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Pb with multimedia timer in an object

Posted on 1997-06-14
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Last Modified: 2010-04-04
What is wrong with this source code ?
I want to use a multimedia timer in an object (TCanal).

At the first event, instead of calling one time the function, TCanal.Lecture is called two time by the timer and cause access violation error : access violation at 0xa8fee0 read of adress 0xffffffff


//*******************************************************************************************
//TCANAL OBJECT
//*******************************************************************************************

TCanal = class(TPanel)
  private
    timer : MMRESULT;
    procedure Lecture(uID : UINT; uMsg : UINT; dwUser : DWORD; dw1 :
DWORD; dw2 : DWORD); pascal;
    procedure StartMesure;
    procedure StopMesure;
  public
  constructor Create(Owner: TComponent); override;
  end;

constructor TCanal.Create(Owner: TComponent);
begin
  inherited Create(Owner);              { Initialisation des parties héritées }
  Parent := TWinControl(Owner);
  StartMesure;
end;

//*******************************************************************************************
// Multimedia timer
//*******************************************************************************************


procedure TCanal.Lecture(uID : UINT; uMsg : UINT; dwUser : DWORD; dw1 :
DWORD; dw2 : DWORD);
begin
     beep;
end;

procedure TCanal.StartMesure;
begin
     timeBeginPeriod(1);
     timer := timeSetEvent(100, 0, @TCanal.Lecture, 0, TIME_PERIODIC );
end;

procedure TCanal.StopMesure;
begin
     timeKillEvent(timer);
     timeEndPeriod(1);
end;

//******************************************************************************************

Thanks in advance.

0
Comment
Question by:turnar
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4 Comments
 
LVL 1

Expert Comment

by:sleach
ID: 1337029
I can find nothing wrong with Canal.Lecture.  However, you should place the suspect code inside a try/except block and see if that fixes the problem.  
example:

procedure TCanal.Lecture(uID : UINT; uMsg : UINT; dwUser : DWORD; dw1 :
DWORD; dw2 : DWORD);
begin
try
beep;
except
{Do nothing}
end;
end;
0
 

Author Comment

by:turnar
ID: 1337030
It cause access violation error, even with the try/except block.
I get this answer in comp.lang.pascal.delphi.misc :

>>>>>>>>>>>>>>>>>>>>>

Lecture() cannot be an object method, for starters. The timer
procedure is used by the Win32 API, which knows nothing about
the Self-pointer passed to all methods.

Secondly, being a Win32 API, you have to use the 'stdcall' calling
convention instead of 'pascal'.

Thirdly (and least seriously), there is a problem with the
definitions in Windows.pas:

type
  TFNTimeCallBack = procedure(uTimerID, uMessage: UINT; dwUser, dw1,
dw2: DWORD) stdcall;

function timeSetEvent(uDelay, uResolution: UINT;
                      lpFunction: TFNTimeCallBack;
                      dwUser: DWORD; uFlags: UINT): MMRESULT stdcall;

(Basically, Delphi forgets the 'stdcall' in TFNTimeCallback. This
isn't fatal because you can work around it using '@')

>>>>>>>>>>>>>>>>>>>

So, how can I do it ? I need a multimedia timer every time a TCanal instance is created.
Thanks in advance.

Christophe.
0
 
LVL 1

Expert Comment

by:sleach
ID: 1337031
I thought that a try/except block might do the trick.  Im not sure what else might help, so grade me an F, and let others have a try.  Wish I could be of more help. ;)
0
 
LVL 3

Accepted Solution

by:
sperling earned 300 total points
ID: 1337032
Go to my homepage, http://www.info-pro.no/sperling and then download my TimerPool units (with source ofcourse). These implement a THiResTimer class, using a single multimedia timer to distribute timer events to multiple THiResTimer instances. The code should get you started if you won't use it directly.


Regards,

Erik.
0

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