How to draw a line using mouse drag?

Question

I need to draw a line in delphi using the cursor, I already have created the line code, but I can\'t get what to do next? How can do that, I push the mouse, when the line ne

Answer 1

Something like this:

unit Unit4;

interface

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

type
  TForm4 = class(TForm)
    procedure FormResize(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormPaint(Sender: TObject);
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
  private
    { Private declarations }
    FStartPoint, FEndPoint: TPoint;
    FDrawingLine: boolean;
    bm: TBitmap;
    procedure AddLineToCanvas;
    procedure SwapBuffers;
  public
    { Public declarations }
  end;

var
  Form4: TForm4;

implementation

{$R *.dfm}

procedure TForm4.FormCreate(Sender: TObject);
begin
  bm := TBitmap.Create;
  FDrawingLine := false;
end;

procedure TForm4.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FStartPoint := Point(X, Y);
  FDrawingLine := true;
end;

procedure TForm4.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if FDrawingLine then
  begin
    SwapBuffers;
    Canvas.MoveTo(FStartPoint.X, FStartPoint.Y);
    Canvas.LineTo(X, Y);
  end;
end;

procedure TForm4.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FDrawingLine := false;
  FEndPoint := Point(X, Y);
  AddLineToCanvas;
  SwapBuffers;
end;

procedure TForm4.AddLineToCanvas;
begin
  bm.Canvas.MoveTo(FStartPoint.X, FStartPoint.Y);
  bm.Canvas.LineTo(FEndPoint.X, FEndPoint.Y);
end;

procedure TForm4.FormPaint(Sender: TObject);
begin
  SwapBuffers;
end;

procedure TForm4.SwapBuffers;
begin
  BitBlt(Canvas.Handle, 0, 0, ClientWidth, ClientHeight,
    bm.Canvas.Handle, 0, 0, SRCCOPY);
end;

procedure TForm4.FormResize(Sender: TObject);
begin
  bm.SetSize(ClientWidth, ClientHeight);
end;

end.

Compiled sample EXE

Notice that this method is simple and robust, but not optimal in terms of performance. This will likely be an issue if you try to run this on a Windows 3.1-era computer.