manage the actual travel logic called FSeamlessTravelHandler.
In FSeamlessTravelHandler::Tick the handler checks against LoadedWorld which represents the instance of the new UWorld to travel to. If it is not null and not equal to the current world, the current world receives a call to BeginTearingDown.
In UWorld::BeginTearingDown, FWorldDelegates::OnWorldBeginTearDown is broadcast, notifying a bunch of systems, including the UGameViewportSubsystem which is a Subsystem for the UEngine. There it calls the method UGameViewportSubsystem::HandleRemoveWorld.
In UGameViewportSubsystem::HandleRemoveWorld, the subsystem iterates over all the existing widgets that are currently added to the viewport and calls UWidget::RemoveFromParent on them.

I think from here on out, you can follow the flow.

The thing you probably want to do instead of modifying the engine code:
You want Widgets to persist between levels. This is tricky, because in the teardown of a world, owners of the UserWidgets may be deleted, resulting in the widgets to be deleted as well. They are removed from their parent and the owner becomes invalid, resulting in them being garbage collected.

So first, you’d have to not add them just to the screen for a player, but to the game instance. This is not that hard, just use it as the owner. But you also want to cache them, so you can add them later when the world was removed. You could do that in your own GameInstanceSubsystem, if you like or directly in the game instance. Just make sure that, after the world you wanted to travel to is properly loaded, you add them back to the viewport.

Following some information from @Max Play, I came up with a solution for this, but it does indeed require to change the source code of the game engine. The changes are easy to make but they do require to be added when you change version of the engine, from 5.3.2 to 5.4.0 for example. For this I added a toggle to “GameInstance”.

Here is a list with all the steps:

Get the source code for the engine, if you know how to do it, you can skip this step

Go to Unreal Engine GitHub repository and change from release to the version you want, in my case was 5.3
After selecting the version click on “Code” then “Download ZIP”, and place it in a path that is less then 50 characters (that is what they recommend)
Unzip the archive and run “Setup.bat”. This will download the dependencies needed
After that is done, run “GenerateProjectFiles.bat” and when it completes, you have the source code and a usable C++ project

Modify game engine

Open UE5.sln and go to “Solution / Engine / UE5 / Source / Runtime / Engine / Classes / Engine / GameInstance.h” and open it
Find “GENERATED_UCLASS_BODY()” that is inside “class UGameInstance : public UObject, public FExec”, and after it add:

    public:
        /** Whether all widgets should be removed when the level/map/world is changed */
        UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = General)
        uint32 bRemoveWidgets : 1;

Now go to “Solution / Engine / UE5 / Source / Runtime / UMG / Private / Blueprint / GameViewportSubsystem.cpp” and open it
Inside it find the function “void UGameViewportSubsystem::HandleRemoveWorld(UWorld* InWorld)” and encompass it’s content in this:

    UGameInstance* gameInstance = InWorld->GetGameInstance();
    bool removeWidgets = true;
    if (gameInstance) {
        removeWidgets = gameInstance->bRemoveWidgets;
    }
    
    if (removeWidgets)
    {
    //Content of the function 
    }

Building the engine and the game

Check if “Development Editor” is selected and “Win64”. For me they were by default, but check it
After all the previews steps are done, right click on “UE5” from “Solution Explorer” inside “Engine” and choose build. This process will take a long time. I have a AMD 5950X and it takes around 30 min
Go where the source code is, and open “UnrealEditor.exe” from inside the folder “EngineBinariesWin64”, and let it finish loading everything and the window to create a new project is opened. Now close that window, you only needed it to compile what it needs and to be visible by Epic for a next step
Now make a backup of the game just in case, if you haven’t done so already
Go to where the backup is and delete the following folders: “.vs”, “Binaries”, “Intermediate”, “Saved”; and the file “.vsconfig”
Right click on your “GameName.uproject” and select the version of engine you just built. It should look something like: “Source build at path”. This will also generate the project files
Now open “GameName.sln”, and go to “Build / Configuration Manager…” and in that list deselect UE5, and then “Build / Build Solution”. The first time you build your game after changing version of the engine, it will take longer, but after that it will take the same amount of time it did when using the game engine got from Epic Launcher

Applying the change for the game

Now that you have the engine and the game compiled, open your game, and find your “CustomGameInstance” and look for “Remove Widgets” inside “Class Defaults”. Deselect it and compile the blueprint. If it is deselected already I recommend checking that box, compile, deselect and compile again
Now all widgets will be kept when “ServerTravel” is called. Don’t forget that before changing the level first call “Remove All Widgets” and after that create your Widget that you want to keep on the screen. If you don’t remove the widgets, you might have some that can cause problems because of the references they contain

//Edit: If you want per map control, you can add the toggle inside GameStateBase.

Filed under: Kiến thức lập trình - @ 13:07

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How to keep Widgets on screen when using ServerTravel