Reset static sizes when DPI changes

include/wx/private/window.h

@@ -11,6 +11,7 @@
 #define _WX_PRIVATE_WINDOW_H_
 
 #include "wx/gdicmn.h"
+#include "wx/dynlib.h"
 
 namespace wxPrivate
 {
@@ -33,6 +34,67 @@ inline wxSize GetAverageASCIILetterSize(const T& of_what)
     return s;
 }
 
+namespace
+{
+
+inline bool SupportsPerMonitorDPI()
+{
+    static bool s_checkDPI =
+#if defined(__WXMSW__) && wxUSE_DYNLIB_CLASS
+        // Only check the DPI when GetDpiForWindow is available because the old
+        // method (GetDeviceCaps) is a lot slower (about 1500 times).
+        // And when GetDpiForWindow is not available (for example older Windows
+        // versions), per-monitor DPI (V2) is also not available.
+        wxLoadedDLL("user32.dll").HasSymbol("GetDpiForWindow");
+#else
+        false;
+#endif
+    return s_checkDPI;
+}
+
+}
+
+template <typename T>
+class DpiDependentValue
+{
+public:
+    // Explicit initialization is needed if T is a primitive type.
+    DpiDependentValue()
+        : m_value(), m_dpi()
+    { }
+
+    bool HasChanged(const wxWindowBase* win)
+    {
+        if ( win && SupportsPerMonitorDPI() )
+        {
+            const wxSize dpi = win->GetDPI();
+            if ( dpi != m_dpi )
+            {
+                m_dpi = dpi;
+                return true;
+            }
+        }
+
+        // Ensure that we return true the first time we're called,
+        // asuming that the value will always be set to a non-default value.
+        return m_value == T();
+    }
+
+    void SetAtNewDPI(const T& value)
+    {
+        m_value = value;
+    }
+
+    T& Get()
+    {
+        return m_value;
+    }
+
+private:
+    T m_value;
+    wxSize m_dpi;
+};
+
 } // namespace wxPrivate
 
 #endif // _WX_PRIVATE_WINDOW_H_