Multi-touch and WPF

Microsoft’s WPF desktop UI framework can suffer a severe lag in input events when multiple fingers are on the touch screen.

This library contains a basic re-implementation of WPF's TouchDevice class based on the data received in WM_TOUCH window messages rather than the Tablet PC API.

This is primarily intended to work around the lag bug, but could be useful to enable other functions that require WM_TOUCH such as App-style focus tracking in Windows 8.

Two steps (or equivalents) are needed, firstly to disable the Tablet PC API, then to use replacement TouchDevice implementations.

Step 1: Disable StylusLogic

WPF uses the Windows 7+ RealTimeStylus API to raise pen and touch events:

PenIMC.dll > StylusLogic > TouchDevice > Events

The the lag is in StylusLogic, which we can't fix but can at least trick into no longer raising events using a by faking disconnection of the touch hardware.

Step 1: Add a call to MessageTouchDevice.RegisterTouchWindow(IntPtr hWnd) during or after your Window's SourceInitialized event.

Your window will still respond to touches after adding this, but you should find that no touch events are being raised - touch input will only be seen by your code as mouse events. WPF touch behaviours such as panning will stop working as they rely on touch events.

Step 2: Re-implement TouchDevice

WPF's TouchDevice is abstract, so a subclass can raise events and continue to manage properties like Focus:

(StylusLogic replacement) > TouchDevice > Events

We could base an implementation on any of:

• PenIMC.dll, seemingly undocumented
• DirectInput, poorly implemented by touch drivers
• RawInput, requiring hardware-specific code
• WM_POINTER window messages, from Windows 8+
• WM_TOUCH window messages, from Windows 7+

For maximum compatibility this library contains an implementation using WM_TOUCH window messages, that need to be forwarded to its handler.

Step 2: Add a call to MessageTouchDevice.WndProc from a function you've attached to your Window using HwndSource.AddHook.

Your window should start raising WPF touch events again after adding this, in turn restoring WPF gestures such as panning.

Having used the RealTimeStylus API prevents a window receiving WM_TOUCH and WM_POINTER window messages (bar WM_POINTERDOWN and UP) but the call in step 1 will have both shut down the RealTimeStylusAPI and called RegisterTouchWindow to undo this.

This won’t quite restore multi-touch visual feedback on Windows 8, but the SetWindowFeedbackSetting function can help.