GDI+ 進階教學：使用 SetDashPattern 自訂畫筆虛線樣式

在 GDI+ 繪圖中，TGPPen 類別提供了豐富的線條樣式控制。除了預設的虛線樣式外，開發者可以透過 SetDashPattern 方法定義完全自訂的虛線模式。此方法接受一個單精度浮點數陣列，陣列元素依序代表「線段長度」與「間隔長度」，陣列大小理論上無上限，可實現極其複雜的線條效果。

核心概念

SetDashPattern 方法的參數是一個 Single 型別陣列，其元素交替定義繪製長度與跳過長度。例如，陣列 [5, 3, 2, 4] 表示：繪製 5 單位線段、跳過 3 單位、繪製 2 單位線段、跳過 4 單位，然後循環重複。本範例使用六個元素的陣列，構成三個線段與三個間隔的複合虛線模式。

程式碼實作

以下完整範例示範如何使用六個 TTrackBar 控制項即時調整虛線各段落長度，並在表單繪製事件中套用至直線與橢圓繪製：

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    Panel1: TPanel;
    TrackBar1: TTrackBar;
    TrackBar2: TTrackBar;
    TrackBar3: TTrackBar;
    TrackBar4: TTrackBar;
    TrackBar5: TTrackBar;
    TrackBar6: TTrackBar;
    procedure FormCreate(Sender: TObject);
    procedure FormPaint(Sender: TObject);
    procedure TrackBar1Change(Sender: TObject);
    procedure TrackBar2Change(Sender: TObject);
    procedure TrackBar3Change(Sender: TObject);
    procedure TrackBar4Change(Sender: TObject);
    procedure TrackBar5Change(Sender: TObject);
    procedure TrackBar6Change(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

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

var
  DashArr: array[0..5] of Single;

procedure TForm1.FormCreate(Sender: TObject);
const
  n = 100;
begin
  TrackBar1.Max := n;
  TrackBar2.Max := n;
  TrackBar3.Max := n;
  TrackBar4.Max := n;
  TrackBar5.Max := n;
  TrackBar6.Max := n;
  TrackBar1.Position := n div 2;
  TrackBar2.Position := n div 2;
  TrackBar3.Position := n div 2;
  TrackBar4.Position := n div 2;
  TrackBar5.Position := n div 2;
  TrackBar6.Position := n div 2;
end;

procedure TForm1.FormPaint(Sender: TObject);
var
  g: TGPGraphics;
  p: TGPPen;
  rect: TRect;
begin
  g := TGPGraphics.Create(Canvas.Handle);
  p := TGPPen.Create(aclRed, 4);
  DashArr[0] := TrackBar1.Position / 10;
  DashArr[1] := TrackBar2.Position / 10;
  DashArr[2] := TrackBar3.Position / 10;
  DashArr[3] := TrackBar4.Position / 10;
  DashArr[4] := TrackBar5.Position / 10;
  DashArr[5] := TrackBar6.Position / 10;
  p.SetDashPattern(@DashArr, Length(DashArr)); { 設定自訂虛線樣式 }
  g.DrawLine(p, 0, 0, ClientWidth - Panel1.Width, ClientHeight);
  rect := Bounds(0, 0, ClientWidth - Panel1.Width, ClientHeight);
  InflateRect(rect, -30, -30);
  g.DrawEllipse(p, MakeRect(rect));
  p.Free;
  g.Free;
end;

procedure TForm1.TrackBar1Change(Sender: TObject);
begin
  Repaint;
end;

procedure TForm1.TrackBar2Change(Sender: TObject);
begin
  Repaint;
end;

procedure TForm1.TrackBar3Change(Sender: TObject);
begin
  Repaint;
end;

procedure TForm1.TrackBar4Change(Sender: TObject);
begin
  Repaint;
end;

procedure TForm1.TrackBar5Change(Sender: TObject);
begin
  Repaint;
end;

procedure TForm1.TrackBar6Change(Sender: TObject);
begin
  Repaint;
end;

end.

表單設計 (DFM)

object Form1: TForm1
  Left = 0
  Top = 0
  Caption = 'Form1'
  ClientHeight = 153
  ClientWidth = 306
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  Position = poDesktopCenter
  OnCreate = FormCreate
  OnPaint = FormPaint
  PixelsPerInch = 96
  TextHeight = 13
  object Panel1: TPanel
    Left = 207
    Top = 0
    Width = 99
    Height = 153
    Align = alRight
    TabOrder = 0
    ExplicitLeft = 200
    ExplicitHeight = 205
    object TrackBar1: TTrackBar
      Left = 0
      Top = 8
      Width = 97
      Height = 20
      ShowSelRange = False
      TabOrder = 0
      OnChange = TrackBar1Change
    end
    object TrackBar2: TTrackBar
      Left = 0
      Top = 31
      Width = 97
      Height = 20
      ShowSelRange = False
      TabOrder = 1
      OnChange = TrackBar2Change
    end
    object TrackBar3: TTrackBar
      Left = 0
      Top = 55
      Width = 97
      Height = 20
      ShowSelRange = False
      TabOrder = 2
      OnChange = TrackBar3Change
    end
    object TrackBar4: TTrackBar
      Left = 0
      Top = 79
      Width = 97
      Height = 20
      ShowSelRange = False
      TabOrder = 3
      OnChange = TrackBar4Change
    end
    object TrackBar5: TTrackBar
      Left = 0
      Top = 103
      Width = 97
      Height = 20
      ShowSelRange = False
      TabOrder = 4
      OnChange = TrackBar5Change
    end
    object TrackBar6: TTrackBar
      Left = 0
      Top = 127
      Width = 97
      Height = 20
      ShowSelRange = False
      TabOrder = 5
      OnChange = TrackBar6Change
    end
  end
end

執行效果

透過拖曳六個軌跡條，即可即時觀察虛線各段落長度變化對線條外觀的影響，直觀理解 SetDashPattern 陣列元素與視覺效果的對應關係。