Advanced MFC Programming

Chapter 11. Sample: Simple Paint
Temporary rectangles are drawn by calling function CDC::DrawFocusRect(…). Because this function
uses XOR drawing mode, it is easy to erase the previous rectangle by simply calling the function twice.
When the left button is released, we erase the previous temporary rectangle if necessary, update the
tracker rectangle, and call function CWnd::InValidate() to let the tracker be updated (along with the client
window).
Because we must keep track of mouse cursor position after its left button is pressed down, the window
capture must be set when a new tracker is being created. Since we share the code implemented for dot and
line drawing here, there is no need to add extra code to set window capture for the client window here.
If Mouse Clicking Hits the Tracker
If mouse clicking hits the tracker (any of the resize buttons, or the middle of the tracker), we must
implement tracking to let the user resize or move the existing tracker. This can be easily implemented by
calling function CRectTracker::Track():
……
……
else
{
::ReleaseCapture();
m_trackerSel.Track(this, point);
Invalidate();
}
Because the window capture is set when a new tracker is being created (also, when a dot or a line is
being drawn), we must first release the capture before trackering mouse movement (Otherwise the tracker
will not be able to receive messages related to mouse moving events). There is no need for us to handle
WM_MOUSEMOVE and WM_LBUTTONUP messages here because after function CRectTracker::Track() is called,
mouse moving events will all be routed to the tracker. After this function exits, variable
CRectTracker::m_rect will be automatically updated to represent the new size and position of the tracker.
So after calling this function, we can update the client window directly to redraw the tracker.
11.5 Moving the Selected Image
Sample 11.5\GDI is based on sample 11.4\GDI. It is implemented with a new feature: when a portion
of the bitmap is selected by the tracker, the user can move the selected image by dragging the tracker to
another place; if the user resizes the tracker, the selected image will also be stretched.
Normalizing Tracker
Since the tracker rectangle is recorded in the zoomed image’s coordinate system, we must first convert
it back to the original bitmap’s own coordinate system (the image with 1:1 ratio) in order to find out which
part of the image is being selected. In the sample, function CGDIView::NormalizeTrackerRect(…) is added
for this purpose. In this function, the current image ratio is retrieved from the document, and the four points
of the tracker rectangle is divided by this ratio. The tracker can be created in two different ways. For
example, the user may click and hold the left mouse button and drag it right-and-downward; also, the
mouse may be dragged up-and-leftward. For the first situation, a normal rectangle will be formed, in which
case member CRectTracker.m_rect.left is always less than member CRectTracker.m_rect.right, and
CRectTracker.m_rect.top is less than CRectTracker::m_rect.bottom. However, in the second situation,
CRectTracker.m_rect.left and CRectTracker.m_rect.top are all grater than their corresponding
variables. So before using variable CRectTracker::m_rect, we must normalize the rectangle.
We can call function CRect::NormalizeRect() to normalize a rectangle implemented by class CRect.
In function CGDIView::NormailizeTrackerRect(…), before the four points of the tracker rectangle are
divided by the ratio, this function is called to first normalize the rectangle.
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