The simplicity and elegance of this program is compelling. (See Figure 1.) Everything is pretty
straightforward. When the left button goes down (
OnLButtonDown
), we capture the cursor (so
that we know if it leaves our window) and record the point at which the button was clicked. For
every mouse move event (
OnMouseMove
) we erase the current line and redraw it in the new posi-
tion. (This is done by drawing in XOR mode. If you don’t understand this, don’t worry about it.)
Then we record the new position. When the left button finally comes back up (
OnLButtonUp
)
we create an instance of a
GraphicLine
from the
GraphicFactory
and place it in the vector
that holds the list of lines. Upon reception of a PAINT event (OnPaint) we simply iterate through
the list of
GraphicObjects
telling each one to draw.
Document/View
The simplicity of this program masks some potential problems. For example, what if we wanted
to show two windows. One with the lines drawn as before, but another text window displaying a
scrolling list of lines in the following format:
Line((0,0),(1,1));
Line((5,2),(8,3));
Each time a new line was added to one window, it would appear on the other. Clearly we are not
currently set up to do that.
Or, what if we wanted to store all the created lines into a file and then read them back later? Pre-
sumably we could do this by adding the appropriate
Save
and
Store
methods to the
LineAp-
plicationWindow
class. However, why should those methods be in the same class with the
event functions such as
OnLButtonDown
? We would rather that the code that knew how to read
and write lines to a file was reusable separately from the code that knows how to create and dis-
play the lines.
This is an instance of a violation of the Open Closed Principle (OCP)
1
. By definition the window
class cannot be closed to gross changes in the way that the data is displayed. However, such
1. The Open Closed Principle, Robert C. Martin, C++ Report, January 1996
// save the current dc parameters that we are changing
int previousROP = dc.SetROP2(R2_XORPEN);
CPen* currentPen = dc.SelectObject(itsBluePen);
// draw the previous line
dc.MoveTo(itsFirstPoint);
dc.LineTo(itsSecondPoint);
// draw the new line
dc.MoveTo(itsFirstPoint);
dc.LineTo(point);
itsSecondPoint = point;
// reset previous parameters
dc.SetROP2(previousROP);
dc.SelectObject(currentPen);
}
return;
}
Listing 2: LineApplicationWindow.cpp (Continued)
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