Visual C++/MFC Tutorial - Lesson 7: Data Viewer

Lesson 7: Data Viewer

I have decided to make this example a data viewing application that takes a text file of data, reads it in, and then displays it. If that isn't enough, we are then going to use the timer to animate the data. Let's first assume that we are doing an experiment tracking the random motion of a drunken bug on a table. Every second we measure its distance from two adjacent sides of the table. These are what we will call the bug's x,y coordinates. Our data file looks like this:

Go cut and paste this to a file named BugPosition.dat if you want to follow along. We first fire up Visual C++ Developers Studio and create a new project. In this case I called the project 'BugTracks'. For the app wizard options select SDI application. Keep the default settings for the rest of the choices, EXCEPT deselect printing preview and docking toolbar.

struct SBugData 
{ 
 float x; 
 float y; 
}; 

Also I want to use one of microsoft's template classes. Templates are C++ things that allow you to write a function for an arbitrary data type. The one I like to use is the CArray class. Include the afxtempl.h before your structure declaration:

#include <afxtempl.h>

and then in a public area of your class declare a CArray template class as follows:

CArray <SBugData, SBugData> m_BugDataArray; 

Yea, it looks funny, but that is the way you want to type it. This is sort of like declaring a data type of CArray but we have to tell the template class what type of data we want to store in the array. That is done between the brackets <,>. The first thing in the brackets is the type of data we want to put in the array. The second thing is what we will pass to the CArray class when we want to add a new element. Since we have a list of data points, it is obvious that we want to have an array of SBugData. The second parameter is also SBugData meaning we will just pass the data to the array. (Alternatively we could have passed a 'reference' to the data, but that is another lesson).

Let's go to the .cpp file for the document and add the code now. Expand the CBugTracksDoc in the ClassView. You should see the member functions for the document. Double click on OnNewDocument(). You will jump to the function in the .cpp file. This function is called every time a new document (file) is opened. All we want to do here is to clear out the array so it will be ready for the new data. Where you see the //TODO comment, add this line of code:

m_BugDataArray.RemoveAll(); 

Now to fill up the array, jump to the Serialize() function. This is a function called when a new file is opened or saved. Instead of the good old FILE pointers you use in C with fopen, we are going to use microsoft's CArchive class. You will notice that a CArchive is passed (by reference) to the Serialize() function. This class has the same functionality as we get with the fread and fwrite functions.

void CBugTracksDoc::Serialize(CArchive& ar) 
{ 
 // if not storing the data, read it 
 if (!ar.IsStoring()) 
 { 
  SBugData Data; 
  CString strOneLine; 

  // read data in, one line at a time 
  while(ar.ReadString(strOneLine)) 
  { 
   // convert the text to floats 
   sscanf(strOneLine,"%g %g\n",&Data.x,&Data.y); 

   // add the data to the array 
   m_BugDataArray.Add(Data); 
  } 
 } 
} 

For the very basic display, we just need to add some code to draw the data. Go to the function OnDraw() in the view class CBugTracksView. This is the function that is called every time the window needs refreshed. All drawing is done through the CDC class. The CDC class has several drawing functions, here we will only use the MoveTo and LineTo calls.

void CBugTracksView::OnDraw(CDC* pDC) 
{ 
 // get a pointer to the document class 
 CBugTracksDoc* pDoc = GetDocument(); 

 // get the total number of data points 
 int N=pDoc->m_BugDataArray.GetSize(); 

 // draw all of the connecting lines 
 for(int i=0; i < N-2; i++) 
 { 
  pDC->MoveTo(pDoc->m_BugDataArray[i].x,   pDoc->m_BugDataArray[i].y); 
  pDC->LineTo(pDoc->m_BugDataArray[i+1].x, pDoc->m_BugDataArray[i+1].y); 
 } 
} 

Well that is it! Compile and run the program. You will get a few warnings since our data is float, but screen coordinates are int, but that is harmless in this case. Once the program is running, go to File, Open and select the data file we created above. It should display the track in the lower-middle part of the window. We could call it quits here, but let's add a couple of more features.

First, I hate the File, Open menu. Lets make our application accept files that are dropped on the main window. Go to the InitInstance() function in our CWinApp class CBugTracksApp. Near the end of the function add this line:

// Enable drag/drop open 
m_pMainWnd->DragAcceptFiles(); 

Now let's take advantage of the status bar and put some useful text in it. The status bar is managed by the CStatusBar class, which is a protected member of CMainFrame. This means that we can't touch it from other classes. We can either move its declaration to a public part of the class or just a public member function to CMainFrame to change the status bar text. We will do the later. Right click on CMainFrame in the class view and select 'Add Member Function'. A dialog will pop up to help add the member function. Type in "void" (without the quotes) for the function type -- this is the return value of the function, and type in "ChangeStatusText(LPCTSTR text)" as the function declaration. Make sure that the 'access' is set to public. Press OK. This will automagically add the declaration to the .h file and a blank function to the .cpp file of CMainFrame. The LPCTSTR is one of many microsoft defines for data types. We could have alternately have typed "ChangeStatusText(const char *text)". LPCTSTR stands for Long Pointer to a Constant T STRing. A T-string is just a string that will work on computers with different character sets (like Japanese). On computers in the US, a T-string is just the same as a char *.

Jump to the new function in the CMainFrame .cpp file and add the code to change the text on the status bar. To do this we'll just use the CWnd function SetWindowText. CStatusBar is derived from CWnd so we can always use any of the CWnd functions with it. A hint on how to find out about all of these strange new functions... use the help and look at the 'class members' for the class, and then look at the class members for all of the base classes from which it was derived. Your function should now look like this:

void CMainFrame::ChangeStatusText(LPCTSTR text) 
{ 
 m_wndStatusBar.SetWindowText(text); 
} 

We have to call this function from somewhere, and I'll do it from the document. Go to the 'File View' which is the view I use most. Under 'Source Files' double-click on your document file "BugTracksDoc.cpp". Go to the top of that file and include the header file for CMainFrame right after the rest of the includes so that we can access the new function we just made.

#include "MainFrm.cpp" 

Next go to our Serialize() function and modify the reading code to spit some text out to the status bar. We first get a pointer to the main window, which is the CMainFrame window in SDI applications. Since the function AfxGetMainWnd() returns a CWnd*, we cast it to a CMainFrame*. Then we use the CString's Format function and CArray's GetSize function to create a text string for the status bar.

void CBugTracksDoc::Serialize(CArchive& ar) 
{ 
 if (!ar.IsStoring()) 
 { 
  SBugData data; 
  CString line; 
  CString strStatus; 

  // get a pointer to the main window 
  // (which is the mainframe for SDI applications) 
  CMainFrame *pMain = (CMainFrame*) AfxGetMainWnd(); 

  while(ar.ReadString(line)) 
  { 
   sscanf(line,"%g %g\n",&data.x,&data.y); 

   // tell the user your reading points 
   strStatus.Format("Reading point %d",m_BugDataArray.GetSize()); 
   pMain->ChangeStatusText(strStatus); 

   m_BugDataArray.Add(data); 
  } 

  // tell the user the total number of points 
  strStatus.Format("Loaded %d points.", m_BugDataArray.GetSize()); 
  pMain->ChangeStatusText(strStatus); 
 } 
} 

If you run the app, you'll notice all of the default menu items. We don't need most of these. Let's clean up the menus and add an item for animating the bug track which we'll code later.

Go to the Resource View. Under the Menu resource, double click on IDR_MAINFRAME. This is the menu resource for the main window. In the edit window, click on File. Then delete the menu entries for New, Save, and Save As. Also delete the main menu headings for Edit and View. Next, go to the empty box at the end of the menu and Add a new heading called 'Track' by selecting the empty box and
typing 'Track'. Drag the Track menu heading so that it is between File and Help. Click on the Track menu and then click on the empty sub menu box. Type in '&Animate\tAlt-A'. The & underlines the 'A' in Animate so that it is the menu Hot Key. The \t is just the scan code for a tab and the Alt-A will be our hot key to start the animation. For the ID, type in `ID_TRACK_ANIMATE', though this will be filled in automatically if you ever forget.

In order to make Alt-A our hot key, go to the Accelerator resources and double-click on IDR_MAINFRAME. In the edit window, double-click on the empty box at the end of the list. From the drop list for the ID, select the ID of your new menu item (ID_TRACK_ANIMATE). Press the `Next Key Typed' button and then press Alt-A. Hit enter to close the dialog.

Before we are done with resources, you should modify the icons to something more suitable for this app. I'm sure you can figure out how to do this. The only hints here are to make user and modify the 32x32 sized icon AND the 16x16 sized icon. If you want part of the icon to be transparent, use that greenish color with the two borders around it on the color palette.

Now we can get back to coding. It's time to add fancier drawing and animating. We will animate the bug track by drawing more and more segments of the path in red as time increases. The rest of the path will be drawn in black.

In order to keep track of the last segment in the path that is to be drawn in red, we have to add a member variable to our document. Go to the Class View, right click on the document class, and select Add Member Variable. Type in 'int' as the data type and 'm_nBugPosition' as the variable name. Make sure that it is public and press OK.

Jump to the OnNewDocument() in the document class. Add this line to initialize the new variable to -1. We will use the value -1 to designate that the track is not being animated.

m_nBugPosition = -1; 

Next let's add the message handler for our 'Animate' hot key and menu. Press Ctrl-W to bring up the class wizard. In the class name drop box select the view class (CBugTracksView) and in the Object ID list, select the ID of our new menu and hot key command (ID_TRACK_ANIMATE). You'll see the two possible choices in the Messages list. Double-click on COMMAND to add a function to handle our new command. You will be prompted for a function name. Just accept the default one OnTrackAnimate() and press OK. You will see the function appear in the Member Function list near the bottom of the dialog. Double-click on the function to jump directly to the code. We set m_nBugPosition to zero and start a timer that will redraw the bug tracks in intervals of 0.2 seconds.

void CBugTracksView::OnTrackAnimate() 
{ 
// get the document 
CBugTracksDoc* pDoc = GetDocument(); 

// set the position to the first data point 
pDoc->m_nBugPosition=0; 

// create a timer with id=1 and delay of 200 milliseconds 
SetTimer(1,200, NULL); 

Next we need to handle the timer message. Ctrl-W back to the class view. Make sure you are looking at the view class, select the class as the Object ID, then double-click WM_TIMER in the message list to handle the timer message. Again, double-click on the function name to jump to the code. In the OnTimer function we will first check the ID of the timer to make sure we are responding to the correct timer. In this case we set the timer ID to 1. Then we will invalidate the window so that it will be repainted.

void CBugTracksView::OnTimer(UINT nIDEvent) 
{ 
 if(nIDEvent==1) 
 { 
  // tell windows the view needs redrawn 
  // note: the last parameter is the erase flag. 
  // if it is TRUE, things will flicker like crazy. 
  InvalidateRect(NULL,FALSE); 
 } 

 CView::OnTimer(nIDEvent); 
} 

All that is left now is to fix up the OnDraw() function in the view class. We need to first draw the red tracks, then the blue ones, then increment the position m_nBugPosition. If m_nBugPosition is larger than the number of positions we will set it to -1 and kill the timer.

One of the new things in this code is the CPen class that is needed to change the color of the line. The way these graphical objects work is that you 'select' the object in to the CDC class. When you are done with it, you select the old one that was in there previously and delete the one you just used.

void CBugTracksView::OnDraw(CDC* pDC) 
{ 
 CBugTracksDoc* pDoc = GetDocument(); 
 ASSERT_VALID(pDoc); 

 // make pens for solid lines of thickness 2 
 CPen RedPen(PS_SOLID, 2, RGB(255,0,0)); 
 CPen BluePen(PS_SOLID, 2, RGB(0,0,255)); 

 CPen *pOldPen = pDC->SelectObject(&RedPen); 

 int i, N=pDoc->m_BugDataArray.GetSize(); 

 // draw any tracks which need animated 
 for(i=0; i < pDoc->m_nBugPosition-1; i++) 
 { 
  pDC->MoveTo(pDoc->m_BugDataArray[i].x,)pDoc->m_BugDataArray[i].y); 
  pDC->LineTo(pDoc->m_BugDataArray[i+1].x,pDoc->m_BugDataArray[i+1].y); 
 } 

 // change pens 
 pDC->SelectObject(&BluePen); 

 // start drawing non animated tracks, but need to check for a 
 // valid starting postion 
 int start=pDoc->m_nBugPosition; 
 if(start<0) start=0; 
 for(i=start; i < N-2; i++) 
 { 
  pDC->MoveTo(pDoc->m_BugDataArray[i].x,)pDoc->m_BugDataArray[i].y); 
  pDC->LineTo(pDoc->m_BugDataArray[i+1].x,pDoc->m_BugDataArray[i+1].y); 
 } 

 // deselect pens and delete them 
 pDC->SelectObject(pOldPen); 
 RedPen.DeleteObject(); 
 BluePen.DeleteObject(); 

 // move to next position or quit animating 
 if(pDoc->m_nBugPosition!=-1) pDoc->m_nBugPosition++; 
 if(pDoc->m_nBugPosition>=N) 
 { 
  pDoc->m_nBugPosition=-1; 

  // stop timer 1 
  KillTimer(1); 

  // redraw and erase so all lines are in initial state (blue) 
  InvalidateRect(NULL); 
 } 
} 

Ctrl-F5 the program to build and run it. Fix any bugs and you are done! Of course many improvements can be made, like scaling and centering the path to better fit the view, printing of the path, etc... but I think you have enough to go on. Good luck! (and don't be afraid of that F1 key)