Question

I have an application that I am working on, and I’m basically self-teaching GUI programming. I asked a fairly involved question over on programmers.stackexchange. This question is about the mechanics of an idea I had not tried.

I have three widgets: a TreeView, a TextField, and a DrawingArea. Each of the three widgets interacts very intimately with events on one necessarily triggering actions on the other. Those three widgets largely do not interact with the rest of the application except (so far) by reading an MVar containing the global application state.

Currently I can think of no case in which the larger application should ever interact directly with any of those three widgets. Further, that identical pattern would be replicated to review other data that has the same form. So, it seems to me that it would make sense to actually bind these three widgets together into a larger composite widget that can interact with GTK’s normal event queue. So, for instance

type MyDataViewWidget = (TreeView, TextField, DrawingArea)
data DataUpdatedSignal a = DataUpdatedSignal a
data RedrawEvent a = RedrawEvent a

So, the widget would use DataUpdatedEvent to indicate to the rest of the application that something inside MyDataViewWidget changed, and RedrawEvent would tell the widget that it needs to redraw or re-read the source data.

(technically, I have not thought through semantically what the various actions in the composite widget would do… whether the widgets would just have a read-only copy of the application data and need to receive new read-only copies with the RedrawEvent or perhaps the widgets would have the MVar itself and be allowed to change the data in the MVar, etc… I’m just interested at the moment in how to actually do this)

Are there any examples of doing something like this? Basically, what instances do I need to implement to create the new widget and the two signals? I’d prefer to stick to Haskell, but I could drop to C in order to build up the new widget.

Answer 1

Unfortunately, there is currently no pure-Haskell way to (correctly) implement the Widget type class. You’ll need to implement your widget in C, then import it via the FFI. There are numerous examples of this — basically all of gtk+/gtk2hs is a collection of hundreds of examples of doing this.