[wxhaskell-users] Coding your own widgets in wxhaskell... and other problems

[dreamer <Ben...@we...>]

Hello Daan, Hello List,

I just tried to create my own custom widget based on HtmlWindow.  I
encountered a few problems which I want to share with you:

    1) Surprisingly to me I had to realize that there is a separate 
       CWXCHtmlWindow hierarchy.  As the corresponding creation function
       wxcHtmlWindowCreate is not documented I first assumed that it is
       used to implemented HtmlWindow, which on a closer look I had to
       realize it is NOT.  My current guess is that it is a left over
       from the automatic wrapping process.  Which normally wouldn't 
       bother me that much but unfortunately the function
       htmlWindowOnHtmlEvent can ONLY be called on a CWXCHtmlWindow! Why
       is that?

    2) A bigger problem: while the attribute handling is really nice to
       have and works well with the wrapped C++ widgets it can be quite
       difficult to provide with new self written widgets.  Let's for
       the sake of argument assume that I want to write a wrapper for the
       HtmlWindow which maps the HtmlLinkClicked event to the select
       event of the class Selecting and provides a attribute
       selectedLink :: Attr w (Maybe String).  

       Iff I got it right one cannnot "derive" from a builtin widget and
       add new "state" to the created value.  That is write something
       like  newtype MyWindow = MyWindow (MyState, HtmlWindow ())
       and then just add some instance declarations to make it work with
       the wxHaskell routines.  

       Therefore the only way I see to associate some state with a
       window is to use the windowGetId function and a table of 
       Int -> MyState.  

       Add this point we hit a kind of open design problem in haskell!

       It is not yet possible (as far as I know) to write a global value
       which originally was part of the IO monad.  Or at least it is not
       possible to do so safely!  Using unsafePerformIO one can do this:

       {-# noinline states #-}
       states :: Var (FiniteMap Int MyState)

       states = unsafePerformIO $ varCreate state

       So this is probably not a really satisfactory solution, but if it
       happens to be the only way one can solve above problem it should
       probably be part of the library.

       So is there a better way / thoughts / suggestions?  Note that at
       the moment I can not even do this as windowGetId can not be
       called on values of type CWXCHtmlWindow.

Cheers,

Bene


PS @Daan: Together with a colleaque of mine I recently found a small bug in
    wxHaskell but not having had any response from you I assume that
    mailing to you directly failed again and therefore I include it here
    again:

In WX.Controls.MultiListBox:

Controls.hs 598:
-- | Abstract type of the 'MultiListBox' class.
data CMultiListBox a  = CMultiListBox

instance Selections (MultiListBox a) where
  selections
    = newAttr "selections" listBoxGetSelectionList setter
    where
      setter w is
        = mapM_ (\i -> listBoxSetSelection w i True) is


This code is obviously incorrect in that it only adds elements
to the selection but never removes elements of it.

this bugfix should do the trick:

-- | Abstract type of the 'MultiListBox' class.
data CMultiListBox a  = CMultiListBox

instance Selections (MultiListBox a) where
  selections
    = newAttr "selections" listBoxGetSelectionList setter
    where
      setter w is
        = do s <- listBoxGetSelectionList w
             mapM_ (\i -> listBoxSetSelection w i False) s
             mapM_ (\i -> listBoxSetSelection w i True) is