gui_setnulleventhandler

#GUI.SetNullEventHandler

##Syntax GUI.SetNullEventHandler (nullHandler : procedure x ())

##Description Sets the new null event handler. The nullHandler parameter is the name of a procedure that is called every time GUI.ProcessEvent is called and there are no mouse button presses or keystrokes to be processed.

This is used by programs that need to call subprograms often, but do not wish to interrupt the action of user widgets.

##Example The following program has a Quit button. When no widgets are being processed, a clock in the corner is updated.

    import GUI
    View.Set ("graphics:220;50")
    
    var oldTime : string := ""
    var button : int
    
    procedure NullHandler
        var newTime : string := Time.Date
        newTime := newTime (11 .. *)
        if newTime not= oldTime then
            Text.Locate (maxrow, maxcol  9)
            put newTime ..
            oldTime := newTime
        end if
    end NullHandler
    
    GUI.SetNullEventHandler (NullHandler)
    button := GUI.CreateButton (10, 5, 100, "Quit", GUI.Quit)
    
    loop
        exit when GUI.ProcessEvent
    end loop

##Status Exported qualified.

This means that you can only call the function by calling GUI.SetNullEventHandler, not by calling SetNullEventHandler.

##See also gui_processevent.html.