Playing with Buttons, Labels and Placement Elements

A Window with a Single Button

This window contains a single button. The button fills the entire window. It is labelled "Click here". When it is clicked the first time, the button label changes to "Hi".

Archive file with this example: Bttn-Example001.tar.gz. Be sure to read the file README.txt for further instructions.

import "gtk3" Graphics.UI.Gtk

destroyEventHandler :: IO ()
destroyEventHandler =
  do mainQuit

buttonClickHandler :: Button  -> IO ()
buttonClickHandler bttn  =
  do
    buttonSetLabel bttn "Hi"

main :: IO ()
main = 
  do
    initGUI
    window <- windowNew
    button <- buttonNewWithLabel "Click here"
    set window [windowDefaultWidth := 250,
                windowDefaultHeight := 200,
                windowWindowPosition := WinPosCenter,
                containerChild := button]
    on button buttonActivated (buttonClickHandler button)
    on window objectDestroy destroyEventHandler
    widgetShow button
    widgetShow window
    mainGUI

To enable your program to process a button click, you have to do two things:

• Program a function that implements the reaction to the button click. This function is called a callback function, because it is called by the button to inform the program about some event.
• Associate the callback function with the buttonActivated-event of the button.

The button handler is not difficult to understand:

It assigns a new label text to the button.

The connection between the button and its event handler is established with the statement:

on button buttonActivated (buttonClickHandler button)

Note that

(buttonClickHandler button)

is a function call that returns a parameterless function with result type:

IO ()

This is due to currying.