Improved plugin to initialize correctly even when started during earlier loading phases:

- Split tick and textures initializations.
- Supporting deferred tick initialization, if Slate application is not ready during module startup.
- Textures are lazy-loaded before the first use.
- ImGui Context Manager initializes font atlas to make sure that ImGui resources are always initialized.

Source/ImGui/Private/ImGuiContextManager.cpp

@@ -72,6 +72,10 @@ namespace
 
 FImGuiContextManager::FImGuiContextManager()
 {
+	unsigned char* Pixels;
+	int Width, Height, Bpp;
+	FontAtlas.GetTexDataAsRGBA32(&Pixels, &Width, &Height, &Bpp);
+
 	FWorldDelegates::OnWorldTickStart.AddRaw(this, &FImGuiContextManager::OnWorldTickStart);
 }
 

Source/ImGui/Private/ImGuiModuleManager.cpp

@@ -7,6 +7,8 @@
 #include "ImGuiInteroperability.h"
 #include "Utilities/WorldContextIndex.h"
 
+#include <ModuleManager.h>
+
 #include <imgui.h>
 
 
@@ -15,15 +17,18 @@ FImGuiModuleManager::FImGuiModuleManager()
 	// Typically we will use viewport created events to add widget to new game viewports.
 	ViewportCreatedHandle = UGameViewportClient::OnViewportCreated().AddRaw(this, &FImGuiModuleManager::OnViewportCreated);
 
-	// Initialize resources and start ticking. Depending on loading phase, this may fail if Slate is not yet ready.
-	Initialize();
+	// Try to register tick delegate (it may fail if Slate application isn't yet ready).
+	RegisterTick();
+
+	// If we failed to register, create an initializer that will do it later.
+	if (!IsTickRegistered())
+	{
+		CreateTickInitializer();
+	}
 
 	// We need to add widgets to active game viewports as they won't generate on-created events. This is especially
 	// important during hot-reloading.
-	if (bInitialized)
-	{
-		AddWidgetToAllViewports();
-	}
+	AddWidgetsToActiveViewports();
 }
 
 FImGuiModuleManager::~FImGuiModuleManager()
@@ -45,33 +50,18 @@ FImGuiModuleManager::~FImGuiModuleManager()
 	}
 
 	// Deactivate this manager.
-	Uninitialize();
-}
-
-void FImGuiModuleManager::Initialize()
-{
-	// We rely on Slate, so we can only continue if it is already initialized.
-	if (!bInitialized && FSlateApplication::IsInitialized())
-	{
-		bInitialized = true;
-		LoadTextures();
-		RegisterTick();
-	}
-}
-
-void FImGuiModuleManager::Uninitialize()
-{
-	if (bInitialized)
-	{
-		bInitialized = false;
+	ReleaseTickInitializer();
 	UnregisterTick();
 }
-}
 
 void FImGuiModuleManager::LoadTextures()
 {
 	checkf(FSlateApplication::IsInitialized(), TEXT("Slate should be initialized before we can create textures."));
 
+	if (!bTexturesLoaded)
+	{
+		bTexturesLoaded = true;
+
 		// Create an empty texture at index 0. We will use it for ImGui outputs with null texture id.
 		TextureManager.CreatePlainTexture(FName{ "ImGuiModule_Plain" }, 2, 2, FColor::White);
 
@@ -87,14 +77,12 @@ void FImGuiModuleManager::LoadTextures()
 		// Set font texture index in ImGui.
 		Fonts.TexID = ImGuiInterops::ToImTextureID(FontsTexureIndex);
 	}
+}
 
 void FImGuiModuleManager::RegisterTick()
 {
-	checkf(FSlateApplication::IsInitialized(), TEXT("Slate should be initialized before we can register tick listener."));
-
-	// We will tick on Slate Post-Tick events. They are quite convenient as they happen at the very end of the frame,
-	// what helps to minimise tearing.
-	if (!TickDelegateHandle.IsValid())
+	// Slate Post-Tick is a good moment to end and advance ImGui frame as it minimises a tearing.
+	if (!TickDelegateHandle.IsValid() && FSlateApplication::IsInitialized())
 	{
 		TickDelegateHandle = FSlateApplication::Get().OnPostTick().AddRaw(this, &FImGuiModuleManager::Tick);
 	}
@@ -112,6 +100,36 @@ void FImGuiModuleManager::UnregisterTick()
 	}
 }
 
+void FImGuiModuleManager::CreateTickInitializer()
+{
+	if (!TickInitializerHandle.IsValid())
+	{
+		// Try to register tick delegate until we finally succeed.
+
+		TickInitializerHandle = FModuleManager::Get().OnModulesChanged().AddLambda([this](FName Name, EModuleChangeReason Reason)
+		{
+			if (Reason == EModuleChangeReason::ModuleLoaded)
+			{
+				RegisterTick();
+			}
+
+			if (IsTickRegistered())
+			{
+				ReleaseTickInitializer();
+			}
+		});
+	}
+}
+
+void FImGuiModuleManager::ReleaseTickInitializer()
+{
+	if (TickInitializerHandle.IsValid())
+	{
+		FModuleManager::Get().OnModulesChanged().Remove(TickInitializerHandle);
+		TickInitializerHandle.Reset();
+	}
+}
+
 bool FImGuiModuleManager::IsInUpdateThread()
 {
 	// We can get ticks from the Game thread and Slate loading thread. In both cases IsInGameThread() is true, so we
@@ -135,9 +153,6 @@ void FImGuiModuleManager::OnViewportCreated()
 {
 	checkf(FSlateApplication::IsInitialized(), TEXT("We expect Slate to be initialized when game viewport is created."));
 
-	// Make sure that all resources are initialized to handle configurations where this module is loaded before Slate.
-	Initialize();
-
 	// Create widget to viewport responsible for this event.
 	AddWidgetToViewport(GEngine->GameViewport);
 }
@@ -147,13 +162,19 @@ 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."));
 
-	// This makes sure that context for this world is created.
-	auto& Proxy = ContextManager.GetWorldContextProxy(*GameViewport->GetWorld());
+	// Make sure that we have a context for this viewport's world and get its index.
+	int32 ContextIndex;
+	auto& Proxy = ContextManager.GetWorldContextProxy(*GameViewport->GetWorld(), ContextIndex);
 
+	// Make sure that textures are loaded before the first Slate widget is created.
+	LoadTextures();
+
+	// Create and initialize the widget.
 	TSharedPtr<SImGuiWidget> SharedWidget;
-	SAssignNew(SharedWidget, SImGuiWidget).ModuleManager(this).GameViewport(GameViewport)
-		.ContextIndex(Utilities::GetWorldContextIndex(GameViewport));
+	SAssignNew(SharedWidget, SImGuiWidget).ModuleManager(this).GameViewport(GameViewport).ContextIndex(ContextIndex);
 
+	// We transfer widget ownerships to viewports but we keep weak references in case we need to manually detach active
+	// widgets during module shutdown (important during hot-reloading).
 	if (TWeakPtr<SImGuiWidget>* Slot = Widgets.FindByPredicate([](auto& Widget) { return !Widget.IsValid(); }))
 	{
 		*Slot = SharedWidget;
@@ -164,11 +185,9 @@ void FImGuiModuleManager::AddWidgetToViewport(UGameViewportClient* GameViewport)
 	}
 }
 
-void FImGuiModuleManager::AddWidgetToAllViewports()
+void FImGuiModuleManager::AddWidgetsToActiveViewports()
 {
-	checkf(FSlateApplication::IsInitialized(), TEXT("Slate should be initialized before we can add widget to game viewports."));
-
-	if (GEngine)
+	if (FSlateApplication::IsInitialized() && GEngine)
 	{
 		// Loop as long as we have a valid viewport or until we detect a cycle.
 		UGameViewportClient* GameViewport = GEngine->GameViewport;

Source/ImGui/Private/ImGuiModuleManager.h

@@ -35,14 +35,15 @@ private:
 	FImGuiModuleManager(FImGuiModuleManager&&) = delete;
 	FImGuiModuleManager& operator=(FImGuiModuleManager&&) = delete;
 
-	void Initialize();
-	void Uninitialize();
-
 	void LoadTextures();
 
+	bool IsTickRegistered() { return TickDelegateHandle.IsValid(); }
 	void RegisterTick();
 	void UnregisterTick();
 
+	void CreateTickInitializer();
+	void ReleaseTickInitializer();
+
 	bool IsInUpdateThread();
 
 	void Tick(float DeltaSeconds);
@@ -50,7 +51,7 @@ private:
 	void OnViewportCreated();
 
 	void AddWidgetToViewport(UGameViewportClient* GameViewport);
-	void AddWidgetToAllViewports();
+	void AddWidgetsToActiveViewports();
 
 	// Event that we call after ImGui is updated.
 	FSimpleMulticastDelegate PostImGuiUpdateEvent;
@@ -64,8 +65,9 @@ private:
 	// Slate widgets that we created.
 	TArray<TWeakPtr<SImGuiWidget>> Widgets;
 
+	FDelegateHandle TickInitializerHandle;
 	FDelegateHandle TickDelegateHandle;
 	FDelegateHandle ViewportCreatedHandle;
 
-	bool bInitialized = false;
+	bool bTexturesLoaded = false;
 };