Paint event example for Delphi

This example uses some Windows API functions to demonstrate the following:

$image\sqrblit.gif$ Paint event

$image\sqrblit.gif$ Click event

$image\sqrblit.gif$ Enabled property

$image\sqrblit.gif$ Window property

1.	Start with the project that you created in Loading and Displaying an Image.
2.	Add these variables for MyRect and MyPoint to the Main form’s private section:

MyRect: TRECT;
MyPoint: TPOINT;

3.	$image\btncmd.gif$ Add a button to your form and name it as follows:
	Name	Caption
	Toggle	Toggle
4.	Add an Edit box to the top of your form, and name it as follows:
	Name	Text
	Directions	Use the Toggle button to turn on or turn off Click-Splat.
5.	Code the Toggle button's Click procedure as the following, This code uses the Enabled property to toggle the LEAD RasterView control's ability to respond to click events.

procedure TForm1.ToggleClick(Sender: TObject);
begin
   if (LEADRasterView1.Enabled = False) Then
   begin
      LEADRasterView1.Enabled:= True;
       Directions.Text:= 'Click on the image for a splat';
       Toggle.Caption:= 'Disable';
   end
     else
   begin
      LEADRasterView1.Enabled := False;
       Directions.Text := 'Click does nothing';
       Toggle.Caption := 'Enable';
     end;
end;

6.	Code the LEADRasterView1 Click procedure as the following. When the Enabled property is true, this code uses a Windows API call to generate a paint event.

procedure TForm1.LEADRasterView1Click (Sender: TObject);
var
   hCtl: HWND;
   SavedMode: RasterScaleModeConstants;
begin
   // Use pixels for API calls
   SavedMode := LEADRasterView1.ScaleMode;
   LEADRasterView1.ScaleMode := 3; //Pixels
   // Use InvalidateRect to generate a paint message
   MyRect.Left := 0;
   MyRect.Top := 0;
   MyRect.Bottom := Trunc(LEADRasterView1.ScaleHeight);
   MyRect.Right := Trunc(LEADRasterView1.ScaleWidth);
   hCtl := LEADRasterView1.Window _;
   InvalidateRect (hCtl, @MyRect, True);
   // Set the ScaleMode back to the default
   LEADRasterView1.ScaleMode := SavedMode;
end;

8.	Code the LEADRasterView1 Paint procedure as the following. It uses Windows GDI functions to draw random lines on the control whenever there is a paint event.

procedure TForm1.LEADRasterView1Paint (Sender: TObject; hDC: Integer);
var
   LeadClientDC: Longint;
   xStart: longint;
   yStart: longint;
   xDelta: Longint;
   yDelta: Longint;
   xEnd: Longint;
   yEnd: Longint;
   nRet: Integer;
   sRet: Smallint;
   i: Longint;
begin
   MyPoint.x := 0;
   MyPoint.y := 0;
   // Create a device context for the control that the GDI functions can access
   LeadClientDC := LEADRasterView1.GetClientDC (nRet);
   // Draw random lines on the control. This overlay does not affect the bitmap.
   xStart := Longint(Trunc((LEADRasterView1.ScaleWidth)*Random+ 1));
   yStart := Longint(Trunc((LEADRasterView1.ScaleHeight)*Random+ 1));
   MoveToEx(LeadClientDC, xStart, yStart, @MyPoint);
   for i := 1 to 50 do
   begin
      xDelta := Longint(Trunc((81 * Random) - 40));
      yDelta := Longint(Trunc((81 * Random) - 40));
      xEnd := xStart + xDelta;
      yEnd := yStart + yDelta;
      LineTo(LeadClientDC, xEnd, yEnd);
      MoveToEx(LeadClientDC, xStart, yStart, @MyPoint);
   end;
   // Remove the lock from the device control
   LEADRasterView1.ReleaseClientDC (sRet);
end;

9.	Run your program to test it.