Implement wxListBox::EnsureVisible() for wxMSW.

Manually check whether the item is currently visible and scroll the listbox
just enough if it isn't.

Closes #3224.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77653 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -102,6 +102,7 @@ wxMSW:
 - Add wxThread::MSWGetHandle() (troelsk).
 - Allow using sizers for laying out wxMDIClientWindow (Artur Wieczorek).
 - Fix updating wxSlider background when its parent background changes.
+- Implement wxListBox::EnsureVisible() (RIVDSL).
 
 wxOSX/Cocoa:
 

include/wx/msw/listbox.h

@@ -93,6 +93,8 @@ public:
     int HitTest(const wxPoint& pt) const { return DoHitTestList(pt); }
     int HitTest(wxCoord x, wxCoord y) const { return DoHitTestList(wxPoint(x, y)); }
 
+    virtual void EnsureVisible(int n);
+
     // ownerdrawn wxListBox and wxCheckListBox support
 #if wxUSE_OWNER_DRAWN
     // override base class virtuals

interface/wx/listbox.h

@@ -285,12 +285,8 @@ public:
     /**
         Ensure that the item with the given index is currently shown.
 
-        Scroll the listbox if necessary.
+        This method scrolls the listbox only if necessary and doesn't do
-
+        anything if this item is already shown, unlike SetFirstItem().
-        This method is currently only implemented in wxGTK and wxOSX and does
-        nothing in other ports.
-
-        @see SetFirstItem()
      */
     virtual void EnsureVisible(int n);
 

src/msw/listbox.cpp

@@ -221,6 +221,38 @@ void wxListBox::MSWOnItemsChanged()
 // implementation of wxListBoxBase methods
 // ----------------------------------------------------------------------------
 
+void wxListBox::EnsureVisible(int n)
+{
+    wxCHECK_RET( IsValid(n),
+                 wxT("invalid index in wxListBox::EnsureVisible") );
+
+    // when item is before the first visible item, make the item the first visible item
+    const int firstItem = SendMessage(GetHwnd(), LB_GETTOPINDEX, 0, 0);
+    if ( n <= firstItem )
+    {
+        DoSetFirstItem(n);
+        return;
+    }
+
+    // retrieve item height in order to compute last visible item and scroll amount
+    const int itemHeight = SendMessage(GetHwnd(), LB_GETITEMHEIGHT, 0, 0);
+    if ( itemHeight == LB_ERR || itemHeight == 0)
+        return;
+
+    // compute the amount of fully visible items
+    int countVisible = GetClientSize().y / itemHeight;
+    if ( !countVisible )
+        countVisible = 1;
+
+    // when item is before the last fully visible item, it is already visible
+    const int lastItem = firstItem + countVisible - 1;
+    if ( n <= lastItem )
+        return;
+
+    // make the item the last visible item by setting the first visible item accordingly
+    DoSetFirstItem(n - countVisible + 1);
+}
+
 void wxListBox::DoSetFirstItem(int N)
 {
     wxCHECK_RET( IsValid(N),