[Lazarus] Event created by program logic rather than user action

[Anthony Walter]

Here's an example which calculates Pi forever in a thread. It's important
to note a few things:

1. Periodically check if the thread should stop by checking the Terminated
property inside Execute
2. If you want to communicate with the user interface, such as refreshing
the screen based on some calculations, use the Synchronize method to do so
3. Do not share data the thread is using in calculations with anything. If
you need access to the data in somewhere else, use Synchronize and make a
copy there.

unit Main;

{$mode delphi}

interface

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

{ TMainForm }

type
  TMainForm = class(TForm)
    OutputLabel: TLabel;
    procedure FormClose(Sender: TObject; var CloseAction: TCloseAction);
    procedure FormCreate(Sender: TObject);
  private
    FThread: TThread;
  end;

var
  MainForm: TMainForm;

implementation

{$R *.lfm}

{ TCalculatePiThread }

type
  TCalculatePiThread = class(TThread)
  private
    FPiDigits: string;
  protected
    procedure Update;
    procedure Execute; override;
  end;

procedure TCalculatePiThread.Update;
begin
  MainForm.OutputLabel.Caption := FPiDigits;
end;

procedure TCalculatePiThread.Execute;
var
  Data: array[0..2801] of Integer;
  I, K, B, D, C: Integer;
begin
  FreeOnTerminate := True;
  while not Terminated do
  begin
    C := 0;
    for I := Low(Data) to High(Data) do
      Data[I] := 2800;
    K := 2800;
    while K > 0 do
    begin
      K := K - 14;
      D := 0;
      I := K;
      while not Terminated do
      begin
        D := D  + Data[I] * 10000;
        B := 2 * I - 1;
        Data[I] := D mod B;
        D := D div  B;
        Dec(I);
        if I < 1 then
          Break;
        D := D * I;
      end;
      FPiDigits := Format('%.4d', [C + D div 10000]);
      Synchronize(Update);
      C := D mod 10000;
    end;
  end;
end;

{ TMainForm }

procedure TMainForm.FormCreate(Sender: TObject);
begin
  FThread := TCalculatePiThread.Create(False);
end;

procedure TMainForm.FormClose(Sender: TObject; var CloseAction:
TCloseAction);
begin
  FThread.Terminate;
end;

end.
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