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 ***************************************************************************
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  Abstract:
    Demo to show, how a Thread waits for another.
        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 WaitForUnit1;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, Forms, Controls, Graphics, Dialogs, Buttons,
  StdCtrls, LCLProc;

type

  { TBaseThread }

  TBaseThread = class(TThread)
  public
    procedure Log(const Msg: string; AppendLineEnd: boolean = true);
  end;

  { TThreadA }

  TThreadA = class(TBaseThread)
  public
    WaitForB: PRtlEvent;
    procedure Execute; override;
  end;

  { TThreadB }

  TThreadB = class(TBaseThread)
  private
    FCounter: integer;
  public
    procedure Execute; override;
    property Counter: integer read FCounter write FCounter;
  end;

  { TForm1 }

  TForm1 = class(TForm)
    Button1: TButton;
    Memo1: TMemo;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    ACriticalSection: TRTLCriticalSection;
    MsgText: string;
    procedure AddMessage;
  public
    ThreadA: TThreadA;
    ThreadB: TThreadB;
  end;

var
  Form1: TForm1; 

implementation

{$R *.lfm}

{ TForm1 }

procedure TForm1.Button1Click(Sender: TObject);
begin
  if ThreadA=nil then
    ThreadA:=TThreadA.Create(false);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  InitCriticalSection(ACriticalSection);
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  DoneCriticalsection(ACriticalSection);
end;

procedure TForm1.AddMessage;
begin
  Memo1.Lines.Text:=MsgText;
end;

{ TThreadA }

procedure TThreadA.Execute;
begin
  Form1.ThreadB:=TThreadB.Create(false);
  // create event
  WaitForB:=RTLEventCreate;
  while not Application.Terminated do begin
    Log('A: wait for B ...');
    // wait infinitely (until B wakes A)
    RtlEventWaitFor(WaitForB);
    Log('A: ThreadB.Counter='+IntToStr(Form1.ThreadB.Counter));
  end;
end;

{ TThreadB }

procedure TThreadB.Execute;
var
  i: Integer;
begin
  Counter:=0;
  while not Application.Terminated do begin
    Log('B: Working ...');
    for i:=1 to 5 do begin
      Sleep(300);
      Log('.....  ',false);
    end;
    Log('');
    Counter := Counter + 1;
    Log('B: Wake A');
    // wake A
    RtlEventSetEvent(Form1.ThreadA.WaitForB);
  end;
end;

{ TBaseThread }

procedure TBaseThread.Log(const Msg: string; AppendLineEnd: boolean);
var
  s: String;
begin
  EnterCriticalsection(Form1.ACriticalSection);
  s:=Msg;
  if AppendLineEnd then
    s:=s+LineEnding;
  dbgout(s);
  Form1.MsgText:=Form1.MsgText+s;
  Synchronize(@Form1.AddMessage);
  LeaveCriticalsection(Form1.ACriticalSection);
end;

end.