Invalidate wxDisplay cache under MSW when the displays change

The cache added in 990c8bfd73 was not
invalidated properly, meaning that wrong information was returned when
displays were [dis]connected after the application startup.

Fix this at least for MSW by invalidating the cache on receiving
WM_DISPLAYCHANGE (which means that sometimes we will do it
unnecessarily, as the change in resolution of an existing display
doesn't require cache invalidation, but this shouldn't be a big problem
in practice as the speed with which the user can change the display
resolution is not very high).

Closes https://github.com/wxWidgets/wxWidgets/pull/1090

include/wx/display.h

@@ -115,6 +115,11 @@ public:
     void ResetMode() { (void)ChangeMode(); }
 #endif // wxUSE_DISPLAY
 
+    // If the implementation caches any information about the displays, calling
+    // this function clears it -- this should be done e.g. after a display
+    // [dis]connection.
+    static void InvalidateCache();
+
 private:
     // returns the factory used to implement our static methods and create new
     // displays

include/wx/private/display.h

@@ -22,7 +22,7 @@ class wxDisplayFactory
 {
 public:
     wxDisplayFactory() { }
-    virtual ~wxDisplayFactory();
+    virtual ~wxDisplayFactory() { ClearImpls(); }
 
     // Create the display if necessary using CreateDisplay(), otherwise just
     // get it from cache.
@@ -48,6 +48,9 @@ public:
     // the window pointer must not be NULL (i.e. caller should check it)
     virtual int GetFromWindow(const wxWindow *window);
 
+    // Trigger recreation of wxDisplayImpl when they're needed the next time.
+    void InvalidateCache() { ClearImpls(); }
+
 protected:
     // create a new display object
     //
@@ -55,6 +58,9 @@ protected:
     virtual wxDisplayImpl *CreateDisplay(unsigned n) = 0;
 
 private:
+    // Delete all the elements of m_impls vector and clear it.
+    void ClearImpls();
+
     // On-demand populated vector of wxDisplayImpl objects.
     wxVector<wxDisplayImpl*> m_impls;
 

src/common/dpycmn.cpp

@@ -113,6 +113,11 @@ wxDisplay::wxDisplay(const wxWindow* window)
     return Factory().GetFromWindow(window);
 }
 
+/* static */ void wxDisplay::InvalidateCache()
+{
+    Factory().InvalidateCache();
+}
+
 // ----------------------------------------------------------------------------
 // functions forwarded to wxDisplayImpl
 // ----------------------------------------------------------------------------
@@ -229,13 +234,15 @@ wxSize wxDisplayImpl::GetPPI() const
 // wxDisplayFactory implementation
 // ============================================================================
 
-wxDisplayFactory::~wxDisplayFactory()
+void wxDisplayFactory::ClearImpls()
 {
     for ( size_t n = 0; n < m_impls.size(); ++n )
     {
         // It can be null, that's ok.
         delete m_impls[n];
     }
+
+    m_impls.clear();
 }
 
 int wxDisplayFactory::GetFromWindow(const wxWindow *window)

src/msw/window.cpp

@@ -58,8 +58,9 @@
     #include "wx/ownerdrw.h"
 #endif
 
-#include "wx/hashmap.h"
+#include "wx/display.h"
 #include "wx/evtloop.h"
+#include "wx/hashmap.h"
 #include "wx/popupwin.h"
 #include "wx/power.h"
 #include "wx/scopeguard.h"
@@ -4692,6 +4693,8 @@ bool wxWindowMSW::HandleSysColorChange()
 
 bool wxWindowMSW::HandleDisplayChange()
 {
+    wxDisplay::InvalidateCache();
+
     wxDisplayChangedEvent event;
     event.SetEventObject(this);