Now that you’ve had an introduction to documents and views, a little hands-on experience should help you better understand how these classes work. In the steps that follow, you build the Rectangles application, which demonstrates the manipulation of documents and views. When you first run this application, it will draw an empty window. Wherever you click in the window, a small rectangle will be drawn. You can resize the window, or minimize and restore it, and the rectangles will be redrawn at all the coordinates where you clicked, because Rectangles keeps an array of coordinate points in the document and uses that array in the view.

First, use AppWizard to create the basic files for the Rectangles program, selecting the options listed in the following table. When you’re done, the New Project Information dialog box appears; it should look like Figure 4.1. Click the OK button to create the project files.

FIG. 4.1 When you create an SDI application with AppWizard, the project information summary confirms your settings.

Now that you have a starter application, it’s time to add code to the document and view classes in order to create an application that actually does something. This application will draw many rectangles in the view and save the coordinates of the rectangles in the document.

Follow these steps to add the code that modifies the document class to handle the application’s data, which is an array of CPoint objects that determine where rectangles should be drawn in the view window:

The m_points[] array holds the locations of rectangles displayed in the view window. The m_pointIndex data member holds the index of the next empty element of the array.

Now you need to get these variables initialized to appropriate values and then use them to draw the view. MFC applications that use the document/view paradigm initialize document data in a function called OnNewDocument(), which is called automatically when the application first runs and whenever the user chooses File, New.

The list of member variables and functions of CRecsDoc should still be displayed in ClassView. Double-click OnNewDocument() in that list to edit the code. Using Listing 4.3 as a guide, remove the comments left by AppWizard and initialize m_pointIndex to zero.

There is no need to initialize the array of points because the index into the array will be used to ensure no code tries to use an uninitialized element of the array. At this point your modifications to the document class are complete. In order to focus on the way documents and views work together, you will not be making those changes to the recs application.

Now turn your attention to the view class. It will use the document data to draw rectangles onscreen. A full discussion of the way that drawing works must wait for next Unit. For now it is enough to know that the OnDraw() function of your view class does the drawing. Expand the CRecsView class in ClassView and double-click OnDraw(). Using Listing 4.4 as a guide, remove the comments left by AppWizard and add code to draw a rectangle at each point in the array.

Your modifications to the starter application generated by AppWizard are almost complete. You have added member variables to the document, initialized those variables in the document’s OnNewDocument() function, and used those variables in the view’s OnDraw() function. All that remains is to enable the user to add points to the array. As discussed in previous Unit "Messages and Commands," you catch the mouse message with ClassWizard and then add code to the message handler. Follow these steps:

The new OnLButtonDown() adds a point to the document’s point array each time the user clicks the left mouse button over the view window. It increments m_pointIndex so that the next click goes into the point on the array after this one.

The call to SetModifiedFlag() marks this document as modified, or "dirty." MFC automatically prompts the user to save any dirty files on exit. Any code you write that changes any document variables should call SetModifiedFlag().

Finally, the call to Invalidate() causes MFC to call the OnDraw() function, where the window’s display is redrawn with the new data. Invalidate() takes a single parameter (with the default value TRUE) that determines if the background is erased before calling OnDraw(). On rare occasions you may choose to call Invalidate(FALSE) so that OnDraw() draws over whatever was already onscreen.

Finally, a call to the base class OnLButtonDown() takes care of the rest of the work involved in handling a mouse click.

You’ve now finished the complete application. Click the toolbar’s Build button, or choose Build, Build from the menu bar, to compile and link the application. After you have the Rectangles application compiled and linked, run it by choosing Build, Execute. When you do, you see the application’s main window. Place your mouse pointer over the window’s client area and click. A rectangle appears. Go ahead and keep clicking. You can place up to 100 rectangles in the window (see Figure 4.2).

FIG. 4.2 The Rectangles application draws rectangles wherever you click.