GDI+ 橢圓繪製與滑鼠互動：DrawEllipse 拖曳移動與縮放

DrawEllipse 在指定矩形邊界內繪製橢圓。本範例示範完整的滑鼠互動邏輯：左鍵拖曳移動橢圓位置，Ctrl+左鍵拖曳調整寬高，實現基本的圖形編輯器操作模式。

核心互動邏輯

• OnMouseDown：記錄起始滑鼠座標，進入拖曳狀態
• OnMouseMove：根據 ssCtrl 判斷移動模式或縮放模式，累加偏移量並 Repaint
• OnMouseUp：結束拖曳狀態
• OnPaint：套用累積偏移量繪製橢圓

程式碼實作

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm1 = class(TForm)
    procedure FormPaint(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(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);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}
uses GDIPOBJ, GDIPAPI;

var
  x: Integer = 20;
  y: Integer = 20;
  w: Integer = 100;
  h: Integer = 50;
  f: Byte;
  a, b: Integer;
  p: TGPPen;

procedure TForm1.FormCreate(Sender: TObject);
begin
  p := TGPPen.Create(MakeColor(255, 0, 0), 2);
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  p.Free;
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  f := 1;
  a := X;
  b := Y;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
begin
  if (f = 0) then Exit;
  if ssCtrl in Shift then
  begin
    f := 2;
    Text := 'Ctrl 鍵被按下，滑鼠可調整大小';
  end
  else
    Text := '沒有按下 Ctrl 鍵，滑鼠可調整位置';
  a := X - a;
  b := Y - b;
  Repaint;
  a := X;
  b := Y;
end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  f := 0;
end;

procedure TForm1.FormPaint(Sender: TObject);
var
  g: TGPGraphics;
begin
  g := TGPGraphics.Create(Canvas.Handle);
  g.Clear(MakeColor(255, 255, 255));
  case f of
    1: begin Inc(x, a); Inc(y, b); end;  { 移動模式 }
    2: begin Inc(w, a); Inc(h, b); end;  { 縮放模式 }
  end;
  g.DrawEllipse(p, x, y, w, h);
  g.Free;
end;

end.

表單設計 (DFM)

object Form1: TForm1
  Left = 0
  Top = 0
  Caption = 'Form1'
  ClientHeight = 176
  ClientWidth = 337
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  Position = poDesktopCenter
  OnCreate = FormCreate
  OnDestroy = FormDestroy
  OnMouseDown = FormMouseDown
  OnMouseMove = FormMouseMove
  OnMouseUp = FormMouseUp
  OnPaint = FormPaint
  PixelsPerInch = 96
  TextHeight = 13
end

執行效果

關鍵技術點總結

• 畫筆 TGPPen 在 FormCreate 建立、FormDestroy 釋放，避免重複建構開銷
• 滑鼠移動時使用 Canvas.DrawFocusRect 或直接 Repaint 回饋
• 座標累加採用相對位移 (dx, dy) 而非絕對座標，避免抖動
• 此模式可擴展至矩形、多邊形、路徑等任意圖形編輯器