如何实现绘图板中，选取区域的效果？？？？

Question

当时我用VC做的onmousedown记下坐标。
onmousemove拖拉鼠标到下一次左击记下另一个坐标，过程中不断重画（XOR）

Answer 1

var
  MDown:Boolean=False;
  SX,SY,EX,EY:Integer;
procedure TForm1.FormActivate(Sender: TObject);
begin
  Paintbox1.Canvas.Pen.Mode:=pmXor;
  Paintbox1.Canvas.Pen.Style:=psDot;
end;procedure TForm1.PaintBox1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  MDown:=True;
  SX:=X;
  SY:=Y;
  EX:=X;
  EY:=Y;
  PaintBox1.Canvas.Polyline([Point(SX,Sy),Point(SX,EY),Point(EX,EY),Point(EX,SY),Point(SX,Sy)]);
end;procedure TForm1.PaintBox1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  MDown:=False;end;procedure TForm1.PaintBox1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if MDown then
  begin
    PaintBox1.Canvas.PolyLine([Point(SX,Sy),Point(SX,EY),Point(EX,EY),Point(EX,SY),Point(SX,Sy)]);
    EX:=X;
    EY:=Y;
    PaintBox1.Canvas.Polyline([Point(SX,Sy),Point(SX,EY),Point(EX,EY),Point(EX,SY),Point(SX,Sy)]);
  end;
end;