Acquire size of ImGUI canvas from game viewport (#27)

- WIP: Does not support multi-PIE. All instances get the same size.
- Setting to enable/disable adaptive size.

Source/ImGui/Private/ImGuiContextProxy.cpp

@@ -139,13 +139,17 @@ void FImGuiContextProxy::Tick(float DeltaSeconds)
 		bHasActiveItem = ImGui::IsAnyItemActive();
 		bIsMouseHoveringAnyWindow = ImGui::IsWindowHovered(ImGuiHoveredFlags_AnyWindow);
 		MouseCursor = ImGuiInterops::ToSlateMouseCursor(ImGui::GetMouseCursor());
-		DisplaySize = ImGuiInterops::ToVector2D(ImGui::GetIO().DisplaySize);
 
 		// Begin a new frame and set the context back to a state in which it allows to draw controls.
 		BeginFrame(DeltaSeconds);
 	}
 }
 
+void FImGuiContextProxy::SetAdaptiveCanvasSize(bool bAdaptive)
+{
+    bAdaptiveCanvasSize = bAdaptive;
+}
+
 void FImGuiContextProxy::BeginFrame(float DeltaTime)
 {
 	if (!bIsFrameStarted)
@@ -156,6 +160,17 @@ void FImGuiContextProxy::BeginFrame(float DeltaTime)
 		ImGuiInterops::CopyInput(IO, InputState);
 		InputState.ClearUpdateState();
 
+		if (bAdaptiveCanvasSize) {
+			if (GEngine && GEngine->GameViewport)
+			{
+				GEngine->GameViewport->GetViewportSize(DisplaySize);
+				IO.DisplaySize = ImVec2(DisplaySize.X, DisplaySize.Y);
+			}
+		} else {
+			IO.DisplaySize = { DEFAULT_CANVAS_WIDTH, DEFAULT_CANVAS_HEIGHT };
+			DisplaySize = ImGuiInterops::ToVector2D(IO.DisplaySize);
+		}
+
 		ImGui::NewFrame();
 
 		bWantsMouseCapture = IO.WantCaptureMouse;

Source/ImGui/Private/ImGuiContextProxy.h

@@ -71,6 +71,9 @@ public:
 	// Tick to advance context to the next frame. Only one call per frame will be processed.
 	void Tick(float DeltaSeconds);
 
+	// Set the adaptive canvas size configuration.
+	void SetAdaptiveCanvasSize(bool);
+
 private:
 
 	void BeginFrame(float DeltaTime = 1.f / 60.f);
@@ -111,4 +114,6 @@ private:
 	FSimpleMulticastDelegate* SharedDrawEvent = nullptr;
 
 	std::string IniFilename;
+
+	bool bAdaptiveCanvasSize = false;
 };

Source/ImGui/Private/ImGuiModuleManager.cpp

@@ -175,6 +175,7 @@ void FImGuiModuleManager::AddWidgetToViewport(UGameViewportClient* GameViewport)
 	// Make sure that we have a context for this viewport's world and get its index.
 	int32 ContextIndex;
 	auto& ContextProxy = ContextManager.GetWorldContextProxy(*GameViewport->GetWorld(), ContextIndex);
+	ContextProxy.SetAdaptiveCanvasSize(Settings.AdaptiveCanvasSize());
 
 	// Make sure that textures are loaded before the first Slate widget is created.
 	LoadTextures();

Source/ImGui/Private/ImGuiModuleSettings.cpp

@@ -103,6 +103,7 @@ void FImGuiModuleSettings::UpdateSettings()
 		SetShareMouseInput(SettingsObject->bShareMouseInput);
 		SetUseSoftwareCursor(SettingsObject->bUseSoftwareCursor);
 		SetToggleInputKey(SettingsObject->ToggleInput);
+		SetAdaptiveCanvasSize(SettingsObject->bAdaptiveCanvasSize);
 	}
 }
 
@@ -160,6 +161,14 @@ void FImGuiModuleSettings::SetToggleInputKey(const FImGuiKeyInfo& KeyInfo)
 	}
 }
 
+void FImGuiModuleSettings::SetAdaptiveCanvasSize(bool bAdaptive)
+{
+	if (bAdaptiveCanvasSize != bAdaptive)
+	{
+		bAdaptiveCanvasSize = bAdaptive;
+	}
+}
+
 #if WITH_EDITOR
 
 void FImGuiModuleSettings::OnPropertyChanged(class UObject* ObjectBeingModified, struct FPropertyChangedEvent& PropertyChangedEvent)

Source/ImGui/Private/ImGuiModuleSettings.h

@@ -103,6 +103,10 @@ protected:
 	UPROPERTY(EditAnywhere, config, Category = "Keyboard Shortcuts")
 	FImGuiKeyInfo ToggleInput;
 
+	// If true, the size of ImGui canvas will be adaptive to game viewport.
+	UPROPERTY(EditAnywhere, config, Category = "Canvas Size")
+	bool bAdaptiveCanvasSize = false;
+
 	// Deprecated name for ToggleInput. Kept temporarily to automatically move old configuration.
 	UPROPERTY(config)
 	FImGuiKeyInfo SwitchInputModeKey_DEPRECATED;
@@ -148,6 +152,9 @@ public:
 	// Get the shortcut configuration for 'ImGui.ToggleInput' command.
 	const FImGuiKeyInfo& GetToggleInputKey() const { return ToggleInputKey; }
 
+	// Get the adaptive canvas size configuration.
+	bool AdaptiveCanvasSize() const { return bAdaptiveCanvasSize; }
+
 	// Delegate raised when ImGui Input Handle is changed.
 	FStringClassReferenceChangeDelegate OnImGuiInputHandlerClassChanged;
 
@@ -164,6 +171,7 @@ private:
 	void SetShareMouseInput(bool bShare);
 	void SetUseSoftwareCursor(bool bUse);
 	void SetToggleInputKey(const FImGuiKeyInfo& KeyInfo);
+	void SetAdaptiveCanvasSize(bool bAdaptive);
 
 #if WITH_EDITOR
 	void OnPropertyChanged(class UObject* ObjectBeingModified, struct FPropertyChangedEvent& PropertyChangedEvent);
@@ -178,4 +186,5 @@ private:
 	bool bShareGamepadInput = false;
 	bool bShareMouseInput = false;
 	bool bUseSoftwareCursor = false;
+	bool bAdaptiveCanvasSize = false;
 };