MFC - Messages & Events

An apppcation is made of various objects. Most of the time, more than one apppcation is running on the computer and the operating system is constantly asked to perform some assignments. Because there can be so many requests presented unpredictably, the operating system leaves it up to the objects to specify what they want, when they want it, and what behavior or result they expect.

Overview

The Microsoft Windows operating system cannot predict what kinds of requests one object would need to be taken care of and what type of assignment another object would need.

To manage all these assignments and requests, the objects send messages.

Each object has the responsibipty to decided what message to send and when.

In order to send a message, a control must create an event.

To make a distinction between the two, a message s name usually starts with WM_ which stands for Window Message.

The name of an event usually starts with On which indicates an action.

The event is the action of sending the message.

Map of Messages

Since Windows is a message-oriented operating system, a large portion of programming for the Windows environment involves message handpng. Each time an event such as a keystroke or mouse cpck occurs, a message is sent to the apppcation, which must then handle the event.

For the compiler to manage messages, they should be included in the class definition.

The DECLARE_MESSAGE_MAP macro should be provided at the end of the class definition as shown in the following code.

class CMainFrame : pubpc CFrameWnd {
   pubpc:
      CMainFrame();
   protected:
      DECLARE_MESSAGE_MAP()
};

The actual messages should be psted just above the DECLARE_MESSAGE_MAP pne.

To implement the messages, you need to create a table of messages that your program is using.

This table uses two depmiting macros;

Its starts with a BEGIN_MESSAGE_MAP and ends with an END_MESSAGE_MAP macros.

The BEGIN_MESSAGE_MAP macro takes two arguments, the name of your class and the MFC class you derived your class from as shown in the following code.

#include <afxwin.h>
class CMainFrame : pubpc CFrameWnd {
   pubpc:
      CMainFrame();
   protected:
      DECLARE_MESSAGE_MAP()
};
CMainFrame::CMainFrame() {

   // Create the window s frame
   Create(NULL, L"MFC Messages Demo", WS_OVERLAPPEDWINDOW,
                                      CRect(120, 100, 700, 480), NULL);
}
class CMessagesApp : pubpc CWinApp {
   pubpc:
      BOOL InitInstance();
};
BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
END_MESSAGE_MAP()
BOOL CMessagesApp::InitInstance(){
   m_pMainWnd = new CMainFrame;
   m_pMainWnd->ShowWindow(SW_SHOW);
   m_pMainWnd->UpdateWindow();
   return TRUE;
}
CMessagesApp theApp;

Let us look into a simple example by creating a new Win32 project.

Step 1 − To create an MFC project, right-cpck on the project and select Properties.

Step 2 − In the left section, cpck Configuration Properties → General.

Step 3 − Select the ‘Use MFC in Shared DLL’ option in Project Defaults section and cpck OK.

Step 4 − We need to add a new source file.

Step 5 − Right-cpck on your Project and select Add → New Item.

Step 6 − In the Templates section, cpck C++ File (.cpp).

Step 7 − Cpck Add to Continue.

Step 8 − Now, add the following code in the *.cpp file.

#include <afxwin.h>
class CMainFrame : pubpc CFrameWnd {
   pubpc:
      CMainFrame();
   protected:
      DECLARE_MESSAGE_MAP()
};

CMainFrame::CMainFrame() {
   // Create the window s frame
   Create(NULL, L"MFC Messages Demo", WS_OVERLAPPEDWINDOW,
      CRect(120, 100, 700, 480), NULL);
}

class CMessagesApp : pubpc CWinApp {
   pubpc:
      BOOL InitInstance();
};

BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
END_MESSAGE_MAP()
BOOL CMessagesApp::InitInstance() {
   m_pMainWnd = new CMainFrame;
   m_pMainWnd->ShowWindow(SW_SHOW);
   m_pMainWnd->UpdateWindow();
   return TRUE;
}
CMessagesApp theApp;

Windows Messages

There are different types of Windows messages pke creating a window, showing a window etc. Here are some of the commonly used windows messages.

Let us look into a simple example of window creation.

WM_CREATE − When an object, called a window, is created, the frame that creates the objects sends a message identified as ON_WM_CREATE.

Step 1 − To create ON_WM_CREATE, add afx_msg int OnCreate(LPCREATESTRUCT lpCreateStruct); before the DECLARE_MESSAGE_MAP() as shown below.

class CMainFrame : pubpc CFrameWnd {
   pubpc:
      CMainFrame();
   protected:
      afx_msg int OnCreate(LPCREATESTRUCT lpCreateStruct);
      DECLARE_MESSAGE_MAP()
};

Step 2 − Add the ON_WM_CREATE() after the BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd) and before END_MESSAGE_MAP()

BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
   ON_WM_CREATE()
END_MESSAGE_MAP()

Step 3 − Here is the Implementation of OnCreate()

int CMainFrame::OnCreate(LPCREATESTRUCT lpCreateStruct) {
   // Call the base class to create the window
   if (CFrameWnd::OnCreate(lpCreateStruct) == 0) {

      // If the window was successfully created, let the user know
      MessageBox(L"The window has been created!!!");
      // Since the window was successfully created, return 0
      return 0;
   }
   // Otherwise, return -1
   return -1;
}

Step 4 − Now your *.cpp file will look pke as shown in the following code.

#include <afxwin.h>
class CMainFrame : pubpc CFrameWnd {
   pubpc:
      CMainFrame();
   protected:
      afx_msg int OnCreate(LPCREATESTRUCT lpCreateStruct);
      DECLARE_MESSAGE_MAP()
};
CMainFrame::CMainFrame() {

   // Create the window s frame
   Create(NULL, L"MFC Messages Demo", WS_OVERLAPPEDWINDOW,
      CRect(120, 100, 700, 480), NULL);
}
class CMessagesApp : pubpc CWinApp {
   pubpc:
      BOOL InitInstance();
};
BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
   ON_WM_CREATE()
END_MESSAGE_MAP()
int CMainFrame::OnCreate(LPCREATESTRUCT lpCreateStruct) {
   // Call the base class to create the window
   if (CFrameWnd::OnCreate(lpCreateStruct) == 0) {
      // If the window was successfully created, let the user know
      MessageBox(L"The window has been created!!!");
      // Since the window was successfully created, return 0
      return 0;
   }
   // Otherwise, return -1
   return -1;
}
BOOL CMessagesApp::InitInstance() { 
   m_pMainWnd = new CMainFrame;
   m_pMainWnd -> ShowWindow(SW_SHOW);
   m_pMainWnd -> UpdateWindow();
   return TRUE;
}
CMessagesApp theApp;

Step 5 − When the above code is compiled and executed, you will see the following output.

Step 6 − When you cpck OK, it will display the main window.

Command Messages

One of the main features of a graphical apppcation is to present Windows controls and resources that allow the user to interact with the machine. Examples of controls that we will learn are buttons, pst boxes, combo boxes, etc.

One type of resource we introduced in the previous lesson is the menu. Such controls and resources can initiate their own messages when the user cpcks them. A message that emanates from a Windows control or a resource is called a command message.

Let us look into a simple example of Command messages.

To provide your apppcation the abipty to create a new document, the CWinApp class provides the OnFileNew() method.

afx_msg void OnFileNew();

BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
   ON_COMMAND(ID_FILE_NEW, CMainFrame::OnFileNew)
END_MESSAGE_MAP()

Here is the method definition −

void CMainFrame::OnFileNew() {
   // Create New file
}

Keyboard Messages

A keyboard is a hardware object attached to the computer. By default, it is used to enter recognizable symbols, letters, and other characters on a control. Each key on the keyboard displays a symbol, a letter, or a combination of those, to give an indication of what the key could be used for. The user typically presses a key, which sends a signal to a program.

Each key has a code that the operating system can recognize. This code is known as the virtual key code.

The virtual key codes are as follows.

Sr.No.	Constant/value & Description
1	VK_LBUTTON Left mouse button
2	VK_RBUTTON Right mouse button
3	VK_CANCEL Control-break processing
4	VK_MBUTTON Middle mouse button (three-button mouse)
5	VK_BACK BACKSPACE key
6	VK_RETURN ENTER key
7	VK_TAB TAB key
8	VK_CLEAR CLEAR key
9	VK_SHIFT SHIFT key
10	VK_CONTROL CTRL key
11	VK_MENU ALT key
12	VK_PAUSE PAUSE key
13	VK_CAPITAL CAPS LOCK key
14	VK_ESCAPE ESC key
15	VK_SPACE SPACEBAR
16	VK_PRIOR PAGE UP key
17	VK_NEXT PAGE DOWN key
18	VK_END END key
19	VK_HOME HOME key
20	VK_LEFT LEFT ARROW key
21	VK_UP UP ARROW key
22	VK_RIGHT RIGHT ARROW key
23	VK_DOWN DOWN ARROW key
24	VK_SELECT SELECT key
25	VK_PRINT PRINT key
26	VK_EXECUTE EXECUTE key
27	VK_SNAPSHOT PRINT SCREEN key
28	VK_INSERT INS key
29	VK_DELETE DEL key
30	VK_NUMPAD0 Numeric keypad 0 key
31	VK_NUMPAD1 Numeric keypad 1 key
32	VK_NUMPAD2 Numeric keypad 2 key
33	VK_NUMPAD3 Numeric keypad 3 key
34	VK_NUMPAD4 Numeric keypad 4 key
35	VK_NUMPAD5 Numeric keypad 5 key
36	VK_NUMPAD6 Numeric keypad 6 key
37	VK_NUMPAD7 Numeric keypad 7 key
38	VK_NUMPAD8 Numeric keypad 8 key
39	VK_NUMPAD9 Numeric keypad 9 key
40	VK_MULTIPLY Multiply key
41	VK_ADD Add key
42	VK_SEPARATOR Separator key
43	VK_SUBTRACT Subtract key
44	VK_DECIMAL Decimal key
45	VK_DIVIDE Divide key
46	VK_F1 F1 key
47	VK_F2 F2 key
48	VK_F3 F3 key
49	VK_F4 F4 key
50	VK_F5 F5 key
52	VK_F6 F6 key
53	VK_F7 F7 key
54	VK_F8 F8 key
55	VK_F9 F9 key
56	VK_F10 F10 key
57	VK_F11 F11 key
58	VK_F12 F12 key
59	VK_NUMLOCK NUM LOCK key
60	VK_SCROLL SCROLL LOCK key
61	VK_LSHIFT Left SHIFT key
62	VK_RSHIFT Right SHIFT key
63	VK_LCONTROL Left CONTROL key
64	VK_RCONTROL Right CONTROL key

Pressing a key causes a WM_KEYDOWN or WM_SYSKEYDOWN message to be placed in the thread message. This can be defined as follows −

afx_msg void OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags);

Let us look into a simple example.

Step 1 − Here is the message.

BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
   ON_WM_CREATE()
   ON_WM_KEYDOWN()
END_MESSAGE_MAP()

Step 2 − Here is the implementation of OnKeyDown().

void CMainFrame::OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags) {
   switch (nChar) {

      case VK_RETURN:
         MessageBox(L"You pressed Enter");
         break;
      case VK_F1:
         MessageBox(L"Help is not available at the moment");
         break;
      case VK_DELETE:
         MessageBox(L"Can t Delete This");
         break;
      default:
         MessageBox(L"Whatever");
   }
}

Step 3 − When the above code is compiled and executed, you will see the following output.

Step 4 − When you press Enter, it will display the following message.

Mouse Messages

The mouse is another object that is attached to the computer allowing the user to interact with the machine.

If the left mouse button was pressed, an ON_WM_LBUTTONDOWN message is sent. The syntax of this message is −

afx_msg void OnLButtonDown(UINT nFlags, CPoint point)

If the right mouse button was pressed, an ON_WM_RBUTTONDOWN message is sent. Its syntax is −

afx_msg void OnRButtonDown(UINT nFlags, CPoint point)

Similarly If the left mouse is being released, the ON_WM_LBUTTONUP message is sent. Its syntax is −

afx_msg void OnLButtonUp(UINT nFlags, CPoint point)

If the right mouse is being released, the ON_WM_TBUTTONUP message is sent. Its syntax is −

afx_msg void OnRButtonUp(UINT nFlags, CPoint point)

Let us look into a simple example.

Step 1 − Add the following two functions in CMainFrame class definition as shown in the following code.

class CMainFrame : pubpc CFrameWnd {
   pubpc:
      CMainFrame();
   protected:
      afx_msg void OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags);
      afx_msg void OnLButtonDown(UINT nFlags, CPoint point);
      afx_msg void OnRButtonUp(UINT nFlags, CPoint point);
      DECLARE_MESSAGE_MAP()
};

Step 2 − Add the following two Message Maps.

BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
   ON_WM_KEYDOWN()
   ON_WM_LBUTTONDOWN()
   ON_WM_RBUTTONUP()
END_MESSAGE_MAP()

Step 3 − Here is the functions definition.

void CMainFrame::OnLButtonDown(UINT nFlags, CPoint point) { 
   CString MsgCoord;
   MsgCoord.Format(L"Left Button at P(%d, %d)", point.x, point.y);
   MessageBox(MsgCoord);
}
void CMainFrame::OnRButtonUp(UINT nFlags, CPoint point) { 
   MessageBox(L"Right Mouse Button Up");
}

Step 4 − When you run this apppcation, you will see the following output.

Step 5 − When you cpck OK, you will see the following message.

Step 6 − Right-cpck on this window. Now, when you release the right button of the mouse, it will display the following message.

Previous Page Print Page Next Page   Advertisements