unit Unit2;

{$mode objfpc}{$H+}

interface

uses
    Classes, SysUtils, Forms, Controls, Graphics, Dialogs, syncobjs, Windows;

const
    WM_USER = 1024;

type

    { TMyThread }

    TMyThread = class(TThread)
    private
        FEvent: TEvent;
        FOwner: TForm;
        FSomePar: integer;
    protected
        procedure Execute; override;
    public
        procedure AfterConstruction; override;
        destructor Destroy; override;
        property Owner: TForm read FOwner write FOwner;
        property SomePar: integer read FSomePar write FSomePar;
        property Event: TEvent read FEvent;
    end;

type

    { TForm2 }

    TForm2 = class(TForm)
        procedure FormCreate(Sender: TObject);
    private
        procedure WMTest(var Msg: TMessage); message WM_USER+1;
    public

    end;

var
    Form2: TForm2;

implementation

{$R *.lfm}

{ TForm2 }

procedure TForm2.FormCreate(Sender: TObject);
var t: TMyThread;
begin
    t := TMyThread.Create(false);
    t.Owner := Self;
    t.SomePar := 10;
    t.Event.SetEvent;
end;

procedure TForm2.WMTest(var Msg: TMessage);
begin
    // тут нам что-то сообщили
    Caption := 'Final!';
end;

{ TMyThread }

procedure TMyThread.Execute;
begin
    while not Terminated do begin
        if not Terminated then FEvent.WaitFor(INFINITE);
        if not Terminated then begin
            if SomePar = 10 then begin
                // тут работу работуем
                Sleep(1000);
                if Assigned(FOwner) then PostMessage(FOwner.Handle, WM_USER+1, 0, 0);
            end;
        end;
    end;
end;

procedure TMyThread.AfterConstruction;
begin
    inherited AfterConstruction;
    FEvent := TEvent.Create(nil, false, false, 'MyEvent');
end;

destructor TMyThread.Destroy;
begin
    Terminate;
    FEvent.SetEvent;
    Sleep(100);
    FEvent.Free;
    inherited Destroy;
end;

end.