Add wxDateTimePickerCtrl::SetNullText()

This allows to customize the string shown when there is no valid date
under MSW (only, for now) and can be notably used to suppress the
unused date completely, which can be useful to lighten up the display
when there are many controls.

Add UI elements to the widgets sample allowing to test the new function.

src/msw/datetimectrl.cpp

@@ -99,6 +99,8 @@ void wxDateTimePickerCtrl::SetValue(const wxDateTime& dt)
         return;
     }
 
+    MSWUpdateFormatIfNeeded(dt.IsValid());
+
     m_date = dt;
 }
 
@@ -107,6 +109,57 @@ wxDateTime wxDateTimePickerCtrl::GetValue() const
     return m_date;
 }
 
+void wxDateTimePickerCtrl::MSWUpdateFormatIfNeeded(bool valid)
+{
+    if ( MSWAllowsNone() && !m_nullText.empty() && valid != m_date.IsValid() )
+        MSWUpdateFormat(valid);
+}
+
+void wxDateTimePickerCtrl::MSWUpdateFormat(bool valid)
+{
+    // We just use NULL to reset to the default format when the date is valid,
+    // as the control seems to remember whichever format was used when it was
+    // created, i.e. this works both with and without wxDP_SHOWCENTURY.
+
+    // Note: due to a bug in MinGW headers, with missing parentheses around the
+    // macro argument (corrected in or before 8.2, but still existing in 5.3),
+    // we have to use a temporary variable here.
+    const TCHAR* const format = valid ? NULL : m_nullText.t_str();
+    DateTime_SetFormat(GetHwnd(), format);
+}
+
+void wxDateTimePickerCtrl::SetNullText(const wxString& text)
+{
+    m_nullText = text;
+    if ( m_nullText.empty() )
+    {
+        // Using empty format doesn't work with the native control, it just
+        // uses the default short date format in this case, so set the format
+        // to the single space which is more or less guaranteed to work as it's
+        // the semi-official way to clear the control contents when it doesn't
+        // have any valid value, according to Microsoft's old KB document
+        // Q238077, which can still be found online by searching for its
+        // number, even if it's not available on Microsoft web site any more.
+        //
+        // Coincidentally, it's also convenient for us, as we can just check if
+        // null text is empty to see if we need to use it elsewhere in the code.
+        m_nullText = wxS(" ");
+    }
+    else
+    {
+        // We need to quote the text, as otherwise format specifiers (e.g.
+        // "d", "m" etc) would be interpreted specially by the control. To make
+        // things simple, we just quote it entirely and always.
+        m_nullText.Replace("'", "''");
+        m_nullText.insert(0, "'");
+        m_nullText.append("'");
+    }
+
+    // Apply it immediately if we don't have any value right now.
+    if ( !m_date.IsValid() )
+        MSWUpdateFormat(false);
+}
+
 wxSize wxDateTimePickerCtrl::DoGetBestSize() const
 {
     wxClientDC dc(const_cast<wxDateTimePickerCtrl *>(this));
@@ -168,7 +221,12 @@ wxDateTimePickerCtrl::MSWOnNotify(int idCtrl, WXLPARAM lParam, WXLPARAM *result)
     switch ( hdr->code )
     {
         case DTN_DATETIMECHANGE:
-            if ( MSWOnDateTimeChange(*(NMDATETIMECHANGE*)(hdr)) )
+            const NMDATETIMECHANGE& dtch = *(NMDATETIMECHANGE*)(hdr);
+
+            // Update the format before showing the new date if necessary.
+            MSWUpdateFormatIfNeeded(dtch.dwFlags == GDT_VALID);
+
+            if ( MSWOnDateTimeChange(dtch) )
             {
                 *result = 0;
                 return true;