How to draw a line using mouse drag? - delphi

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 needs to start and drag my mouse or simple I release mouse button and I draw the line.
procedure TForm1.Button1Click(Sender: TObject);
var
x0, y0, x1, y1: Integer;
begin
x0 := StrToInt(Edit1.Text);
y0 := StrToInt(Edit2.Text);
x1 := StrToInt(Edit3.Text);
y1 := StrToInt(Edit4.Text);
Brezenhems(x0 , Y0 , X1 , Y1);
end;
I hope that someone helps me
Thanks

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.

Answer 2

Another technique that can be used, without the need to create a bitmap, is using the Pen.Mode property. Something like this:
TForm2 = class(TForm)
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);
private
PO,LP: TPoint;
draw: boolean;
public
{ Public declarations }
end;
procedure TForm2.FormMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
PO.X:= X;
PO.Y:= Y;
LP.X:= X;
LP.Y:= Y;
draw:= true;
Canvas.Pen.Mode:= pmNotXor;
end;
procedure TForm2.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
Y: Integer);
begin
if draw then
begin
if (LP.X <> PO.X) or (LP.Y <> PO.Y) then
begin
Canvas.MoveTo(PO.X,PO.Y);
Canvas.LineTo(LP.X,LP.Y);
end;
LP.X:= X;
LP.Y:= Y;
Canvas.MoveTo(PO.X,PO.Y);
Canvas.LineTo(LP.X,LP.Y);
end;
end;
procedure TForm2.FormMouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
if draw then draw:= false;
end;