[Gtk2hs-users] How to create new widget types

[Ben F. <ben...@on...>]

Hello Experts

this is probably a FAQ: what is the proper way to create new widget types?

For concreteness, let's say, you want a (horizontal) composition of two
widgets. It should act and behave just like a built-in widget. The obvious
idea is to 'derive' from HBox:

  newtype MyWidget = MyWidget HBox

In order to make it apparent that this is a widget and that it is derived
from HBox, I do

  -- no instance HBoxClass MyWidget, interface is too general
  -- no instance BoxClass MyWidget, interface is too general
  instance ContainerClass MyWidget
  instance WidgetClass    MyWidget
  instance ObjectClass    MyWidget
  instance GObjectClass   MyWidget where
    toGObject (MyWidget o) = toGObject o
    unsafeCastGObject = MyWidget . unsafeCastGObject

So far, so good. BUT: how can I give MyWidget additional state? Say I have
implemented a feature where one of the two sub-widgets can be 'selected',
and I want client code to find out which. The obvious thing to do is

  data MyWidget MyWidget {
    box :: HBox,
    selection :: IORef (Maybe Widget)
  }

But then, how do I write the GObjectClass methods? I think casting would be
a very bad idea! OTOH, if I stick to the newtype, how can I add attributes?

There is one solution I found after searching for a while, but it has a
certain ugliness...

In System.Glib.GObject there is

  objectCreateAttribute :: GObjectClass o => IO (Attr o (Maybe a))

The problem is that this is an IO action. But you want to call this only
once, right? Remember that I want MyWidget to be indistinguishable from
built-in ones. So what I do is this:

  newtype MyWidget = MyWidgetTableView HBox

  data MyWidgetState = MyWidgetState {
    selection :: IORef (Maybe Widget),
    ...
  }

  -- not exported, at least:
  {-# NOINLINE myWidgetState #-}
  myWidgetState :: Attr MyWidget (Maybe MyWidgetState)
  myWidgetState = unsafePerformIO objectCreateAttribute

  -- exported:
  myWidgetSelection :: ReadAttr MyWidget (Maybe Widget)
  myWidgetSelection = readAttr getter where
    getter mw = do
      maybeState <- mw `get` myWidgetState
      case maybeState of
        Nothing -> error "GAAA! Wrong object type!"
        Just state -> readIORef (selection state)

  myWidgetNew = do
    hbox <- ...
    let myState = ...
    ...
    let mw = MyWidget hbox
    mw `set` [myWidgetState := Just myState]
    return mw

This works, but note the evil global variable (myWidgetState) which I'd
rather like to get rid of. Is there any way?

Do we need (or rather, want) a Gtk monad? (I think not.)

Cheers
Ben