Added support for session reloading and updated contexts and widgets management:

- Added to ImGui Context Proxy a name that is mapped to ini file set in ImGui context.
- ImGui Context Manager generates unique context names from world type and context index.
- Refactored ImGui Context Manager to have a cleaner separation between editor and non-editor bits.
- Fixed context update rules in ImGui Context Manager.
- Changed widgets management in ImGui Module Manager to allow automatic garbage collection after viewports are closed and manual removal when module is shutting down.
- ImGui Widgets are in full control of communication with context proxies.
- Added basic world context utilities.
- Refactored world context index utilities and replaced ambiguous 'default context index' with 'editor' and 'game' ones.

Source/ImGui/Private/ImGuiContextManager.cpp

@@ -4,68 +4,141 @@
 
 #include "ImGuiContextManager.h"
 
+#include "ImGuiImplementation.h"
+#include "Utilities/WorldContext.h"
+#include "Utilities/WorldContextIndex.h"
 
-FImGuiContextProxy& FImGuiContextManager::GetWorldContextProxy(UWorld& World)
+#include <imgui.h>
+
+
+namespace
 {
-	const int32 Index = Utilities::GetWorldContextIndex(World);
-	checkf(Index != Utilities::INVALID_CONTEXT_INDEX, TEXT("Couldn't resolve context index for world %s: WorldType = %d"),
-		*World.GetName(), World.WorldType);
-
 #if WITH_EDITOR
-	// Make sure that PIE worlds don't try to use editor context.
-	checkf(!GEngine->IsEditor() || Index != Utilities::DEFAULT_CONTEXT_INDEX, TEXT("Index for world %s "
-		"was resolved to the default context index %d, which in editor is reserved for editor context. PIE worlds "
-		"should use values that start from 1. WorldType = %d, NetMode = %d"), *World.GetName(),
-		Utilities::DEFAULT_CONTEXT_INDEX, World.WorldType, World.GetNetMode());
+
+	// Name for editor ImGui context.
+	FORCEINLINE FString GetEditorContextName()
+	{
+		return TEXT("Editor");
+	}
+
+	// Name for world ImGui context.
+	FORCEINLINE FString GetWorldContextName(const UWorld& World)
+	{
+		using namespace Utilities;
+
+		const FWorldContext* WorldContext = GetWorldContext(World);
+		switch (WorldContext->WorldType)
+		{
+		case EWorldType::PIE:
+			return FString::Printf(TEXT("PIEContext%d"), GetWorldContextIndex(*WorldContext));
+		case EWorldType::Game:
+			return TEXT("Game");
+		case EWorldType::Editor:
+			return TEXT("Editor");
+		default:
+			return TEXT("Other");
+		}
+	}
+
+#else
+
+	FORCEINLINE FString GetWorldContextName()
+	{
+		return TEXT("Game");
+	}
+
+	FORCEINLINE FString GetWorldContextName(const UWorld&)
+	{
+		return TEXT("Game");
+	}
+
 #endif // WITH_EDITOR
+}
 
-	FContextData& Data = FindOrAddContextData(Index);
-
-	// Track worlds to make sure that different worlds don't try to use the same context in the same time.
-	if (!Data.World.IsValid())
-	{
-		Data.World = &World;
-	}
-	else
-	{
-		checkf(Data.World == &World, TEXT("Two different worlds, %s and %s, resolved to the same world context index %d."),
-			*Data.World->GetName(), *World.GetName(), Index);
-	}
-
-	return Data.ContextProxy;
+FImGuiContextManager::~FImGuiContextManager()
+{
+	Contexts.Empty();
+	ImGui::Shutdown();
 }
 
 void FImGuiContextManager::Tick(float DeltaSeconds)
 {
-	FContextData* Data = Contexts.Find(1);
-	if (!Data || !Data->World.IsValid())
+	// In editor, worlds can get invalid. We could remove corresponding entries, but that would mean resetting ImGui
+	// context every time when PIE session is restarted. Instead we freeze contexts until their worlds are re-created.
+
+	for (auto& Pair : Contexts)
 	{
-		return;
-	}
-
-	for (auto& Entry : Contexts)
-	{
-		FImGuiContextProxy& ContextProxy = Entry.Value.ContextProxy;
-
-		ContextProxy.SetAsCurrent();
-
-		// Tick context proxy to end the old frame and starts a new one.
-		ContextProxy.Tick(DeltaSeconds);
+		auto& ContextData = Pair.Value;
+		if (ContextData.CanTick())
+		{
+			ContextData.ContextProxy.SetAsCurrent();
+			ContextData.ContextProxy.Tick(DeltaSeconds);
+		}
 	}
 }
 
-FImGuiContextManager::FContextData& FImGuiContextManager::FindOrAddContextData(int32 Index)
+#if WITH_EDITOR
+FImGuiContextManager::FContextData& FImGuiContextManager::GetEditorContextData()
 {
-	FContextData* Data = Contexts.Find(Index);
+	FContextData* Data = Contexts.Find(Utilities::EDITOR_CONTEXT_INDEX);
 
-	if (!Data)
+	if (UNLIKELY(!Data))
 	{
-		Data = &Contexts.Add(Index);
-		if (!Data->ContextProxy.OnDraw().IsBoundToObject(&ImGuiDemo))
-		{
-			Data->ContextProxy.OnDraw().AddRaw(&ImGuiDemo, &FImGuiDemo::DrawControls);
-		}
+		Data = &Contexts.Emplace(Utilities::EDITOR_CONTEXT_INDEX, FContextData{ GetEditorContextName(), ImGuiDemo });
 	}
 
 	return *Data;
 }
+#endif // WITH_EDITOR
+
+#if !WITH_EDITOR
+FImGuiContextManager::FContextData& FImGuiContextManager::GetStandaloneWorldContextData()
+{
+	FContextData* Data = Contexts.Find(Utilities::STANDALONE_GAME_CONTEXT_INDEX);
+
+	if (UNLIKELY(!Data))
+	{
+		Data = &Contexts.Emplace(Utilities::STANDALONE_GAME_CONTEXT_INDEX, FContextData{ GetWorldContextName(), ImGuiDemo });
+	}
+
+	return *Data;
+}
+#endif // !WITH_EDITOR
+
+FImGuiContextManager::FContextData& FImGuiContextManager::GetWorldContextData(const UWorld& World)
+{
+	using namespace Utilities;
+
+	const FWorldContext* WorldContext = GetWorldContext(World);
+	const int32 Index = GetWorldContextIndex(*WorldContext);
+
+	checkf(Index != Utilities::INVALID_CONTEXT_INDEX, TEXT("Couldn't find context index for world %s: WorldType = %d"),
+		*World.GetName(), World.WorldType);
+
+#if WITH_EDITOR
+	checkf(!GEngine->IsEditor() || Index != Utilities::EDITOR_CONTEXT_INDEX,
+		TEXT("Context index %d is reserved for editor and cannot be used for world %s: WorldType = %d, NetMode = %d"),
+		Index, *World.GetName(), World.WorldType, World.GetNetMode());
+#endif
+
+	FContextData* Data = Contexts.Find(Index);
+
+#if WITH_EDITOR
+	if (UNLIKELY(!Data))
+	{
+		Data = &Contexts.Emplace(Index, FContextData{ GetWorldContextName(World), ImGuiDemo, WorldContext->PIEInstance });
+	}
+	else
+	{
+		// Because we allow (for the sake of continuity) to map different PIE instances to the same index.
+		Data->PIEInstance = WorldContext->PIEInstance;
+	}
+#else
+	if (UNLIKELY(!Data))
+	{
+		Data = &Contexts.Emplace(Index, FContextData{ GetWorldContextName(World), ImGuiDemo });
+	}
+#endif
+
+	return *Data;
+}

Source/ImGui/Private/ImGuiContextManager.h

@@ -4,7 +4,6 @@
 
 #include "ImGuiContextProxy.h"
 #include "ImGuiDemo.h"
-#include "Utilities/WorldContextIndex.h"
 
 
 // Manages ImGui context proxies.
@@ -20,17 +19,20 @@ public:
 	FImGuiContextManager(FImGuiContextManager&&) = delete;
 	FImGuiContextManager& operator=(FImGuiContextManager&&) = delete;
 
-	// Get or create default ImGui context proxy. In editor this is the editor context proxy and in standalone game
-	// context proxy for the only world and the same value as returned from GetWorldContextProxy.
-	//
-	// If proxy doesn't exist then it will be created and initialized.
-	FImGuiContextProxy& GetDefaultContextProxy() { return FindOrAddContextData(Utilities::DEFAULT_CONTEXT_INDEX).ContextProxy; }
+	~FImGuiContextManager();
+
+#if WITH_EDITOR
+	// Get or create editor ImGui context proxy.
+	FORCEINLINE FImGuiContextProxy& GetEditorContextProxy() { return GetEditorContextData().ContextProxy; }
+#endif
+
+#if !WITH_EDITOR
+	// Get or create standalone game ImGui context proxy.
+	FORCEINLINE FImGuiContextProxy& GetWorldContextProxy() { return GetStandaloneWorldContextData().ContextProxy; }
+#endif
 
 	// Get or create ImGui context proxy for given world.
-	//
-	// If proxy doesn't yet exist then it will be created and initialized. If proxy already exists then associated
-	// world data will be updated.
-	FImGuiContextProxy& GetWorldContextProxy(UWorld& World);
+	FORCEINLINE FImGuiContextProxy& GetWorldContextProxy(const UWorld& World) { return GetWorldContextData(World).ContextProxy; }
 
 	// Get context proxy by index, or null if context with that index doesn't exist.
 	FORCEINLINE FImGuiContextProxy* GetContextProxy(int32 ContextIndex)
@@ -43,13 +45,49 @@ public:
 
 private:
 
+#if WITH_EDITOR
+
 	struct FContextData
 	{
-		TWeakObjectPtr<UWorld> World;
+		FContextData(const FString& ContextName, FImGuiDemo& Demo, int32 InPIEInstance = -1)
+			: PIEInstance(InPIEInstance)
+			, ContextProxy(ContextName)
+		{
+			ContextProxy.OnDraw().AddRaw(&Demo, &FImGuiDemo::DrawControls);
+		}
+
+		FORCEINLINE bool CanTick() const { return PIEInstance < 0 || GEngine->GetWorldContextFromPIEInstance(PIEInstance); }
+
+		int32 PIEInstance = -1;
 		FImGuiContextProxy ContextProxy;
 	};
 
-	FContextData& FindOrAddContextData(int32 Index);
+#else
+
+	struct FContextData
+	{
+		FContextData(const FString& ContextName, FImGuiDemo& Demo)
+			: ContextProxy(ContextName)
+		{
+			ContextProxy.OnDraw().AddRaw(&Demo, &FImGuiDemo::DrawControls);
+		}
+
+		FORCEINLINE bool CanTick() const { return true; }
+
+		FImGuiContextProxy ContextProxy;
+	};
+
+#endif // WITH_EDITOR
+
+#if WITH_EDITOR
+	FContextData& GetEditorContextData();
+#endif
+
+#if !WITH_EDITOR
+	FContextData& GetStandaloneWorldContextData();
+#endif
+
+	FContextData& GetWorldContextData(const UWorld& World);
 
 	TMap<int32, FContextData> Contexts;
 

Source/ImGui/Private/ImGuiContextProxy.cpp

@@ -11,7 +11,29 @@
 static constexpr float DEFAULT_CANVAS_WIDTH = 3840.f;
 static constexpr float DEFAULT_CANVAS_HEIGHT = 2160.f;
 
-FImGuiContextProxy::FImGuiContextProxy()
+
+namespace
+{
+	FString GetSaveDirectory()
+	{
+		FString Directory = FPaths::Combine(*FPaths::GameSavedDir(), TEXT("ImGui"));
+
+		// Make sure that directory is created.
+		IPlatformFile::GetPlatformPhysical().CreateDirectory(*Directory);
+
+		return Directory;
+	}
+
+	FString GetIniFile(const FString& Name)
+	{
+		static FString SaveDirectory = GetSaveDirectory();
+		return FPaths::Combine(SaveDirectory, Name + TEXT(".ini"));
+	}
+}
+
+FImGuiContextProxy::FImGuiContextProxy(const FString& InName)
+	: Name(InName)
+	, IniFilename(TCHAR_TO_ANSI(*GetIniFile(InName)))
 {
 	// Create context.
 	Context = ImGui::CreateContext();
@@ -22,6 +44,9 @@ FImGuiContextProxy::FImGuiContextProxy()
 	// Start initialization.
 	ImGuiIO& IO = ImGui::GetIO();
 
+	// Set session data storage.
+	IO.IniFilename = IniFilename.c_str();
+
 	// Use pre-defined canvas size.
 	IO.DisplaySize = { DEFAULT_CANVAS_WIDTH, DEFAULT_CANVAS_HEIGHT };
 
@@ -53,18 +78,26 @@ FImGuiContextProxy::FImGuiContextProxy(FImGuiContextProxy&& Other)
 	, DrawEvent(std::move(Other.DrawEvent))
 	, InputState(std::move(Other.InputState))
 	, DrawLists(std::move(Other.DrawLists))
+	, Name(std::move(Other.Name))
+	, IniFilename(std::move(Other.IniFilename))
 {
 	Other.Context = nullptr;
 }
 
 FImGuiContextProxy& FImGuiContextProxy::operator=(FImGuiContextProxy&& Other)
 {
-	Context = std::move(Other.Context);
-	Other.Context = nullptr;
+	// Swapping context so it can be destroyed with the other object.
+	using std::swap;	
+	swap(Context, Other.Context);
+
+	// Just moving remaining data that doesn't affect cleanup.
 	bHasActiveItem = Other.bHasActiveItem;
 	DrawEvent = std::move(Other.DrawEvent);
 	InputState = std::move(Other.InputState);
 	DrawLists = std::move(Other.DrawLists);
+	Name = std::move(Other.Name);
+	IniFilename = std::move(Other.IniFilename);
+
 	return *this;
 }
 
@@ -75,14 +108,12 @@ FImGuiContextProxy::~FImGuiContextProxy()
 		// Set this context in ImGui for de-initialization (any de-allocations will be tracked in this context).
 		SetAsCurrent();
 
-		// Shutdown to save data etc.
-		ImGui::Shutdown();
-
-		// Destroy the context.
+		// Save context data and destroy.
+		ImGuiImplementation::SaveCurrentContextIniSettings(IniFilename.c_str());
 		ImGui::DestroyContext(Context);
 
 		// Set default context in ImGui to keep global context pointer valid.
-		ImGui::SetCurrentContext(&GetDefaultContext());
+		ImGui::SetCurrentContext(&ImGuiImplementation::GetDefaultContext());
 	}
 }
 

Source/ImGui/Private/ImGuiContextProxy.h

@@ -6,6 +6,8 @@
 
 #include <imgui.h>
 
+#include <string>
+
 
 class FImGuiInputState;
 
@@ -15,7 +17,7 @@ class FImGuiContextProxy
 {
 public:
 
-	FImGuiContextProxy();
+	FImGuiContextProxy(const FString& Name);
 	~FImGuiContextProxy();
 
 	FImGuiContextProxy(const FImGuiContextProxy&) = delete;
@@ -24,6 +26,9 @@ public:
 	FImGuiContextProxy(FImGuiContextProxy&& Other);
 	FImGuiContextProxy& operator=(FImGuiContextProxy&& Other);
 
+	// Get the name of this context.
+	const FString& GetName() const { return Name; }
+
 	// Get draw data from the last frame.
 	const TArray<FImGuiDrawList>& GetDrawData() const { return DrawLists; }
 
@@ -33,6 +38,9 @@ public:
 	// Set input state to be used by this context.
 	void SetInputState(const FImGuiInputState* SourceInputState) { InputState = SourceInputState; }
 
+	// If context is currently using input state to remove then remove that binding.
+	void RemoveInputState(const FImGuiInputState* InputStateToRemove) { if (InputState == InputStateToRemove) InputState = nullptr; }
+
 	// Is this context the current ImGui context.
 	bool IsCurrentContext() const { return ImGui::GetCurrentContext() == Context; }
 
@@ -63,4 +71,7 @@ private:
 	const FImGuiInputState* InputState = nullptr;
 
 	TArray<FImGuiDrawList> DrawLists;
+
+	FString Name;
+	std::string IniFilename;
 };

Source/ImGui/Private/ImGuiImplementation.cpp

@@ -22,9 +22,17 @@
 #endif // PLATFORM_WINDOWS
 
 
-// This is exposing ImGui default context for the whole module.
-// This is assuming that we don't define custom GImGui and therefore have GImDefaultContext defined in imgui.cpp.
-ImGuiContext& GetDefaultContext()
+namespace ImGuiImplementation
 {
-	return GImDefaultContext;
+	// This is exposing ImGui default context for the whole module.
+	// This is assuming that we don't define custom GImGui and therefore have GImDefaultContext defined in imgui.cpp.
+	ImGuiContext& GetDefaultContext()
+	{
+		return GImDefaultContext;
+	}
+
+	void SaveCurrentContextIniSettings(const char* Filename)
+	{
+		SaveIniSettingsToDisk(Filename);
+	}
 }

Source/ImGui/Private/ImGuiImplementation.h

@@ -5,5 +5,12 @@
 #include <imgui.h>
 
 
-// Get default context created by ImGui framework.
-ImGuiContext& GetDefaultContext();
+// Gives access to selected ImGui implementation features.
+namespace ImGuiImplementation
+{
+	// Get default context created by ImGui framework.
+	ImGuiContext& GetDefaultContext();
+
+	// Save current context settings.
+	void SaveCurrentContextIniSettings(const char* Filename);
+}

Source/ImGui/Private/ImGuiModuleManager.cpp

@@ -5,15 +5,13 @@
 #include "ImGuiModuleManager.h"
 
 #include "ImGuiInteroperability.h"
+#include "Utilities/WorldContextIndex.h"
 
 #include <imgui.h>
 
 
 FImGuiModuleManager::FImGuiModuleManager()
 {
-	// Make sure that default ImGui context is setup.
-	ContextManager.GetDefaultContextProxy();
-
 	// Typically we will use viewport created events to add widget to new game viewports.
 	ViewportCreatedHandle = UGameViewportClient::OnViewportCreated().AddRaw(this, &FImGuiModuleManager::OnViewportCreated);
 
@@ -37,6 +35,15 @@ FImGuiModuleManager::~FImGuiModuleManager()
 		ViewportCreatedHandle.Reset();
 	}
 
+	// Remove still active widgets (important during hot-reloading).
+	for (auto& Widget : Widgets)
+	{
+		if (Widget.IsValid())
+		{
+			Widget.Pin()->Detach();
+		}
+	}
+
 	// Deactivate this manager.
 	Uninitialize();
 }
@@ -141,24 +148,21 @@ void FImGuiModuleManager::AddWidgetToViewport(UGameViewportClient* GameViewport)
 	checkf(GameViewport, TEXT("Null game viewport."));
 	checkf(FSlateApplication::IsInitialized(), TEXT("Slate should be initialized before we can add widget to game viewports."));
 
-	const int32 ContextIndex = Utilities::GetWorldContextIndex(GameViewport);
-
 	// This makes sure that context for this world is created.
 	auto& Proxy = ContextManager.GetWorldContextProxy(*GameViewport->GetWorld());
 
-	// Get widget for this world.
-	auto& ViewportWidget = ViewportWidgets.FindOrAdd(ContextIndex);
-	if (!ViewportWidget.IsValid())
+	TSharedPtr<SImGuiWidget> Widget;
+	SAssignNew(Widget, SImGuiWidget).ModuleManager(this).GameViewport(GameViewport)
+		.ContextIndex(Utilities::GetWorldContextIndex(GameViewport));
+
+	if (TWeakPtr<SImGuiWidget>* Slot = Widgets.FindByPredicate([](auto& Widget) { return !Widget.IsValid(); }))
 	{
-		SAssignNew(ViewportWidget, SImGuiWidget).ModuleManager(this).ContextIndex(ContextIndex);
-		check(ViewportWidget.IsValid());
+		*Slot = Widget;
+	}
+	else
+	{
+		Widgets.Emplace(Widget);
 	}
-
-	// Bind widget's input to context for this world.
-	Proxy.SetInputState(&ViewportWidget->GetInputState());
-
-	// We should always have one viewport per context index at a time (this will be validated by widget).
-	ViewportWidget->AttachToViewport(GameViewport);
 }
 
 void FImGuiModuleManager::AddWidgetToAllViewports()

Source/ImGui/Private/ImGuiModuleManager.h

@@ -61,8 +61,8 @@ private:
 	// Manager for textures resources.
 	FTextureManager TextureManager;
 
-	// Slate widgets that we attach to game viewports.
-	TMap<int32, TSharedPtr<SImGuiWidget>> ViewportWidgets;
+	// Slate widgets that we created.
+	TArray<TWeakPtr<SImGuiWidget>> Widgets;
 
 	FDelegateHandle TickDelegateHandle;
 	FDelegateHandle ViewportCreatedHandle;

Source/ImGui/Private/SImGuiWidget.cpp

@@ -47,10 +47,16 @@ BEGIN_SLATE_FUNCTION_BUILD_OPTIMIZATION
 void SImGuiWidget::Construct(const FArguments& InArgs)
 {
 	checkf(InArgs._ModuleManager, TEXT("Null Module Manager argument"));
+	checkf(InArgs._GameViewport, TEXT("Null Game Viewport argument"));
 
 	ModuleManager = InArgs._ModuleManager;
+	GameViewport = InArgs._GameViewport;
 	ContextIndex = InArgs._ContextIndex;
 
+	// NOTE: We could allow null game viewports (for instance to attach to non-viewport widgets) but we would need
+	// to modify a few functions that assume valid viewport pointer.
+	GameViewport->AddViewportWidgetContent(SharedThis(this), IMGUI_WIDGET_Z_ORDER);
+
 	// Disable mouse cursor over this widget as we will use ImGui to draw it.
 	SetCursor(EMouseCursor::None);
 
@@ -60,10 +66,11 @@ void SImGuiWidget::Construct(const FArguments& InArgs)
 	// Register to get post-update notifications, so we can clean frame updates.
 	ModuleManager->OnPostImGuiUpdate().AddRaw(this, &SImGuiWidget::OnPostImGuiUpdate);
 
-	// Register self-debug function.
+	// Bind this widget to its context proxy.
 	auto* ContextProxy = ModuleManager->GetContextManager().GetContextProxy(ContextIndex);
 	checkf(ContextProxy, TEXT("Missing context during widget construction: ContextIndex = %d"), ContextIndex);
 	ContextProxy->OnDraw().AddRaw(this, &SImGuiWidget::OnDebugDraw);
+	ContextProxy->SetInputState(&InputState);
 }
 END_SLATE_FUNCTION_BUILD_OPTIMIZATION
 
@@ -73,27 +80,20 @@ SImGuiWidget::~SImGuiWidget()
 	if (auto* ContextProxy = ModuleManager->GetContextManager().GetContextProxy(ContextIndex))
 	{
 		ContextProxy->OnDraw().RemoveAll(this);
+		ContextProxy->RemoveInputState(&InputState);
 	}
 
 	// Unregister from post-update notifications.
 	ModuleManager->OnPostImGuiUpdate().RemoveAll(this);
 }
 
-void SImGuiWidget::AttachToViewport(UGameViewportClient* InGameViewport, bool bResetInput)
+void SImGuiWidget::Detach()
 {
-	checkf(InGameViewport, TEXT("Null InGameViewport"));
-	checkf(!GameViewport.IsValid() || GameViewport.Get() == InGameViewport,
-		TEXT("Widget is attached to another game viewport and will be available for reuse only after this session ")
-		TEXT("ends. ContextIndex = %d, CurrentGameViewport = %s, InGameViewport = %s"),
-		ContextIndex, *GameViewport->GetName(), InGameViewport->GetName());
-
-	if (bResetInput)
+	if (GameViewport.IsValid())
 	{
-		ResetInputState();
+		GameViewport->RemoveViewportWidgetContent(SharedThis(this));
+		GameViewport.Reset();
 	}
-
-	GameViewport = InGameViewport;
-	GameViewport->AddViewportWidgetContent(SNew(SWeakWidget).PossiblyNullContent(SharedThis(this)), IMGUI_WIDGET_Z_ORDER);
 }
 
 void SImGuiWidget::Tick(const FGeometry& AllottedGeometry, const double InCurrentTime, const float InDeltaTime)
@@ -284,13 +284,6 @@ bool SImGuiWidget::IgnoreKeyEvent(const FKeyEvent& KeyEvent) const
 	return false;
 }
 
-void SImGuiWidget::ResetInputState()
-{
-	bInputEnabled = false;
-	SetVisibilityFromInputEnabled();
-	UpdateInputMode(false, false);
-}
-
 void SImGuiWidget::SetVisibilityFromInputEnabled()
 {
 	// If we don't use input disable hit test to make this widget invisible for cursors hit detection.
@@ -479,12 +472,14 @@ void SImGuiWidget::OnDebugDraw()
 	bool bDebug = CVars::DebugWidget.GetValueOnGameThread() > 0;
 	if (bDebug)
 	{
-		ImGui::SetNextWindowSize(ImVec2(380, 340), ImGuiSetCond_Once);
+		ImGui::SetNextWindowSize(ImVec2(380, 360), ImGuiSetCond_Once);
 		if (ImGui::Begin("ImGui Widget Debug", &bDebug))
 		{
 			ImGui::Columns(2, nullptr, false);
 
 			TwoColumns::Value("Context Index", ContextIndex);
+			FImGuiContextProxy* ContextProxy = ModuleManager->GetContextManager().GetContextProxy(ContextIndex);
+			TwoColumns::Value("Context Name", ContextProxy ? *ContextProxy->GetName() : TEXT("< Null >"));
 			TwoColumns::Value("Game Viewport", *GameViewport->GetName());
 
 			ImGui::Separator();

Source/ImGui/Private/SImGuiWidget.h

@@ -19,6 +19,7 @@ public:
 	SLATE_BEGIN_ARGS(SImGuiWidget)
 	{}
 	SLATE_ARGUMENT(FImGuiModuleManager*, ModuleManager)
+	SLATE_ARGUMENT(UGameViewportClient*, GameViewport)
 	SLATE_ARGUMENT(int32, ContextIndex)
 	SLATE_END_ARGS()
 
@@ -35,12 +36,8 @@ public:
 	// Get the game viewport to which this widget is attached.
 	const TWeakObjectPtr<UGameViewportClient>& GetGameViewport() const { return GameViewport; }
 
-	// Attach this widget to a target game viewport.
-	// Widget can be attached to only one viewport at a time but can be reused after its last viewport becomes invalid
-	// at the end of a session. Widgets are weakly attached, so once destroyed they are automatically removed.
-	// @param InGameViewport - Target game viewport
-	// @param bResetInput - If true (default), input will be reset back to a default state
-	void AttachToViewport(UGameViewportClient* InGameViewport, bool bResetInput = true);
+	// Detach widget from viewport assigned during construction (effectively allowing to dispose this widget). 
+	void Detach();
 
 	//----------------------------------------------------------------------------------------------------
 	// SWidget overrides
@@ -92,8 +89,6 @@ private:
 
 	bool IgnoreKeyEvent(const FKeyEvent& KeyEvent) const;
 
-	void ResetInputState();
-
 	// Update visibility based on input enabled state.
 	void SetVisibilityFromInputEnabled();
 

Source/ImGui/Private/Utilities/WorldContext.cpp

@@ -0,0 +1,24 @@
+// Distributed under the MIT License (MIT) (see accompanying LICENSE file)
+
+#include "ImGuiPrivatePCH.h"
+
+#include "WorldContext.h"
+
+
+namespace Utilities
+{
+	const FWorldContext* GetWorldContextFromNetMode(ENetMode NetMode)
+	{
+		checkf(GEngine, TEXT("GEngine required to get list of worlds."));
+
+		for (const FWorldContext& WorldContext : GEngine->GetWorldContexts())
+		{
+			if (WorldContext.World() && WorldContext.World()->GetNetMode() == NetMode)
+			{
+				return &WorldContext;
+			}
+		}
+
+		return nullptr;
+	}
+}

Source/ImGui/Private/Utilities/WorldContext.h

@@ -0,0 +1,41 @@
+// Distributed under the MIT License (MIT) (see accompanying LICENSE file)
+
+#pragma once
+
+#include <Core.h>
+#include <Engine.h>
+
+
+// Utilities helping to get a World Context.
+
+namespace Utilities
+{
+	template<typename T>
+	FORCEINLINE const FWorldContext* GetWorldContext(const TWeakObjectPtr<T>& Obj)
+	{
+		return Obj.IsValid() ? GetWorldContext(*Obj.Get()) : nullptr;
+	}
+
+	template<typename T>
+	FORCEINLINE const FWorldContext* GetWorldContext(const T* Obj)
+	{
+		return Obj ? GetWorldContext(*Obj) : nullptr;
+	}
+
+	FORCEINLINE const FWorldContext* GetWorldContext(const UGameInstance& GameInstance)
+	{
+		return GameInstance.GetWorldContext();
+	}
+
+	FORCEINLINE const FWorldContext* GetWorldContext(const UGameViewportClient& GameViewportClient)
+	{
+		return GetWorldContext(GameViewportClient.GetGameInstance());
+	}
+
+	FORCEINLINE const FWorldContext* GetWorldContext(const UWorld& World)
+	{
+		return GetWorldContext(World.GetGameInstance());
+	}
+
+	const FWorldContext* GetWorldContextFromNetMode(ENetMode NetMode);
+}

Source/ImGui/Private/Utilities/WorldContextIndex.h

@@ -2,8 +2,7 @@
 
 #pragma once
 
-#include <Core.h>
-#include <Engine.h>
+#include "Utilities/WorldContext.h"
 
 
 // Utilities mapping worlds to indices that we use to identify ImGui contexts.
@@ -12,51 +11,34 @@
 
 namespace Utilities
 {
-	// Default context index for non PIE worlds.
-	static constexpr int32 DEFAULT_CONTEXT_INDEX = 0;
-
 	// Invalid context index for parameters that cannot be resolved to a valid world.
 	static constexpr int32 INVALID_CONTEXT_INDEX = -1;
 
+	// Standalone context index.
+	static constexpr int32 STANDALONE_GAME_CONTEXT_INDEX = 0;
+
 #if WITH_EDITOR
 
-	template<typename T>
-	FORCEINLINE int32 GetWorldContextIndex(const TWeakObjectPtr<T>& Obj)
-	{
-		return Obj.IsValid() ? GetWorldContextIndex(*Obj.Get()) : INVALID_CONTEXT_INDEX;
-	}
+	// Editor context index.
+	static constexpr int32 EDITOR_CONTEXT_INDEX = 0;
 
 	template<typename T>
-	FORCEINLINE int32 GetWorldContextIndex(const T* Obj)
+	FORCEINLINE int32 GetWorldContextIndex(const T& Obj)
 	{
-		return Obj ? GetWorldContextIndex(*Obj) : INVALID_CONTEXT_INDEX;
+		const FWorldContext* WorldContext = GetWorldContext(Obj);
+		return WorldContext ? GetWorldContextIndex(*WorldContext) : INVALID_CONTEXT_INDEX;
 	}
 
 	FORCEINLINE int32 GetWorldContextIndex(const FWorldContext& WorldContext)
 	{
-		// In standalone game (WorldType = Game) we have only one context with index 0 (see DEFAULT_CONTEXT_INDEX).
+		// In standalone game (WorldType = Game) we have only one context with index 0 (see GAME_CONTEXT_INDEX).
 
 		// In editor, we keep 0 for editor and use PIEInstance to index worlds. In simulation or standalone single-PIE
 		// sessions PIEInstance is 0, but since there is only one world we can change it without causing any conflicts.
 		// In single-PIE with dedicated server or multi-PIE sessions worlds have PIEInstance starting from 1 for server
 		// and 2+ for clients, what maps directly to our index.
 
-		return WorldContext.WorldType == EWorldType::PIE ? FMath::Max(WorldContext.PIEInstance, 1) : DEFAULT_CONTEXT_INDEX;
-	}
-
-	FORCEINLINE int32 GetWorldContextIndex(const UGameInstance& GameInstance)
-	{
-		return GetWorldContextIndex(GameInstance.GetWorldContext());
-	}
-
-	FORCEINLINE int32 GetWorldContextIndex(const UGameViewportClient& GameViewportClient)
-	{
-		return GetWorldContextIndex(GameViewportClient.GetGameInstance());
-	}
-
-	FORCEINLINE int32 GetWorldContextIndex(const UWorld& World)
-	{
-		return GetWorldContextIndex(World.GetGameInstance());
+		return WorldContext.WorldType == EWorldType::PIE ? FMath::Max(WorldContext.PIEInstance, 1) : STANDALONE_GAME_CONTEXT_INDEX;
 	}
 
 #else
@@ -65,7 +47,7 @@ namespace Utilities
 	constexpr int32 GetWorldContextIndex(const T&)
 	{
 		// The only option is standalone game with one context.
-		return DEFAULT_CONTEXT_INDEX;
+		return STANDALONE_GAME_CONTEXT_INDEX;
 	}
 
 #endif // #if WITH_EDITOR