Legal Use of CallBack Function?

[Gary Beene]

I was looking at the serial port sample from PowerBASIC Inc and noticed several callback functions in use:

CallBack Function Dialog_Callback () As Long
CallBack Function Send_Callback () As Long
CallBack Function SendFile_Callback() As Long
CallBack Function ReceiveFile_Callback () As Long
CallBack Function Quit_Callback () As Long

I was surprised because only one of them is associated with the dialog. The others are simply called like any other function. Also, the extra callback functions use CB.HNDL (but none of the other CB statements).

I didn't know you could use a callback not tied to a dialog/control, nor did I know that an "un-tied" callback would be given the CB values - but from which message? And suppose there were 2 dialogs in the apps, what would CB.hndl return?

Can someone clarify why/how a Callback is used in this fashion?

[Scott Slater]

The other ones are more than likely tied to controls. See Control Add Button in the help file. It should explain the optional Call keyword usage there as well as on other controls.

In a nutshell you can assign a callback to individual controls as well as the dialog.

[Gary Beene]

Yep that was it. I knew controls could have callbacks too, but I thought I saw code which called the Callback functions directly. I was wrong - my eyes told my brain something that wasn't true.

[Cliff Nichols]

No it is true.

For example

                    eStream.pfnCallback = CODEPTR(RichEditStreamInString)                                     'pointer to RichEdit callback procedure
                    gPos = 1
                    gTxt = BUILD$(RtfHeader, StaticRtfText, RtfEnd)
'if len(gTxt) > = 65535 then msgbox "Hold It, I am at 65535"
                    gPtr = STRPTR(gTxt)
                    SendMessage HwndRichEdit, %EM_STREAMIN, %SF_RTF, VARPTR(eStream)                          'Send Message to Window

and

'¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
' Rich Edit stream in callback - for streaming in string contents
'¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
     FUNCTION RichEditStreamInString (BYVAL dwCookie AS DWORD, BYVAL pbBuff AS BYTE PTR, BYVAL cb AS LONG, pcb AS LONG) AS DWORD
          pcb = MIN&(cb, LEN(gTxt) - (gPos - 1)) 'number of bytes to copy
          IF pcb > 0 THEN 'copy block from global string directly into Richedit's buffer.
                  CopyMemory pbBuff, (gPtr + gPos - 1), pcb 'could use POKE$ too, but this is a bit faster
                  gPos = gPos + pcb 'incr pos for next callback position.
         ELSE
              FUNCTION = %True    'Break the action
         END IF
     END FUNCTION

[Michael Mattias]

but I thought I saw code which called the Callback functions directly. I was wrong - my eyes told my brain something that wasn't true.

Well, you may well have seen that, and done carefully it will work OK, but I can't accept directly calling a callback function of any type (not just a PB "CALLBACK FUNCTION...") as good programming practice.

AFAIK, the PB syntax "CALLBACK FUNCTION FOO()" is just a kind of shorthand for....

FUNCTION FOO (BYVAL hWnd AS LONG, BYVAL uMSG AS LONG, _ 
                     BYVAL wParam AS LONG, BYVAL lParam AS LONG ) AS LONG

...with the additional proviso that the "CALLBACK FUNCTION FOO()" syntax must be used to describe the callback functions used in any DIALOG SHOW or CONTROL ADD statement.

MCM