Re: [Gtk2hs-users] Creating a new widget type

[Axel S. <Axe...@in...>]

Hi Vivian,

On Sep 19, 2010, at 5:59, Vivian McPhail wrote:

> Hi,
>
> I'd like to be able to create a new widget type in Haskell.  Is this  
> possible?
>

We don't provide a way to construct new widgets. But you can take a  
DrawingArea and add some new attributes to it which is usually all  
that is needed.

> The widget draws (using cairo) a graph to a drawArea inside a  
> container.  I would like to have the widget store the graph data  
> structure so that it can be get/set'ted.  Unfortunately I am unclear  
> as to how the Attribute system works with respect to defining new  
> attributes.  Does each widget instance have some sort of unique ID?
>

No, it's simpler than that. An individual widget has an instance with  
an individual pointer. You can simply compare the handles to a widget  
using == . In order to store additional information inside a widget  
you use
objectCreateAttribute (with unsafePerformIO) to add a new attribute to  
a widget, e.g.:

myInt :: (Attr DrawingArea (Maybe Int))
myInt = unsafePerformIO $ objectCreateAttribute

Then later you can access this integer using set and get as you would  
with any other attributes. If this integer has never been set on a  
particular instance, you get Nothing back.

Hope this helps,
Axel

> Are there any examples I could be pointed to, or somewhere to  
> start?  (I have seen how to derive a new widget in C++, but that  
> does not help).
>
> This is essentially the code that works at the moment, but the  
> canvas is not its own widget type:
>
> \begin{code}
> plotNew :: Frame -> Figure () -> IO Plot
> plotNew frame figure = do
>    canvas <- drawingAreaNew
>    containerAdd frame canvas
>    widgetModifyBg canvas StateNormal (Color 65535 65535 65535)
>
>    -- unclear here
>    {-
>      newAttr (\this -> getFigureData this->uniqueID)
>              (\this fig -> setFigureData this->uniqueID fig)
>    -}
>
>    on canvas exposeEvent $ tryEvent $ do s <- liftIO $ widgetGetSize  
> canvas
>                                                              drw <-  
> liftIO $ widgetGetDrawWindow canvas
>                                                              liftIO  
> $ renderWithDrawable drw (render figure s)
>    return canvas
> \end{code}
>
> Thanks,
>
> Vivian
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