Question

The following program does not work as expected:

module Main where

import Graphics.UI.Gtk
import Control.Monad.IO.Class (liftIO)
import Control.Concurrent (forkIO)

main = do
    unsafeInitGUIForThreadedRTS
    window <- windowNew
    windowSetDefaultSize window 200 200
    label <- labelNew (Just "Hello")
    forkIO (postGUISync (containerAdd window label))
    on window deleteEvent (liftIO mainQuit >> return True)
    widgetShowAll window
    mainGUI

The label is not added to the window. I compile with -threaded. Am I doing it wrong?

Answer 1

I believe that widgetShowAll only shows the children at the time of execution, so if your containerAdd happens after that, it won’t appear. I suggest explicitly showing the label in the action you pass to postGUISync, like this:

forkIO (postGUISync (containerAdd window label >> widgetShow label))