Paint event example for C++ Builder

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 Unit1.h private section:

RECT MyRect;

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.

void __fastcall TForm1::ToggleClick(TObject *Sender)
{
if (LEADRasterView1->Enabled == false)
{
LEADRasterView1->Enabled= true;
Directions->Text= "Click on the image for a splat";
Toggle->Caption= "Disable";
}
else
{
LEADRasterView1->Enabled = false;
Directions->Text= "Click does nothing";
Toggle->Caption= "Enable";
}
}

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.

void __fastcall TForm1::LEADRasterView1Click(TObject *Sender)
{
HWND hCtl;
RasterScaleModeConstants SavedMode;

// Use pixels for API calls
SavedMode = (RasterScaleModeConstants)LEADRasterView1->ScaleMode;
LEADRasterView1->ScaleMode = 3; //Pixels
// Use InvalidateRect to generate a paint message
MyRect.left = 0;
MyRect.top = 0;
MyRect.bottom = LEADRasterView1->ScaleHeight;
MyRect.right = LEADRasterView1->ScaleWidth;
hCtl = (HWND)LEADRasterView1->Window_;
InvalidateRect (hCtl, &MyRect, True);
// Set the ScaleMode back to the default
LEADRasterView1->ScaleMode = SavedMode;
}

7.	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.

void __fastcall TForm1::LEADRasterView1Paint(TObject *Sender, long hDC)
{
HDC LeadClientDC;
int xStart;
int yStart;
int xDelta;
int yDelta;
int xEnd;
int yEnd;
short i;

srand((unsigned) time(NULL));

// Create a device context for the control that the GDI functions can access
LeadClientDC = (HDC)LEADRasterView1->GetClientDC ();
// Draw random lines on the control. This overlay does not affect the bitmap.

xStart = (int)((rand() * (int)LEADRasterView1->ScaleWidth / RAND_MAX) + 1);
yStart = (int)((rand() * (int)LEADRasterView1->ScaleHeight / RAND_MAX) + 1);
MoveToEx(LeadClientDC, xStart, yStart, NULL);
for (i = 1; i <=50; i ++)
{
xDelta = (int)(81L * rand()/ RAND_MAX) - 40;
yDelta = (int)(81L * rand()/ RAND_MAX) - 40;
xEnd = xStart + xDelta;
yEnd = yStart + yDelta;
LineTo(LeadClientDC, xEnd, yEnd);
MoveToEx(LeadClientDC, xStart, yStart, NULL);
}
// Remove the lock from the device control
LEADRasterView1->ReleaseClientDC ();
}

8.	Run your program to test it.