Extract LOGPIXELS* usage to FontDialog::getDPI #62

This change refactors the usage of LOGPIXELS* in FontDialog to make it
clear why we depend on the System DPI aware value while using
DPI_AWARENESS_CONTEXT_UNAWARE_GDISCALED and extracts it into a new
method FontDialog::getDPI.

Contributes to
#62 and
#127

Author: amartya4256

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/FontDialog.java

@@ -200,9 +200,7 @@ public FontData open () {
 	if (fontData != null && fontData.data != null) {
 		LOGFONT logFont = fontData.data;
 		int lfHeight = logFont.lfHeight;
-		long hDC = OS.GetDC (0);
-		int pixels = -(int)(0.5f + (fontData.height * OS.GetDeviceCaps(hDC, OS.LOGPIXELSY) / 72));
-		OS.ReleaseDC (0, hDC);
+		int pixels = -(int)(0.5f + (fontData.height * getDPI() / 72));
 		logFont.lfHeight = pixels;
 		lpcf.Flags |= OS.CF_INITTOLOGFONTSTRUCT;
 		OS.MoveMemory (lpLogFont, logFont, LOGFONT.sizeof);
@@ -256,9 +254,9 @@ public FontData open () {
 			 * This will not work on multiple screens or for printing. Should use DC for the
 			 * proper device.
 			 */
-			long hDC = OS.GetDC(0);
-			int logPixelsY = OS.GetDeviceCaps(hDC, OS.LOGPIXELSY);
+			int logPixelsY = getDPI();
 			int pixels = 0;
+			long hDC = OS.GetDC(0);
 			if (logFont.lfHeight > 0) {
 				/*
 				 * Feature in Windows. If the lfHeight of the LOGFONT structure is positive, the
@@ -308,6 +306,17 @@ public FontData open () {
 	return fontData;
 }
 
+private int getDPI() {
+	long hDC = OS.GetDC (0);
+	// We need to use OS.GetDeviceCaps, which is static throughout application
+	// lifecycle (System DPI Aware), because we use
+	// DPI_AWARENESS_CONTEXT_UNAWARE_GDISCALED which always depends on the DPI at
+	// application startup
+	int dpi = OS.GetDeviceCaps(hDC, OS.LOGPIXELSY);
+	OS.ReleaseDC (0, hDC);
+	return dpi;
+}
+
 /**
  * Sets the effects selection controls in the dialog visible if the
  * argument is <code>true</code>, and invisible otherwise.