More of a text snippet than a tutorial, this is how it's possible to call a function when the game loses focus.

In this example we have put the callback function in a custom APlayerController subclass, but it could be just about anywhere.

Also it might be worth wrapping this feature in a UUserGameSetting property so players can disable it if they don't want it.

First we need to add Slate to our MyProject.Build.cs file:

MyProject.Build.cs

public class MyProject : ModuleRules
{
	public MyProject(ReadOnlyTargetRules Target) : base(Target)
	{
		PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
	
		PublicDependencyModuleNames.AddRange(new string[] {
			"Core",
			"CoreUObject",
			"Engine",
			"InputCore",
			"Slate" // Add this!
		});

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

Next, we need to add to our player controller in both the .h and the .cpp:

BUIPlayerController.h

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

#pragma once

#include "CoreMinimal.h"
#include "BUIPlayerController.generated.h"

UCLASS()
class ABUIPlayerController : public APlayerController
{
	GENERATED_BODY()
protected:
	virtual void BeginPlay() override;
	void OnWindowFocusChanged( bool bIsFocused );
};

BUIPlayerController.cpp

#include "BUIPlayerController.h"
#include "Framework/Application/SlateApplication.h"

void ABUIPlayerController::BeginPlay()
{
	FSlateApplication::Get().OnApplicationActivationStateChanged()
		.AddUObject( this, &ABUIPlayerController::OnWindowFocusChanged );
}

void ABUIPlayerController::OnWindowFocusChanged( bool bIsFocused )
{
// Don't pause in the editor, it's annoying
#if !WITH_EDITOR
	if ( bIsFocused )
	{
		// Unlimit game FPS
		GEngine->SetMaxFPS( 0 );

		// Unpause the game
		// MyHUD->SetPause( false );
	}
	else
	{
		// Reduce FPS to max 10 while in the background
		GEngine->SetMaxFPS( 10.0f );

		// Pause the game, using your "show pause menu" function
		// MyHUD->SetPause( true );
	}
#endif
}

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