unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ComCtrls, ExtCtrls, thread2;

type
  TForm1 = class(TForm)
    TrackBar1: TTrackBar;
    TrackBar2: TTrackBar;
    TrackBar3: TTrackBar;
    CheckBox1: TCheckBox;
    CheckBox2: TCheckBox;
    CheckBox3: TCheckBox;
    PaintBox1: TPaintBox;
    PaintBox2: TPaintBox;
    PaintBox3: TPaintBox;
    procedure FormCreate(Sender: TObject);
    procedure TrackBar1Change(Sender: TObject);
    procedure CheckBox1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  Th1: TLineThread;
  Th2: TRectThread;
  Th3: TCircThread;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  Th1 := TLineThread.Create(PaintBox1, tpNormal);
  Th2 := TRectThread.Create(PaintBox2, tpNormal);
  Th3 := TCircThread.Create(PaintBox3, tpNormal);
end;

procedure TForm1.TrackBar1Change(Sender: TObject);
var
  tH: TThread;
begin
  if Sender is TTrackBar then
    with TTrackBar(Sender) do begin
      case Tag of
        1: Th := Th1;
        2: Th := Th2;
        3: Th := Th3;
      end;
      case Position of
        1: Th.Priority := tpIdle;
        2: Th.Priority := tpLowest;
        3: Th.Priority := tpLower;
        4: Th.Priority := tpNormal;
        5: Th.Priority := tpHigher;
        6: Th.Priority := tpHighest;
        7: Th.Priority := tpTimeCritical;
      end;
    end;
end;

procedure TForm1.CheckBox1Click(Sender: TObject);
var
  Th: TThread;
begin
  if Sender is TCheckBox then
    with TCheckBox(Sender) do begin
      case Tag of
        1: Th := Th1;
        2: Th := Th2;
        3: Th := Th3;
      end;
      if Th.Suspended then Th.Resume else Th.Suspend;
    end;
end;

end.

unit thread2;

interface

uses Classes, ExtCtrls, Windows;

type
  TPaintThread = class(TThread)
  private
    fX1, fX2, fY1, fY2, fColor: Integer;
    fBox: TPaintBox;
  protected
    procedure Execute; override;
    procedure Paint(x1, y1, x2, y2, Color: Integer);
    procedure DoPaint; virtual; abstract;
  public
    constructor Create(Box: TPaintBox; ThreadPriority: TThreadPriority);
  end;

  TLineThread = class(TPaintThread)
  protected
    procedure DoPaint; override;
  end;

  TRectThread = class(TPaintThread)
  protected
    procedure DoPaint; override;
  end;

  TCircThread = class(TPaintThread)
  protected
    procedure DoPaint; override;
  end;

implementation

uses Graphics;

constructor TPaintThread.Create(Box: TPaintBox; ThreadPriority: TThreadPriority);
begin
  inherited Create(false);
  Priority := ThreadPriority;
  fColor := 0;
  fBox := Box;
  fX1 := 0;
  fX2 := 0;
  fY1 := 0;
  fY2 := 0;
end;

procedure TPaintThread.Execute;
begin
  randomize;
  while true do
    Paint(Random(200), Random(200), Random(200), Random(200), Random($FFFF));
end;

procedure TPaintThread.Paint(x1, y1, x2, y2, Color: Integer);
begin
  fX1 := x1;
  fX2 := x2;
  fY1 := y1;
  fY2 := y2;
  fColor := Color;
  Synchronize(DoPaint);
end;

procedure TLineThread.DoPaint;
begin
  with fBox.Canvas do
  begin
    Pen.Color := fColor;
    MoveTo(fX1, fY1);
    LineTo(fX2, fY2);
  end;
end;

procedure TRectThread.DoPaint;
begin
  with fBox.Canvas do
  begin
    Brush.Style := bsClear;
    Pen.Color := fColor;
    Rectangle(fX1, fY1, fX2, fY2);
  end;
end;

procedure TCircThread.DoPaint;
begin
  with fBox.Canvas do
  begin
    Brush.Style := bsClear;
    Pen.Color := fColor;
    Ellipse(fX1, fY1, fX2, fY2);
  end;
end;

end.