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

Question

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          


        

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.