Moved ImGui Draw events, so they are called during World Tick Start. Old behaviour, where those events were called during Post-Tick, can be restored by setting console variable ImGui.DebugDrawOnWorldTick to zero.

Source/ImGui/Private/ImGuiContextManager.cpp

@@ -12,6 +12,20 @@
 #include <imgui.h>
 
 
+namespace CVars
+{
+	TAutoConsoleVariable<int> DebugDrawOnWorldTick(TEXT("ImGui.DebugDrawOnWorldTick"), 1,
+		TEXT("If this is enabled then all the ImGui debug draw events will be called during World Tick Start.\n")
+		TEXT("This has an advantage that all the global variables like GWorld are set to correct values.\n")
+		TEXT("A disadvantage is that objects are not yet updated. That can be changed by disabling this feature,")
+		TEXT("but preferable solution is to call debug code from object's own Tick function.\n")
+		TEXT("NOTE: Order of multi-context and world draw events depends on this value and is arranged in a way ")
+		TEXT("that world draw events and objects updates are closer together.\n")
+		TEXT("0: disabled, ImGui Debug Draw is called during Post-Tick\n")
+		TEXT("1: enabled (default), ImGui Debug Draw is called during World Tick Start"),
+		ECVF_Default);
+}
+
 namespace
 {
 #if WITH_EDITOR
@@ -58,17 +72,13 @@ namespace
 
 FImGuiContextManager::FImGuiContextManager()
 {
-#if WITH_EDITOR
 	FWorldDelegates::OnWorldTickStart.AddRaw(this, &FImGuiContextManager::OnWorldTickStart);
-#endif
 }
 
 FImGuiContextManager::~FImGuiContextManager()
 {
-#if WITH_EDITOR
 	// Order matters because contexts can be created during World Tick Start events.
 	FWorldDelegates::OnWorldTickStart.RemoveAll(this);
-#endif
 	Contexts.Empty();
 	ImGui::Shutdown();
 }
@@ -88,15 +98,18 @@ void FImGuiContextManager::Tick(float DeltaSeconds)
 	}
 }
 
-#if WITH_EDITOR
 void FImGuiContextManager::OnWorldTickStart(ELevelTick TickType, float DeltaSeconds)
 {
 	if (GWorld)
 	{
-		GetWorldContextProxy(*GWorld).SetAsCurrent();
+		FImGuiContextProxy& ContextProxy = GetWorldContextProxy(*GWorld);
+		ContextProxy.SetAsCurrent();
+		if (CVars::DebugDrawOnWorldTick.GetValueOnGameThread() > 0)
+		{
+			ContextProxy.Draw();
+		}
 	}
 }
-#endif // WITH_EDITOR
 
 #if WITH_EDITOR
 FImGuiContextManager::FContextData& FImGuiContextManager::GetEditorContextData()

Source/ImGui/Private/ImGuiContextManager.h

@@ -86,9 +86,7 @@ private:
 
 #endif // WITH_EDITOR
 
-#if WITH_EDITOR
 	void OnWorldTickStart(ELevelTick TickType, float DeltaSeconds);
-#endif
 
 #if WITH_EDITOR
 	FContextData& GetEditorContextData();

Source/ImGui/Private/ImGuiContextProxy.cpp

@@ -14,6 +14,11 @@ static constexpr float DEFAULT_CANVAS_WIDTH = 3840.f;
 static constexpr float DEFAULT_CANVAS_HEIGHT = 2160.f;
 
 
+namespace CVars
+{
+	extern TAutoConsoleVariable<int> DebugDrawOnWorldTick;
+}
+
 namespace
 {
 	FString GetSaveDirectory()
@@ -99,24 +104,26 @@ FImGuiContextProxy::~FImGuiContextProxy()
 
 void FImGuiContextProxy::Draw()
 {
-	// Comparing to LastTickFrameNumber rather than GFrameNumber to make sure that this is driven by our own update cycle.
-	if (LastDrawFrameNumber < LastTickFrameNumber)
+	if (bIsFrameStarted && !bIsDrawCalled)
 	{
-		LastDrawFrameNumber = LastTickFrameNumber;
+		bIsDrawCalled = true;
 
-		if (bIsFrameStarted)
+		SetAsCurrent();
+
+		const bool bSharedFirst = (CVars::DebugDrawOnWorldTick.GetValueOnGameThread() > 0);
+
+		// Broadcast draw event to allow listeners to draw their controls to this context.
+		if (bSharedFirst && SharedDrawEvent && SharedDrawEvent->IsBound())
 		{
-			SetAsCurrent();
-
-			// Broadcast draw event to allow listeners to draw their controls to this context.
-			if (DrawEvent.IsBound())
-			{
-				DrawEvent.Broadcast();
-			}
-			if (SharedDrawEvent && SharedDrawEvent->IsBound())
-			{
-				SharedDrawEvent->Broadcast();
-			}
+			SharedDrawEvent->Broadcast();
+		}
+		if (DrawEvent.IsBound())
+		{
+			DrawEvent.Broadcast();
+		}
+		if (!bSharedFirst && SharedDrawEvent && SharedDrawEvent->IsBound())
+		{
+			SharedDrawEvent->Broadcast();
 		}
 	}
 }
@@ -124,9 +131,9 @@ void FImGuiContextProxy::Draw()
 void FImGuiContextProxy::Tick(float DeltaSeconds)
 {
 	// Making sure that we tick only once per frame.
-	if (LastTickFrameNumber < GFrameNumber)
+	if (LastFrameNumber < GFrameNumber)
 	{
-		LastTickFrameNumber = GFrameNumber;
+		LastFrameNumber = GFrameNumber;
 
 		SetAsCurrent();
 
@@ -165,6 +172,7 @@ void FImGuiContextProxy::BeginFrame(float DeltaTime)
 		ImGui::NewFrame();
 
 		bIsFrameStarted = true;
+		bIsDrawCalled = false;
 	}
 }
 

Source/ImGui/Private/ImGuiContextProxy.h

@@ -76,12 +76,13 @@ private:
 	bool bHasActiveItem = false;
 
 	bool bIsFrameStarted = false;
+	bool bIsDrawCalled = false;
+
+	uint32 LastFrameNumber = 0;
+
 	FSimpleMulticastDelegate DrawEvent;
 	FSimpleMulticastDelegate* SharedDrawEvent = nullptr;
 
-	uint32 LastTickFrameNumber = 0;
-	uint32 LastDrawFrameNumber = 0;
-
 	const FImGuiInputState* InputState = nullptr;
 
 	TArray<FImGuiDrawList> DrawLists;