[WPF] Improve GetScaleFactor and return device-independent pixels from GetMouseLocation

This enables us to use the value of Desktop.MouseLocation to accurately set the
ScreenBounds of a window on WPF.

1 files changed, 44 insertions, 18 deletions

diff --git a/Xwt.WPF/Xwt.WPFBackend/WpfDesktopBackend.cs b/Xwt.WPF/Xwt.WPFBackend/WpfDesktopBackend.cs
index 1a0fa7e9..6eb8b7c7 100644
--- a/Xwt.WPF/Xwt.WPFBackend/WpfDesktopBackend.cs
+++ b/Xwt.WPF/Xwt.WPFBackend/WpfDesktopBackend.cs
@@ -23,7 +23,8 @@
 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
-using System;
+using System;

+using System.Reflection;
 using System.Runtime.InteropServices;
 using Xwt.Backends;
 using SWF = System.Windows.Forms;
@@ -42,31 +43,54 @@ namespace Xwt.WPFBackend
 			{
 				System.Windows.Application.Current.Dispatcher.BeginInvoke (new Action (OnScreensChanged));
 			};
-		}
+		}

 
+		static bool cannotCallGetDpiForMonitor;
 		public override double GetScaleFactor (object backend)
-		{
-			var hdc = GetDC (IntPtr.Zero);
-			if (hdc == IntPtr.Zero) {
-				// GetDC failed for some reason
-				return base.GetScaleFactor (backend);
-			}
-			try {
-				//FIXME: Is it possible for the Y dpi to differ from the X dpi,
-				//  and if so, what should we do about it?
-				var dpiX = GetDeviceCaps (hdc, LOGPIXELSX);
-				return dpiX / BASELINE_DPI;
-			} finally {
-				ReleaseDC (IntPtr.Zero, hdc);
-			}
+		{

+			//FIXME: Is it possible for the Y dpi to differ from the X dpi,

+			//  and if so, what should we do about it?

+			int dpi = (int)BASELINE_DPI;
+
+			// In Windows 8.1, there can be a different dpi per monitor

+			if (!cannotCallGetDpiForMonitor) {

+				// .. I wish there was a less hacky way of getting the HMONITOR from the SWF.Screen :/

+				var hmonitorField = typeof (SWF.Screen).GetField ("hmonitor", BindingFlags.Instance | BindingFlags.NonPublic);

+				if (hmonitorField == null) {

+					cannotCallGetDpiForMonitor = true;

+				} else {

+					try {

+						int dpiY;

+						GetDpiForMonitor ((IntPtr)hmonitorField.GetValue (backend), MDT_Effective_DPI, out dpi, out dpiY);

+					} catch {

+						cannotCallGetDpiForMonitor = true;

+					}

+				}

+			}

+			if (cannotCallGetDpiForMonitor) {

+				// Get system-wide dpi

+				var hdc = GetDC (IntPtr.Zero);

+				if (hdc != IntPtr.Zero) {

+					try {

+						dpi = GetDeviceCaps (hdc, LOGPIXELSX);

+					} finally {

+						ReleaseDC (IntPtr.Zero, hdc);

+					}

+				}
+			}

+			return dpi / BASELINE_DPI;
 		}
 
 		#region implemented abstract members of DesktopBackend
 
 		public override Point GetMouseLocation()
 		{
-			var loc = SWF.Cursor.Position;
-			return new Point (loc.X, loc.Y);
+			var loc = SWF.Cursor.Position;

+			var screen = SWF.Screen.FromPoint (loc);

+			var scale = GetScaleFactor (screen);
+
+			// We need to convert the device pixels into WPF's device-independent pixels..
+			return new Point (loc.X / scale, loc.Y / scale);
 		}
 
 		public override IEnumerable<object> GetScreens ()
@@ -102,10 +126,12 @@ namespace Xwt.WPFBackend
 
 		const int LOGPIXELSX = 88;
 		const int LOGPIXELSY = 90;
+		const int MDT_Effective_DPI = 0;
 
 		[DllImport ("user32")] static extern IntPtr GetDC (IntPtr hWnd);
 		[DllImport ("user32")] static extern int ReleaseDC (IntPtr hWnd, IntPtr hdc);
 		[DllImport ("gdi32")]  static extern int GetDeviceCaps (IntPtr hdc, int nIndex);
+		[DllImport ("Shcore")] static extern int GetDpiForMonitor (IntPtr hmonitor, int dpiType, out int dpiX, out int dpiY);
 		#endregion
 	}
 }