Quick_Tut 101: What is an Event Handler - DDT style

[Brad D Byrne]

First, start with a Modeless Dialog Box

'EXAMPLE 5: A BASIC MODELESS DIALOG - DDT style
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#COMPILE EXE
#INCLUDE "WIN32API.INC"
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
CALLBACK FUNCTION My_DlgProc() AS LONG
    SELECT CASE CBMSG    '----------- start message handler ----------
    END SELECT     '-----------  end message handler  ----------
END FUNCTION ' (My_DlgProc)
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
FUNCTION PBMAIN
    DIM hDlg AS LONG, Count AS LONG
    DIALOG NEW %HWND_DESKTOP, "A BASIC MODELESS DIALOG - DDT style",100,100,250,100, _
                              %WS_OVERLAPPEDWINDOW, TO hDlg&
    DIALOG SHOW MODELESS hDlg&, CALL My_DlgProc&()
    DO		'----------- start message loop ----------
        DIALOG DOEVENTS TO Count&
    LOOP WHILE Count&	  '-----------  end message loop  ----------
END FUNCTION ' (PBMAIN)
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now, a lot of programmers new to windows want to place thier code
inside of the PBMAIN() function block, let's try it:

#COMPILE EXE
#INCLUDE "WIN32API.INC"
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
FUNCTION PBMAIN  
    DIM hDlg as LONG
    DIALOG NEW %HWND_DESKTOP, "Demo-ing improper placement of code",100,100,250,100, _
                              %WS_OVERLAPPEDWINDOW, TO hDlg
    CONTROL ADD LABEL,  hDlg, 101,"This is a label",10,10,170,11, %SS_SUNKEN or %SS_CENTER                          
    CONTROL ADD LABEL,  hDlg, 102,"This is a label",10,30,170,11, %SS_SUNKEN or %SS_CENTER
    DIALOG SHOW MODELESS hDlg&, 

   'a demonstration of improperly attempting to use code
   'outside of an Event Handler
		CONTROL SET TEXT hDlg,102,
		"This is a label"    
		SLEEP 1500
		CONTROL SET TEXT hDlg,102,
		"Changing the text "
		SLEEP 1500
		CONTROL SET TEXT hDlg,102,
		"This is the last change"
   '~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   
    DO     '----------- start message loop ----------
        DIALOG DOEVENTS TO Count&
    LOOP WHILE Count&   '----------- end message loop ----------
        
END FUNCTION ' (PBMAIN)

Now, as you run it, you will see the labels not completly drawn until the end
this is because in order for windows to support multi-tasking it needs to process
many messages (or commands) coming from different programs at the same time.
Windows does this by putting messages on a queue (a circular list) and processes
them in any order they are recieved, how-ever some messages must have priority
over others, so windows uses a procedure that manages the messages.

This procedure, (or callback function) is called an event handler.
let's try a proper version that uses the callback function:

#COMPILE EXE
#INCLUDE "WIN32API.INC"
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
CALLBACK FUNCTION My_DlgProc
STATIC My_TextFlg as LONG
  SELECT CASE CBMSG  '----------- start message handling ----------
	CASE %WM_INITDIALOG
	   SetTimer CBHNDL, 1, 1000, BYVAL 0
	   FUNCTION=%TRUE
		
	CASE %WM_TIMER
            SELECT CASE My_TextFlg
               CASE 0
		  CONTROL SET TEXT CBHNDL,102,
		  "This is a label"
		  INCR My_TextFlg
               CASE 1
		  CONTROL SET TEXT CBHNDL,102,
		  "Changing the text "
		  BEEP
                   INCR My_TextFlg
               CASE 2
		  CONTROL SET TEXT CBHNDL,102,
		  "This is the last change"
                    My_TextFlg = 0
             END SELECT

 	CASE %WM_DESTROY
            KillTimer CBHNDL, 1    
  END SELECT   '----------- end message handling ----------
END FUNCTION '(My_DlgProc)
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
FUNCTION PBMAIN  
    DIM hDlg as LONG
    DIALOG NEW %HWND_DESKTOP, "Demo: using the Event Handler",100,100,250,100, _
                              %WS_OVERLAPPEDWINDOW, TO hDlg
    CONTROL ADD LABEL,  hDlg, 101,"This is a label",10,10,170,11, %SS_SUNKEN or %SS_CENTER                          
    CONTROL ADD LABEL,  hDlg, 102,"This is a label",10,30,170,11, %SS_SUNKEN or %SS_CENTER
          
    DIALOG SHOW MODAL hDlg, CALL My_DlgProc&()
END FUNCTION ' (PBMAIN)

SIMPLE !!!

Also note that we didn't need to use a Modeless Dialog to process the messages
The Modal Dialog worked fine.

[Michael Mattias]

Also note that we didn't need to use a Modeless Dialog to process the messages
The Modal Dialog worked fine

It might be a little easier to understand if you said...

All dialogs require a message loop.

DIALOG SHOW MODAL statement creates its own message loop, and does not return until DIALOG END is executed

DIALOG SHOW MODELESS statement returns more or less immediately and requires the programmer supply a message loop.

MCM

[Chris Holbrook]

What happens to %WM_COMMAND messages if you don't give a callback function to DIALOG SHOW?

[Brad D Byrne]

What happens to %WM_COMMAND messages if you don't give a callback function to DIALOG SHOW?

it's been a while since I've programmed windows, but as I recall windows has it's own callback function in the background that processes all the messages that we don't provide code for in our callbacks.

Michael, I appreciate all the critique!

let's fix this demo up, I remember having a very difficult time understanding how it all worked when I first attempted windows, maybe very beginner tut's like this will help a lot!