How to pass a method as callback to a Windows API call?

Question

Hi,

I'd like to pass a method of a class as callback to a WinAPI function. Is this possible and if yes, how?

Example case for setting a timer:

TMyClass = class
public
  procedure TimerProc(Wnd:HWND; uMsg:DWORD; idEvent:PDWORD; dwTime:DWORD);
  procedure DoIt;
end;
[...]
procedure TMyClass.DoIt;
begin
  SetTimer(0, 0, 8, @TimerProc);  // <-???- that's what I want to do (last param)
end;

Thanks for your help!

Edit: The goal is to specify a method of this class as callback. No procedure outside the class.

Edit2: I appreciate all your help but as long as the method has no "TMyClass." in front of its name it is not what I am searching for. I used to do it this way but wondered if could stay fully in the object oriented world. Pointer magic welcome.

Answer 1

The TimerProc procedure should be a standard procedure, not a method pointer.

A method pointer is really a pair of pointers; the first stores the address of a method, and the second stores a reference to the object the method belongs to

Edit

This might be as much OOP as you are going to get it. All the nasty stuff is hidden from anyone using your TMyClass.

unit Unit2;

interface

type
  TMyClass = class
  private
    FTimerID: Integer;
    FPrivateValue: Boolean;
  public
    constructor Create;
    destructor Destroy; override;
    procedure DoIt;
  end;

implementation

uses
  Windows, Classes;

var
  ClassList: TList;

constructor TMyClass.Create;
begin
  inherited Create;
  ClassList.Add(Self);
end;

destructor TMyClass.Destroy;
var
  I: Integer;
begin
  I := ClassList.IndexOf(Self);
  if I <> -1 then
    ClassList.Delete(I);
  inherited;
end;

procedure TimerProc(Wnd:HWND; uMsg:DWORD; idEvent:PDWORD; dwTime:DWORD); stdcall;
var
  I: Integer;
  myClass: TMyClass;
begin
  for I := 0 to Pred(ClassList.Count) do
  begin
    myClass := TMyClass(ClassList[I]);
    if myClass.FTimerID = Integer(idEvent) then
      myClass.FPrivateValue := True;
  end;
end;

procedure TMyClass.DoIt;
begin
  FTimerID := SetTimer(0, 0, 8, @TimerProc);  // <-???- that's what I want to do (last param)
end;

initialization
  ClassList := TList.Create;

finalization
  ClassList.Free;

end.

Edit: (as mentioned by glob)

Don't forget to add the stdcall calling convention.

Answer 2

Madshi has a MethodToProcedure procedure. It's in the "madTools.pas" which is in the "madBasic" package. If you use it, you should change the calling convention for "TimerProc" to stdcall and DoIt procedure would become,

TMyClass = class
private
  Timer: UINT;
  SetTimerProc: Pointer;
[...]

procedure TMyClass.DoIt;
begin
  SetTimerProc := MethodToProcedure(Self, @TMyClass.TimerProc);
  Timer := SetTimer(0, 0, 8, SetTimerProc);
end;
// After "KillTimer(0, Timer)" is called call:
// VirtualFree(SetTimerProc, 0, MEM_RELEASE);

I've never tried but I think one could also try to duplicate the code in the "classses.MakeObjectInstance" for passing other procedure types than TWndMethod.

Answer 3

Response to your second edit:

If you want a reply that includes a pointer to a TMyClass instance, you may be out of luck. Fundamentally, the procedure Windows will call has a certain signature and is not an object method. You cannot directly work around that, not even with __closure or procedure of object magic, except as described below and in other answers. Why?

• Windows has no knowledge of it being an object method, and wants to call a procedure with a specific signature.

• The pointer is no longer a simple pointer - it has two halves, the object instance and the method. It needs to save the Self, as well as the method.

By the way, I don't understand what is wrong with a short dip outside the object-oriented world. Non-OO code is not necessarily dirty if used well.

Original, pre-your-edit answer:

It's not possible exactly as you are trying to do it. The method that SetTimer wants must exactly follow the TIMERPROC signature - see the MSDN documentation. This is a simple, non-object procedure.

However, the method TMyClass.DoIt is an object method. It actually has two parts: the object on which it is called, and the method itself. In Delphi, this is a "procedure of object" or a "closure" (read about procedural types here). So, the signatures are not compatible, and you cannot store the object instance, which you need in order to call an object method. (There are also calling convention problems - standard Delphi methods are implemented using the fastcall convention, whereas TIMERPROC specifies CALLBACK which, from memory, is a macro that expands to stdcall. Read more about calling conventions and especially fastcall.)

So, what do you do? You need to map your non-object-oriented callback into object-oriented code.

There are several ways, and the simplest is this:

If you only have one timer ever, then you know that when your timer callback is called it is that specific timer that fired. Save a method pointer in a variable that is of type procedure of object with the appropriate signature. See the Embarcadero documentation link above for more details. It will probably look like:

type TMyObjectProc = procedure of object;
var pfMyProc : TMyObjectProc;

Then, initialise pfMyProc to nil. In TMyClass.DoIt, set pfMyProc to @DoIt - that is, it is now pointing at the DoIt procedure in the context of that specific TMyClass instantiation. Your callback can then call that method.

(If you're interested, class variables that are of a procedural type like this are how event handlers are stored internally. The OnFoo properties of a VCL object are pointers to object procedures.)

Unfortunately this procedural architecture is not object-oriented, but it's how it has to be done.

Here's what some full code might look like (I'm not at a compiler, so it may not work as written, but it should be close):

type TMyObjectProc = procedure of object;
var pfMyProc : TMyObjectProc;

initialization
  pfMyProc = nil;

procedure MyTimerCallback(hWnd : HWND; uMsg : DWORD; idEvent : PDWORD; dwTime : DWORD); stdcall;
begin
  if Assigned(pfMyProc) then begin
    pfMyProc(); // Calls DoIt, for the object that set the timer
    pfMyProc = nil;
  end;
end;

procedure TMyClass.MyOOCallback;
begin
  // Handle your callback here
end;

procedure TMyClass.DoIt;
begin
  pfMyProc = @MyOOCallback;
  SetTimer(0, 0, 8, @ MyTimerCallback);
end;

Another way would be to take advantage of the fact your timer has a unique ID. Save a mapping between the timer ID and the the object. In the callback, convert from the ID to the pointer, and call the object's method.

Edit: I've noticed a comment to another answer suggesting using the address of your object as the timer ID. This will work, but is a potentially dangerous hack if you end up having two objects at the same address at different times, and you don't call KillTimer. I've used that method but don't personally like it - I think the extra bookkeeping of keeping a (timer ID, object pointer) map is better. It really comes down to personal style, though.

Answer 4

TMyClass = class
public
  procedure DoIt;
  procedure DoOnTimerViaMethod;
end;

var MyReceiverObject: TMyClass;

[...]

procedure TimerProc(Wnd:HWND; uMsg:DWORD; idEvent:PDWORD; dwTime:DWORD); stdcall:
begin
  if Assigned(MyReceiverObject) then 
    MyReceiverObject.DoOnTimerViaMethod;
end;

procedure TMyClass.DoIt;
begin
  MyReceiverObject := Self;
  SetTimer(0, 0, 8, @TimerProc);  // <-???- that's what I want to do (last param)
end;

Not perfect. Watch for the threads, variable overwriting etc. But it does the job.

Answer 5

Which version of Delphi are you using?

In recent ones you can use static class methods for this:

TMyClass = class
public
  class procedure TimerProc(Wnd:HWND; uMsg:DWORD; idEvent:PDWORD; dwTime:DWORD); stdcall; static;
  procedure DoIt;
end;
[...]
procedure TMyClass.DoIt;
begin
  SetTimer(0, 0, 8, @TimerProc);  
end;

Answer 6

I've used MakeObjectInstance a few times to do the same. Here's an article on the subject: How to use a VCL class member-function as a Win32 callback