delphi 根据区域剪切画板

unit Unit1;
interface
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
type
  TForm1 = class(TForm)
    CheckBox1: TCheckBox;
    procedure FormCreate(Sender: TObject);
    procedure FormPaint(Sender: TObject);
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure CheckBox1Click(Sender: TObject);
  end;
var
  Form1: TForm1;
implementation
{$R *.dfm}
uses GDIPOBJ, GDIPAPI;
var
  rt: TRect;
  flag: Boolean;
  x1,y1: Integer;
procedure TForm1.FormCreate(Sender: TObject);
begin
  Self.Color := clWhite;
  rt := ClientRect;
  InflateRect(rt, -ClientWidth div 3, -ClientHeight div 3);
  CheckBox1.Caption := '剪切区域';
  CheckBox1.Checked := True;
  DoubleBuffered := True;
end;
procedure TForm1.FormPaint(Sender: TObject);
var
  g : TGPGraphics;
  b: TGPSolidBrush;
  rgn: TGPRegion;
  str: string;
  font: TGPFont;
  RectF: TGPRectF;
  i: Integer;
begin
  RectF := MakeRect(0.0, 0, ClientWidth+10, ClientHeight);
  str := '';
  for i := 0 to 800 do str := str + IntToStr(Random(2));
  g := TGPGraphics.Create(Canvas.Handle);
  b := TGPSolidBrush.Create(aclBlue);
  rgn := TGPRegion.Create(MakeRect(rt));
  if CheckBox1.Checked then g.SetClip(rgn); {用区域剪切画板}
  font := TGPFont.Create(Canvas.Handle);
  g.DrawString(str, -1, font, RectF, nil, b);
  rgn.Free;
  font.Free;
  b.Free;
  g.Free;
end;
procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if PtInRect(rt, Point(X,Y)) then
  begin
    x1 := X;
    y1 := Y;
    flag := True;
  end;
end;
procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if not flag then Exit;
  OffsetRect(rt, X-x1, Y-y1);
  x1 := X;
  y1 := Y;
  Repaint;
end;
procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  flag := False;
end;
procedure TForm1.CheckBox1Click(Sender: TObject);
begin
  Repaint;
end;
end.