C++ API

Add the module to your .Build.cs:

PrivateDependencyModuleNames.AddRange(new string[] { "CommonLoadingScreen" });

Keep the loading screen up from an Actor or Component

Implement ILoadingProcessInterface and override ShouldShowLoadingScreen. This works on the Game State, a Player Controller and their components without any registration.

#include "LoadingProcessInterface.h"

UCLASS()
class AMyGameState : public AGameStateBase, public ILoadingProcessInterface
{
	GENERATED_BODY()

public:
	virtual bool ShouldShowLoadingScreen(FString& OutReason) const override
	{
		if (!bWorldReady)
		{
			OutReason = TEXT("Waiting for the world to be ready");
			return true;
		}
		return false;
	}

private:
	bool bWorldReady = false;
};

warning

Always set OutReason when you return true. An empty reason triggers an ensure.

Any other object must be registered with the manager:

#include "LoadingScreenManager.h"

if (ULoadingScreenManager* Manager = ULoadingScreenManager::Get(this))
{
	Manager->RegisterLoadingProcessor(this);
}

Call UnregisterLoadingProcessor(this) when it is done. The manager holds the object weakly, so a destroyed object is skipped automatically.

Show the loading screen on demand

#include "LoadingProcessTask.h"

ULoadingProcessTask* Task = ULoadingProcessTask::CreateLoadingScreenProcessTask(this, TEXT("Saving the game"));

// later
Task->Unregister();

The manager keeps the task alive until Unregister is called.

React to the loading screen

if (ULoadingScreenManager* Manager = ULoadingScreenManager::Get(this))
{
	Manager->OnLoadingScreenVisibilityChangedDelegate().AddUObject(this, &ThisClass::HandleLoadingScreenVisibilityChanged);
}

void AMyActor::HandleLoadingScreenVisibilityChanged(bool bIsVisible)
{
}

ULoadingScreenManager

Function	Description
static ULoadingScreenManager* Get(UObject* WorldContextObject)	Returns the manager, or nullptr on a dedicated server.
void RegisterLoadingProcessor(TScriptInterface<ILoadingProcessInterface>)	Registers an object that is asked whether the loading screen should be shown.
void UnregisterLoadingProcessor(TScriptInterface<ILoadingProcessInterface>)	Unregisters it.
void AddLoadingScreenWatcher(UObject*)	Adds a Watcher.
void RemoveLoadingScreenWatcher(UObject*)	Removes a Watcher.
void SetCustomLoadingScreenText(FString)	Sets the custom text.
bool GetLoadingScreenDisplayStatus() const	True while the loading screen is shown.
FString GetDebugReasonForShowingOrHidingLoadingScreen() const	The current reason.
FString GetLoadingScreenCustomText() const	The custom text.
FString GetCurrentLoadingMap() const	The map that is loading, or was loaded last.
FOnLoadingScreenVisibilityChangedDelegate& OnLoadingScreenVisibilityChangedDelegate()	Native delegate for visibility changes.