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  Abstract:
    Demo to show, how to start a thread and how synchronize with the main
    thread.
    Important: In most (all?) Unix like systems, the cthread unit must be added
               to the uses section of the .lpr file. Further, cmem is likely to
               be significently faster so add it as well. Due to how the units
               work a sensible order is cmem, cthreads and then perhaps cwstrings.
               But note that heaptrc does not work with cmem so comment it out
               while testing/debugging.
}
unit MainUnit;

{$mode objfpc}{$H+}

interface

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

type

  { TMyThread }

  TMyThread = class(TThread)
  private
    fStatusText: string;
    procedure ShowStatus;
  protected
    procedure Execute; override;
  public
    constructor Create(CreateSuspended: boolean);
  end;

  { TForm1 }

  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
  public
  end;

var
  Form1: TForm1; 

implementation

{$R *.lfm}

{ TForm1 }

procedure TForm1.FormCreate(Sender: TObject);
var
  MyThread : TMyThread;
begin
  MyThread := TMyThread.Create(True); // With the True parameter it doesn't start automatically
  if Assigned(MyThread.FatalException) then
    raise MyThread.FatalException;
      
  // Here the code initialises anything required before the threads starts executing

  MyThread.Start;
end;

{ TMyThread }

procedure TMyThread.ShowStatus;
// this method is only called by Synchronize(@ShowStatus) and therefore
// executed by the main thread
// The main thread can access GUI elements, for example Form1.Caption.
begin
  Form1.Caption := fStatusText;
end;

procedure TMyThread.Execute;
var
  newStatus : string;
begin
  fStatusText := 'TMyThread Starting ...';
  Synchronize(@Showstatus);
  fStatusText := 'TMyThread Running ...';
  while (not Terminated) and (true {any condition required}) do begin

    //here goes the code of the main thread loop
    newStatus:='TMyThread Time: '+FormatDateTime('YYYY-MM-DD HH:NN:SS',Now);
    
    if NewStatus <> fStatusText then begin
      fStatusText := newStatus;
      Synchronize(@Showstatus);
    end;
    sleep(50); // alternatively the thread may wait for an event. E.g., external I/O
  end;
end;

constructor TMyThread.Create(CreateSuspended: boolean);
begin
  FreeOnTerminate := True;
  inherited Create(CreateSuspended);
end;

end.