Added support for touch input (experimental and right now only non-shared):

- Added to SImGuiWidget and FImGuiInputHandler handling of touch started, move and ended events.
- Events are passed to FImGuiInputState to be copied to ImGui IO as a mouse move and left button down.
- Added one frame lag between touch ending event and ending processing, to allow ImGui handle mouse-up event.

Source/ImGui/Private/ImGuiInputHandler.cpp

@@ -145,6 +145,25 @@ FReply UImGuiInputHandler::OnMouseMove(const FVector2D& MousePosition)
 	return ToReply(true);
 }
 
+FReply UImGuiInputHandler::OnTouchStarted(const FVector2D& CursorPosition, const FPointerEvent& TouchEvent)
+{
+	InputState->SetTouchDown(true);
+	InputState->SetTouchPosition(CursorPosition);
+	return ToReply(true);
+}
+
+FReply UImGuiInputHandler::OnTouchMoved(const FVector2D& CursorPosition, const FPointerEvent& TouchEvent)
+{
+	InputState->SetTouchPosition(CursorPosition);
+	return ToReply(true);
+}
+
+FReply UImGuiInputHandler::OnTouchEnded(const FVector2D& CursorPosition, const FPointerEvent& TouchEvent)
+{
+	InputState->SetTouchDown(false);
+	return ToReply(true);
+}
+
 void UImGuiInputHandler::OnKeyboardInputEnabled()
 {
 	bKeyboardInputEnabled = true;

Source/ImGui/Private/ImGuiInputState.cpp

@@ -93,6 +93,8 @@ void FImGuiInputState::ClearUpdateState()
 	MouseButtonsUpdateRange.SetEmpty();
 
 	MouseWheelDelta = 0.f;
+
+	bTouchProcessed = bTouchDown;
 }
 
 void FImGuiInputState::ClearCharacters()

Source/ImGui/Private/ImGuiInputState.h

@@ -82,11 +82,11 @@ public:
 	// @param DeltaValue - Mouse wheel delta to add
 	void AddMouseWheelDelta(float DeltaValue) { MouseWheelDelta += DeltaValue; }
 
-	// Get the current mouse position.
+	// Get the mouse position.
 	const FVector2D& GetMousePosition() const { return MousePosition; }
 
-	// Set mouse position.
+	// Set the mouse position.
-	// @param Position - New mouse position
+	// @param Position - Mouse position
 	void SetMousePosition(const FVector2D& Position) { MousePosition = Position; }
 
 	// Check whether input has active mouse pointer.
@@ -96,6 +96,24 @@ public:
 	// @param bHasPointer - True, if input has active mouse pointer
 	void SetMousePointer(bool bInHasMousePointer) { bHasMousePointer = bInHasMousePointer; }
 
+	// Check whether touch input is in progress. True, after touch is started until one frame after it has ended.
+	// One frame delay is used to process mouse release in ImGui since touch-down is simulated with mouse-down.
+	bool IsTouchActive() const { return bTouchDown || bTouchProcessed; }
+
+	// Check whether touch input is down.
+	bool IsTouchDown() const { return bTouchDown; }
+
+	// Set whether touch input is down.
+	// @param bIsDown - True, if touch is down (or started) and false, if touch is up (or ended)
+	void SetTouchDown(bool bIsDown) { bTouchDown = bIsDown; }
+
+	// Get the touch position.
+	const FVector2D& GetTouchPosition() const { return TouchPosition; }
+
+	// Set the touch position.
+	// @param Position - Touch position
+	void SetTouchPosition(const FVector2D& Position) { TouchPosition = Position; }
+
 	// Get Control down state.
 	bool IsControlDown() const { return bIsControlDown; }
 
@@ -197,6 +215,7 @@ private:
 	void ClearNavigationInputs();
 
 	FVector2D MousePosition = FVector2D::ZeroVector;
+	FVector2D TouchPosition = FVector2D::ZeroVector;
 	float MouseWheelDelta = 0.f;
 
 	FMouseButtonsArray MouseButtonsDown;
@@ -211,6 +230,8 @@ private:
 	FNavInputArray NavigationInputs;
 
 	bool bHasMousePointer = false;
+	bool bTouchDown = false;
+	bool bTouchProcessed = false;
 
 	bool bIsControlDown = false;
 	bool bIsShiftDown = false;

Source/ImGui/Private/ImGuiInteroperability.cpp

@@ -247,16 +247,6 @@ namespace ImGuiInterops
 		static const uint32 LeftAlt = GetKeyIndex(EKeys::LeftAlt);
 		static const uint32 RightAlt = GetKeyIndex(EKeys::RightAlt);
 
-		// Check whether we need to draw cursor.
-		IO.MouseDrawCursor = InputState.HasMousePointer();
-
-		// Copy mouse position.
-		IO.MousePos.x = InputState.GetMousePosition().X;
-		IO.MousePos.y = InputState.GetMousePosition().Y;
-
-		// Copy mouse wheel delta.
-		IO.MouseWheel += InputState.GetMouseWheelDelta();
-
 		// Copy key modifiers.
 		IO.KeyCtrl = InputState.IsControlDown();
 		IO.KeyShift = InputState.IsShiftDown();
@@ -287,5 +277,28 @@ namespace ImGuiInterops
 		SetFlag(IO.ConfigFlags, ImGuiConfigFlags_NavEnableKeyboard, InputState.IsKeyboardNavigationEnabled());
 		SetFlag(IO.ConfigFlags, ImGuiConfigFlags_NavEnableGamepad, InputState.IsGamepadNavigationEnabled());
 		SetFlag(IO.BackendFlags, ImGuiBackendFlags_HasGamepad, InputState.HasGamepad());
+
+		// Check whether we need to draw cursor.
+		IO.MouseDrawCursor = InputState.HasMousePointer();
+
+		// If touch is enabled and active, give it a precedence.
+		if (InputState.IsTouchActive())
+		{
+			// Copy the touch position to mouse position.
+			IO.MousePos.x = InputState.GetTouchPosition().X;
+			IO.MousePos.y = InputState.GetTouchPosition().Y;
+
+			// With touch active one frame longer than it is down, we have one frame to processed touch up.
+			IO.MouseDown[0] = InputState.IsTouchDown();
+		}
+		else
+		{
+			// Copy the mouse position.
+			IO.MousePos.x = InputState.GetMousePosition().X;
+			IO.MousePos.y = InputState.GetMousePosition().Y;
+
+			// Copy mouse wheel delta.
+			IO.MouseWheel += InputState.GetMouseWheelDelta();
+		}
 	}
 }

Source/ImGui/Private/Widgets/SImGuiWidget.cpp

@@ -189,8 +189,7 @@ FReply SImGuiWidget::OnMouseWheel(const FGeometry& MyGeometry, const FPointerEve
 
 FReply SImGuiWidget::OnMouseMove(const FGeometry& MyGeometry, const FPointerEvent& MouseEvent)
 {
-	const FSlateRenderTransform ImGuiToScreen = ImGuiTransform.Concatenate(MyGeometry.GetAccumulatedRenderTransform());
+	return InputHandler->OnMouseMove(TransformScreenPointToImGui(MyGeometry, MouseEvent.GetScreenSpacePosition()));
-	return InputHandler->OnMouseMove(ImGuiToScreen.Inverse().TransformPoint(MouseEvent.GetScreenSpacePosition()));
 }
 
 FReply SImGuiWidget::OnFocusReceived(const FGeometry& MyGeometry, const FFocusEvent& FocusEvent)
@@ -235,6 +234,22 @@ void SImGuiWidget::OnMouseLeave(const FPointerEvent& MouseEvent)
 	InputHandler->OnMouseInputDisabled();
 }
 
+FReply SImGuiWidget::OnTouchStarted(const FGeometry& MyGeometry, const FPointerEvent& TouchEvent)
+{
+	return InputHandler->OnTouchStarted(TransformScreenPointToImGui(MyGeometry, TouchEvent.GetScreenSpacePosition()), TouchEvent);
+}
+
+FReply SImGuiWidget::OnTouchMoved(const FGeometry& MyGeometry, const FPointerEvent& TouchEvent)
+{
+	return InputHandler->OnTouchMoved(TransformScreenPointToImGui(MyGeometry, TouchEvent.GetScreenSpacePosition()), TouchEvent);
+}
+
+FReply SImGuiWidget::OnTouchEnded(const FGeometry& MyGeometry, const FPointerEvent& TouchEvent)
+{
+	UpdateVisibility();
+	return InputHandler->OnTouchEnded(TransformScreenPointToImGui(MyGeometry, TouchEvent.GetScreenSpacePosition()), TouchEvent);
+}
+
 void SImGuiWidget::CreateInputHandler(const FStringClassReference& HandlerClassReference)
 {
 	ReleaseInputHandler();
@@ -446,8 +461,7 @@ void SImGuiWidget::UpdateTransparentMouseInput(const FGeometry& AllottedGeometry
 	{
 		if (!GameViewport->GetGameViewportWidget()->HasMouseCapture())
 		{
-			const FSlateRenderTransform ImGuiToScreen = ImGuiTransform.Concatenate(AllottedGeometry.GetAccumulatedRenderTransform());
+			InputHandler->OnMouseMove(TransformScreenPointToImGui(AllottedGeometry, FSlateApplication::Get().GetCursorPos()));
-			InputHandler->OnMouseMove(ImGuiToScreen.Inverse().TransformPoint(FSlateApplication::Get().GetCursorPos()));
 		}
 	}
 }
@@ -490,6 +504,12 @@ void SImGuiWidget::OnPostImGuiUpdate()
 	UpdateMouseCursor();
 }
 
+FVector2D SImGuiWidget::TransformScreenPointToImGui(const FGeometry& MyGeometry, const FVector2D& Point) const
+{
+	const FSlateRenderTransform ImGuiToScreen = ImGuiTransform.Concatenate(MyGeometry.GetAccumulatedRenderTransform());
+	return ImGuiToScreen.Inverse().TransformPoint(Point);
+}
+
 namespace
 {
 	FORCEINLINE FSlateRenderTransform RoundTranslation(const FSlateRenderTransform& Transform)

Source/ImGui/Private/Widgets/SImGuiWidget.h

@@ -70,6 +70,12 @@ public:
 
 	virtual void OnMouseLeave(const FPointerEvent& MouseEvent) override;
 
+	virtual FReply OnTouchStarted(const FGeometry& MyGeometry, const FPointerEvent& TouchEvent) override;
+
+	virtual FReply OnTouchMoved(const FGeometry& MyGeometry, const FPointerEvent& TouchEvent) override;
+
+	virtual FReply OnTouchEnded(const FGeometry& MyGeometry, const FPointerEvent& TouchEvent) override;
+
 private:
 
 	void CreateInputHandler(const FStringClassReference& HandlerClassReference);
@@ -101,6 +107,8 @@ private:
 
 	void OnPostImGuiUpdate();
 
+	FVector2D TransformScreenPointToImGui(const FGeometry& MyGeometry, const FVector2D& Point) const;
+
 	virtual int32 OnPaint(const FPaintArgs& Args, const FGeometry& AllottedGeometry, const FSlateRect& MyClippingRect, FSlateWindowElementList& OutDrawElements, int32 LayerId, const FWidgetStyle& WidgetStyle, bool bParentEnabled) const override;
 
 	virtual FVector2D ComputeDesiredSize(float) const override;

Source/ImGui/Public/ImGuiInputHandler.h

@@ -82,11 +82,35 @@ public:
 
 	/**
 	 * Called to handle mouse move events.
-	 * @param Mouse position (in ImGui space)
+	 * @param MousePosition Mouse position (in ImGui space)
 	 * @returns Response whether the event was handled
 	 */
 	virtual FReply OnMouseMove(const FVector2D& MousePosition);
 
+	/**
+	 * Called to handle touch started event.
+	 * @param TouchPosition Touch position (in ImGui space)
+	 * @param TouchEvent Touch event passed from Slate
+	 * @returns Response whether the event was handled
+	 */
+	virtual FReply OnTouchStarted(const FVector2D& TouchPosition, const FPointerEvent& TouchEvent);
+
+	/**
+	 * Called to handle touch moved event.
+	 * @param TouchPosition Touch position (in ImGui space)
+	 * @param TouchEvent Touch event passed from Slate
+	 * @returns Response whether the event was handled
+	 */
+	virtual FReply OnTouchMoved(const FVector2D& TouchPosition, const FPointerEvent& TouchEvent);
+
+	/**
+	 * Called to handle touch ended event.
+	 * @param TouchPosition Touch position (in ImGui space)
+	 * @param TouchEvent Touch event passed from Slate
+	 * @returns Response whether the event was handled
+	 */
+	virtual FReply OnTouchEnded(const FVector2D& TouchPosition, const FPointerEvent& TouchEvent);
+
 	/** Called to handle activation of the keyboard input. */
 	virtual void OnKeyboardInputEnabled();