How to do something repeatedly while button is pressed in gtk2hs? - haskell

Question

I need to do some action while button is pressed. How can I do it?
I have version 0.12.4.
P. S.:
For some reason, onButtonActivate in
import Graphics.UI.Gtk
import Control.Concurrent
main = do
initGUI
window <- windowNew
but <-buttonNewWithLabel "Write A"
onButtonActivate but $ do
putStr "A"
threadDelay 1000000
return()
containerAdd window but
widgetShowAll window
onDestroy window mainQuit
mainGUI
do not do anything.
Also, it's good to go, if action will be done repeatedly while pressed some key on keyboard.

Answer 1

According to the docs onButtonActivate is depreciated so you probably shouldn't use it. Im having trouble finding the correct way though, there probably are some generics signals somewhere that you should use. You can try my solution that uses onPressed and onRelease (these are also noted as depreciated). You could do as suggested in the comment and fork a thread:
import Control.Concurrent (forkIO, killThread, threadDelay)
import Control.Monad (void, when)
whilePressed button action = do
onPressed button $ do
threadId <- forkIO go
onReleased button (killThread threadId)
where
go = do
action
threadDelay 1000000
go
Then rewrite your main to do:
whilePressed but (putStr "A")
Im not sure if this is safe though, as it seems it could be possible for the buttonReleased event to be fired before killThread is registered. It might be safer to use an IORef:
import Data.IORef
whilePressed button action = do
isPressed <- newIORef False
onPressed button $ do
writeIORef isPressed True
void $ forkIO $ go isPressed
onReleased button (writeIORef isPressed False)
where
go isPressed = do
c <- readIORef isPressed
when c $ do
action
threadDelay 1000000
void $ forkIO $ go isPressed
Unfortunately I haven't compiled or tested the code sine I cannot install GTK on this computer, but let me know if you have any issues.

Answer 2

This uses a ToggleButton, a "timeout" object (see timeoutAdd in the module System.Glib.MainLoop in the docs) and an IORef. A timer is started when the ToggleButton is pressed down, but only if no other timer is currently running (that's the purpose of the IORef). If the button is released, the timer is stopped. The timer's callback function returns IO False to stop and destroy the timer object.
import Graphics.UI.Gtk
import Data.IORef
import Control.Monad(void, when)
action but idle = do
butDown <- get but toggleButtonActive
if butDown
then do
putStrLn "A"
writeIORef idle False
return True
else do
writeIORef idle True
return False
main = do
initGUI
window <- windowNew
but <-toggleButtonNewWithLabel "Write A"
idle <- newIORef True
on but toggled $ do
butDown <- get but toggleButtonActive
isIdle <- readIORef idle
when (butDown && isIdle) $ void $ timeoutAdd (action but idle) 1000
containerAdd window but
widgetShowAll window
on window objectDestroy mainQuit
mainGUI
The preferred way of registering signal callbacks is using "on". Also note that "on window objectDestroy mainQuit" correctly destroys the window and stops the Gtk main loop (you version didn't destroy the timers in GHCi, they kept running after calling "main" again).