Add wxDisplay::GetStdPPIValue() and GetStdPPI()

Provide public way to access the default platform DPI, which was
previously a constant private to src/common/wincmn.cpp.

include/wx/display.h

@@ -87,6 +87,21 @@ public:
     // get the resolution of this monitor in pixels per inch
     wxSize GetPPI() const;
 
+    // get the default resolution for displays on this platform
+    static int GetStdPPIValue()
+    {
+#ifdef __WXOSX__
+        return 72;
+#else
+        return 96;
+#endif
+    }
+
+    static wxSize GetStdPPI()
+    {
+        return wxSize(GetStdPPIValue(), GetStdPPIValue());
+    }
+
     // name may be empty
     wxString GetName() const;
 

interface/wx/display.h

@@ -143,6 +143,33 @@ public:
      */
     wxSize GetPPI() const;
 
+    /**
+        Returns default display resolution for the current platform in pixels
+        per inch.
+
+        This function mostly used internally, use GetPPI() to get the actual
+        display resolution.
+
+        Currently the standard PPI is the same in both horizontal and vertical
+        directions on all platforms and its value is 96 everywhere except under
+        Apple devices (those running macOS, iOS, watchOS etc), where it is 72.
+
+        @see GetStdPPI()
+
+        @since 3.1.5
+     */
+    static int GetStdPPIValue();
+
+    /**
+        Returns default display resolution for the current platform as wxSize.
+
+        This function is equivalent to constructing wxSize object with both
+        components set to GetStdPPIValue().
+
+        @since 3.1.5
+     */
+    static wxSize GetStdPPI();
+
     /**
         Returns @true if the display is the primary display. The primary
         display is the one whose index is 0.

src/common/wincmn.cpp

@@ -101,13 +101,6 @@ bool IsInCaptureStack(wxWindowBase* win);
 
 } // wxMouseCapture
 
-// Most platforms use 96 DPI by default, but Mac traditionally uses 72.
-#ifdef __WXOSX__
-static const int BASELINE_DPI = 72;
-#else
-static const int BASELINE_DPI = 96;
-#endif
-
 // ----------------------------------------------------------------------------
 // static data
 // ----------------------------------------------------------------------------
@@ -805,7 +798,7 @@ static wxSize GetDPIHelper(const wxWindowBase* w)
     if ( !dpi.x || !dpi.y )
         dpi = wxScreenDC().GetPPI();
     if ( !dpi.x || !dpi.y )
-        dpi = wxSize(BASELINE_DPI, BASELINE_DPI);
+        dpi = wxDisplay::GetStdPPI();
 
     return dpi;
 }
@@ -829,7 +822,7 @@ double wxWindowBase::GetDPIScaleFactor() const
     // We use just the vertical component of the DPI because it's the one
     // that counts most and, in practice, it's equal to the horizontal one
     // anyhow.
-    return dpi.y / (double)BASELINE_DPI;
+    return dpi.y / (double)wxDisplay::GetStdPPIValue();
 }
 
 // helper of GetWindowBorderSize(): as many ports don't implement support for
@@ -2900,10 +2893,12 @@ wxWindowBase::FromDIP(const wxSize& sz, const wxWindowBase* w)
 {
     const wxSize dpi = GetDPIHelper(w);
 
+    const int baseline = wxDisplay::GetStdPPIValue();
+
     // Take care to not scale -1 because it has a special meaning of
     // "unspecified" which should be preserved.
-    return wxSize(sz.x == -1 ? -1 : wxMulDivInt32(sz.x, dpi.x, BASELINE_DPI),
-                  sz.y == -1 ? -1 : wxMulDivInt32(sz.y, dpi.y, BASELINE_DPI));
+    return wxSize(sz.x == -1 ? -1 : wxMulDivInt32(sz.x, dpi.x, baseline),
+                  sz.y == -1 ? -1 : wxMulDivInt32(sz.y, dpi.y, baseline));
 }
 
 /* static */
@@ -2912,10 +2907,12 @@ wxWindowBase::ToDIP(const wxSize& sz, const wxWindowBase* w)
 {
     const wxSize dpi = GetDPIHelper(w);
 
+    const int baseline = wxDisplay::GetStdPPIValue();
+
     // Take care to not scale -1 because it has a special meaning of
     // "unspecified" which should be preserved.
-    return wxSize(sz.x == -1 ? -1 : wxMulDivInt32(sz.x, BASELINE_DPI, dpi.x),
-                  sz.y == -1 ? -1 : wxMulDivInt32(sz.y, BASELINE_DPI, dpi.y));
+    return wxSize(sz.x == -1 ? -1 : wxMulDivInt32(sz.x, baseline, dpi.x),
+                  sz.y == -1 ? -1 : wxMulDivInt32(sz.y, baseline, dpi.y));
 }
 
 #endif // !wxHAVE_DPI_INDEPENDENT_PIXELS