Improved hot-reload stability and support for reloading after recompiling outside of the editor:

- Hot-reloading from inside of the editor and after recompiling from the outside should be equally supported and work together.
- Improved robustness of the delegates redirections. There should be no lost delegates when reloading (either after in-editor or outside recompilation).
- Refactored code to use the same redirection template for context and delegates container.

CHANGES.md

@@ -8,6 +8,8 @@ Change History
 Version: 1.21 (2020/07)
 Improving stability
 - Fixed a crash in the input handler caused by invalidated by hot-reload instance trying to unregister a delegate.
+- Improved hot-reload stability and support for reloading after recompiling outside of the editor. Both methods should be equally supported and work together.
+- Improved behaviour of delegates when hot-reloading.
 
 Version: 1.20 (2020/06)
 Transition to IWYU and maintenance:

Source/ImGui/Private/ImGuiDelegatesContainer.cpp

@@ -2,25 +2,49 @@
 
 #include "ImGuiDelegatesContainer.h"
 
+#include "ImGuiModule.h"
+#include "Utilities/RedirectingHandle.h"
 #include "Utilities/WorldContextIndex.h"
 
 
-FImGuiDelegatesContainer FImGuiDelegatesContainer::DefaultInstance;
+// Redirecting handle which will automatically bind to another one, if a different instance of the module is loaded.
+struct FImGuiDelegatesContainerHandle : Utilities::TRedirectingHandle<FImGuiDelegatesContainer>
+{
+	FImGuiDelegatesContainerHandle(FImGuiDelegatesContainer& InDefaultContainer)
+		: Utilities::TRedirectingHandle<FImGuiDelegatesContainer>(InDefaultContainer)
+	{
+		if (FImGuiModule* Module = FModuleManager::GetModulePtr<FImGuiModule>("ImGui"))
+		{
+			SetParent(&Module->GetDelegatesContainerHandle());
+		}
+	}
+};
 
-FImGuiDelegatesContainer* FImGuiDelegatesContainer::InstancePtr = &FImGuiDelegatesContainer::DefaultInstance;
+static FImGuiDelegatesContainer DelegatesContainer;
+static FImGuiDelegatesContainerHandle DelegatesHandle(DelegatesContainer);
 
-void FImGuiDelegatesContainer::MoveContainer(FImGuiDelegatesContainer& Dst)
+FImGuiDelegatesContainer& FImGuiDelegatesContainer::Get()
+{
+	return GetHandle().Get();
+}
+
+FImGuiDelegatesContainerHandle& FImGuiDelegatesContainer::GetHandle()
+{
+	return DelegatesHandle;
+}
+
+void FImGuiDelegatesContainer::MoveContainer(FImGuiDelegatesContainerHandle& OtherContainerHandle)
 {
 	// Only move data if pointer points to default instance, otherwise our data has already been moved and we only
 	// keep pointer to a more recent version.
-	if (InstancePtr == &DefaultInstance)
+	if (GetHandle().IsDefault())
 	{
-		Dst = MoveTemp(DefaultInstance);
-		DefaultInstance.Clear();
+		OtherContainerHandle.Get() = MoveTemp(GetHandle().Get());
+		GetHandle().Get().Clear();
 	}
 
 	// Update pointer to the most recent version.
-	InstancePtr = &Dst;
+	GetHandle().SetParent(&OtherContainerHandle);
 }
 
 int32 FImGuiDelegatesContainer::GetContextIndex(UWorld* World)

Source/ImGui/Private/ImGuiDelegatesContainer.h

@@ -6,15 +6,20 @@
 #include <Delegates/Delegate.h>
 
 
+struct FImGuiDelegatesContainerHandle;
+
 struct FImGuiDelegatesContainer
 {
 public:
 
 	// Get the current instance (can change during hot-reloading).
-	static FImGuiDelegatesContainer& Get() { return *InstancePtr; }
+	static FImGuiDelegatesContainer& Get();
 
-	// If this is an active container move its data to a destination and redirect all future calls to that instance.
-	static void MoveContainer(FImGuiDelegatesContainer& Dst);
+	// Get the handle to the container instance (can attach to other handles in hot-reloaded modules).
+	static FImGuiDelegatesContainerHandle& GetHandle();
+
+	// Redirect to the other container and if this one is still active move its data to the other one.
+	static void MoveContainer(FImGuiDelegatesContainerHandle& OtherContainerHandle);
 
 	// Get delegate to ImGui world early debug event from known world instance.
 	FSimpleMulticastDelegate& OnWorldEarlyDebug(UWorld* World) { return OnWorldEarlyDebug(GetContextIndex(World)); }
@@ -44,10 +49,4 @@ private:
 	TMap<int32, FSimpleMulticastDelegate> WorldDebugDelegates;
 	FSimpleMulticastDelegate MultiContextEarlyDebugDelegate;
 	FSimpleMulticastDelegate MultiContextDebugDelegate;
-
-	// Default container instance.
-	static FImGuiDelegatesContainer DefaultInstance;
-
-	// Pointer to the container instance that can be overwritten during hot-reloading.
-	static FImGuiDelegatesContainer* InstancePtr;
 };

Source/ImGui/Private/ImGuiImplementation.cpp

@@ -18,12 +18,28 @@
 #endif // PLATFORM_WINDOWS
 
 #if WITH_EDITOR
-// Global ImGui context pointer.
-ImGuiContext* GImGuiContextPtr = nullptr;
-// Handle to the global ImGui context pointer.
-ImGuiContext** GImGuiContextPtrHandle = &GImGuiContextPtr;
+
+#include "ImGuiModule.h"
+#include "Utilities/RedirectingHandle.h"
+
+// Redirecting handle which will automatically bind to another one, if a different instance of the module is loaded.
+struct FImGuiContextHandle : public Utilities::TRedirectingHandle<ImGuiContext*>
+{
+	FImGuiContextHandle(ImGuiContext*& InDefaultContext)
+		: Utilities::TRedirectingHandle<ImGuiContext*>(InDefaultContext)
+	{
+		if (FImGuiModule* Module = FModuleManager::GetModulePtr<FImGuiModule>("ImGui"))
+		{
+			SetParent(&Module->GetImGuiContextHandle());
+		}
+	}
+};
+
+static ImGuiContext* ImGuiContextPtr = nullptr;
+static FImGuiContextHandle ImGuiContextPtrHandle(ImGuiContextPtr);
+
 // Get the global ImGui context pointer (GImGui) indirectly to allow redirections in obsolete modules.
-#define GImGui (*GImGuiContextPtrHandle)
+#define GImGui (ImGuiContextPtrHandle.Get())
 #endif // WITH_EDITOR
 
 #include "imgui.cpp"
@@ -41,14 +57,14 @@ ImGuiContext** GImGuiContextPtrHandle = &GImGuiContextPtr;
 namespace ImGuiImplementation
 {
 #if WITH_EDITOR
-	ImGuiContext** GetImGuiContextHandle()
+	FImGuiContextHandle& GetContextHandle()
 	{
-		return GImGuiContextPtrHandle;
+		return ImGuiContextPtrHandle;
 	}
 
-	void SetImGuiContextHandle(ImGuiContext** Handle)
+	void SetParentContextHandle(FImGuiContextHandle& Parent)
 	{
-		GImGuiContextPtrHandle = Handle;
+		ImGuiContextPtrHandle.SetParent(&Parent);
 	}
 #endif // WITH_EDITOR
 }

Source/ImGui/Private/ImGuiImplementation.h

@@ -2,16 +2,16 @@
 
 #pragma once
 
-struct ImGuiContext;
+struct FImGuiContextHandle;
 
 // Gives access to selected ImGui implementation features.
 namespace ImGuiImplementation
 {
 #if WITH_EDITOR
 	// Get the handle to the ImGui Context pointer.
-	ImGuiContext** GetImGuiContextHandle();
+	FImGuiContextHandle& GetContextHandle();
 
 	// Set the ImGui Context pointer handle.
-	void SetImGuiContextHandle(ImGuiContext** Handle);
+	void SetParentContextHandle(FImGuiContextHandle& Parent);
 #endif // WITH_EDITOR
 }

Source/ImGui/Private/ImGuiModule.cpp

@@ -149,9 +149,9 @@ void FImGuiModule::ShutdownModule()
 			if (&LoadedModule != this)
 			{
 				// Statically bound functions will be still made to the obsolete module so we need to  
-				ImGuiImplementation::SetImGuiContextHandle(LoadedModule.GetImGuiContextHandle());
+				ImGuiImplementation::SetParentContextHandle(LoadedModule.GetImGuiContextHandle());
 
-				FImGuiDelegatesContainer::MoveContainer(LoadedModule.GetDelegatesContainer());
+				FImGuiDelegatesContainer::MoveContainer(LoadedModule.GetDelegatesContainerHandle());
 
 				if (bMoveProperties)
 				{
@@ -170,14 +170,14 @@ void FImGuiModule::SetProperties(const FImGuiModuleProperties& Properties)
 	ImGuiModuleManager->GetProperties() = Properties;
 }
 
-ImGuiContext** FImGuiModule::GetImGuiContextHandle()
+FImGuiContextHandle& FImGuiModule::GetImGuiContextHandle()
 {
-	return ImGuiImplementation::GetImGuiContextHandle();
+	return ImGuiImplementation::GetContextHandle();
 }
 
-FImGuiDelegatesContainer& FImGuiModule::GetDelegatesContainer()
+FImGuiDelegatesContainerHandle& FImGuiModule::GetDelegatesContainerHandle()
 {
-	return FImGuiDelegatesContainer::Get();
+	return FImGuiDelegatesContainer::GetHandle();
 }
 #endif
 

Source/ImGui/Private/Utilities/RedirectingHandle.h

@@ -0,0 +1,96 @@
+// Distributed under the MIT License (MIT) (see accompanying LICENSE file)
+
+#pragma once
+
+#include <Delegates/Delegate.h>
+#include <Delegates/DelegateCombinations.h>
+
+
+namespace Utilities
+{
+	// Handle initialized as a pointer to a default value, but once attached it will follow the parent handle.
+	// When detached it will revert back to the default value. Intended for cross-module redirections.
+	template<typename T>
+	struct TRedirectingHandle
+	{
+		TRedirectingHandle(T& InDefaultValue)
+			: Handle(&InDefaultValue)
+			, DefaultHandle(&InDefaultValue)
+		{
+		}
+
+		~TRedirectingHandle()
+		{
+			// Broadcast null pointer as a request to detach.
+			OnRedirectionUpdate.Broadcast(nullptr);
+		}
+
+		// Check whether this handle points to the default value.
+		bool IsDefault() const
+		{
+			return (Handle == DefaultHandle);
+		}
+
+		// Get the current value.
+		T& Get() const
+		{
+			return *Handle;
+		}
+
+		// Set the other handle as a parent to this one. Passing null or itself will result with detaching from
+		// the current parent (if any) and reverting back to the default value.
+		void SetParent(TRedirectingHandle* InParent)
+		{
+			if (InParent != Parent)
+			{
+				if (Parent)
+				{
+					Parent->OnRedirectionUpdate.RemoveAll(this);
+				}
+
+				// Protecting from setting itself as a parent.
+				Parent = (Parent != this) ? InParent : nullptr;
+
+				if (Parent)
+				{
+					Parent->OnRedirectionUpdate.AddRaw(this, &TRedirectingHandle::UpdateRedirection);
+				}
+
+				SetHandle((Parent) ? Parent->Handle : DefaultHandle);
+			}
+		}
+
+	protected:
+
+		void UpdateRedirection(T* InHandle)
+		{
+			if (InHandle)
+			{
+				SetHandle(InHandle);
+			}
+			else
+			{
+				// Interpreting null as a signal to detach.
+				SetParent(nullptr);
+			}
+		}
+
+		void SetHandle(T* InHandle)
+		{
+			if (InHandle != Handle)
+			{
+				Handle = InHandle;
+				OnRedirectionUpdate.Broadcast(Handle);
+			}
+		}
+
+		T* Handle;
+		T* DefaultHandle;
+
+		TRedirectingHandle* Parent = nullptr;
+
+		// Event with a new handle value or null pointer as a signal to detach.
+		DECLARE_MULTICAST_DELEGATE_OneParam(FRedirectionUpdateDelegate, T*);
+		FRedirectionUpdateDelegate OnRedirectionUpdate;
+	};
+}

Source/ImGui/Public/ImGuiModule.h

@@ -164,11 +164,13 @@ public:
 	virtual void StartupModule() override;
 	virtual void ShutdownModule() override;
 
-	private:
+private:
 
 #if WITH_EDITOR
+	friend struct FImGuiContextHandle;
+	friend struct FImGuiDelegatesContainerHandle;
 	virtual void SetProperties(const FImGuiModuleProperties& Properties);
-	virtual struct ImGuiContext** GetImGuiContextHandle();
-	virtual struct FImGuiDelegatesContainer& GetDelegatesContainer();
+	virtual FImGuiContextHandle& GetImGuiContextHandle();
+	virtual FImGuiDelegatesContainerHandle& GetDelegatesContainerHandle();
 #endif
 };