Add accessibility support to SwitchButton on Windows (#804)

There is no reliable way for screen reader users to find out the fuzzy (needs
work) status. After some research, the most likely cause is as follows:

1. SwitchButton is owner drawn and therefore sets the BS_OWNERDRAW window style

2. When BS_OWNERDRAW is set, all other window styles should be ignored
   according to the docs

3. The default MSAA implementation in Windows ignores other window styles. As
   it relies on window style to find out whether the button is either a push
   button or a check box, this information is lost, and MSAA treats the toggle
   as a plain button without checkable state.

Fixed by implementing wxAccessible for the custom button.

Fixes #693.

src/custom_buttons.cpp

@@ -407,9 +407,24 @@ SwitchButton::SwitchButton(wxWindow *parent, wxWindowID winid, const wxString& l
     SetBackgroundColour(parent->GetBackgroundColour());
     MakeOwnerDrawn();
     Bind(wxEVT_LEFT_DOWN, &SwitchButton::OnMouseClick, this);
-#endif
+#if wxUSE_ACCESSIBILITY
+    Bind(wxEVT_TOGGLEBUTTON, [=](wxCommandEvent& e) {
+        wxAccessible::NotifyEvent(wxACC_EVENT_OBJECT_STATECHANGE, this, wxOBJID_CLIENT, wxACC_SELF);
+        e.Skip();
+    });
+#endif // wxUSE_ACCESSIBILITY
+#endif // __WXMSW__
 }
 
+#ifdef __WXMSW__
+#if wxUSE_ACCESSIBILITY
+wxAccessible* SwitchButton::CreateAccessible() 
+{
+    return new accessible(this);
+}
+#endif // wxUSE_ACCESSIBILITY
+#endif // __WXMSW__
+
 void SwitchButton::SetColors(const wxColour& on, const wxColour& offLabel)
 {
     (void)on;
@@ -546,6 +561,49 @@ bool SwitchButton::MSWOnDraw(WXDRAWITEMSTRUCT *wxdis)
     return true;
 }
 
+#if wxUSE_ACCESSIBILITY
+wxAccStatus SwitchButton::accessible::GetRole(int childId, wxAccRole* role)
+{
+    if (childId != wxACC_SELF)
+    {
+        return wxAccessible::GetRole(childId, role);
+    }
+    *role = wxROLE_SYSTEM_CHECKBUTTON;
+    return wxACC_OK;
+}
+
+wxAccStatus SwitchButton::accessible::GetState(int childId, long* state)
+{
+    if (childId != wxACC_SELF)
+    {
+        return wxAccessible::GetState(childId, state);
+    }
+    auto window = dynamic_cast<SwitchButton*>(this->GetWindow());
+    if (window->IsFocusable())
+    {
+        *state |= wxACC_STATE_SYSTEM_FOCUSABLE;
+    }
+    if (!window->IsShown())
+    {
+        *state |= wxACC_STATE_SYSTEM_INVISIBLE;
+    }
+    if (window->GetValue())
+    {
+        *state |= wxACC_STATE_SYSTEM_CHECKED;
+    }
+    if (!window->IsEnabled())
+    {
+        *state |= wxACC_STATE_SYSTEM_UNAVAILABLE;
+    }
+    if (window->HasFocus())
+    {
+        *state |= wxACC_STATE_SYSTEM_FOCUSED;
+    }
+    return wxACC_OK;
+}
+
+#endif // wxUSE_ACCESSIBILITY
+
 #endif // __WXMSW__
 
 #endif // !__WXOSX__

src/custom_buttons.h

@@ -114,12 +114,24 @@ class SwitchButton : public wxToggleButton
     void SetColors(const wxColour& on, const wxColour& offLabel);
 
 #ifdef __WXMSW__
+#if wxUSE_ACCESSIBILITY
+    wxAccessible* CreateAccessible() override;
+#endif // wxUSE_ACCESSIBILITY
     bool ShouldInheritColours() const override { return true; }
     void OnMouseClick(wxMouseEvent& e);
     bool MSWOnDraw(WXDRAWITEMSTRUCT *wxdis) override;
     wxSize DoGetBestSize() const override;
 
 private:
+#if wxUSE_ACCESSIBILITY
+    class accessible : public wxAccessible
+    {
+    public:
+        accessible(SwitchButton* win) : wxAccessible(wxDynamicCast(win, wxWindow)) {}
+        wxAccStatus GetRole(int childId, wxAccRole* role) override;
+        wxAccStatus GetState(int childId, long* state) override;
+    };
+#endif // wxUSE_ACCESSIBILITY
     wxColour m_clrOn, m_clrOffLabel;
 #endif // __WXMSW__
 };