CMPT 212 Object-Oriented Applications Design in C++ Fall 1997

Outputting Text to a Window (continued)

• To output text on the next line, we must calculate the Y-coordinate of the next line based on the size of the current font.
  • Example: This example displays the numbers 1 to 10 on successive lines.

       char       buffer[10];
       HDC        hdc;
       TEXTMETRIC tm;
       int        x = 1;
       int        y = 1;
    
       hdc = GetDC(hwnd);
       GetTextMetrics(hdc, &tm);
       for (int i=0; i<=10; i++) {
          sprintf(buffer, "%d", i);
          TextOut(hdc, x, y, buffer, strlen(buffer));
          y = y + tm.tmHeight + tm.tmExternalLeading;
          }
       ReleaseDC(hwnd, hdc);
    	       

  • A TEXTMETRIC structure contains information about the current font. (See page 161 of Schildt.)

  • GetTextMetrics will fill in a TEXTMETRICS structure.

  • tm.tmHeight is the height of the font, and tm.tmExternalLeading is the amount of space between lines of text.

The WM_PAINT Message

• Question: Who (MS-Windows or your application) decides when the contents of a window should be updated on the screen and who does the updating?
  • Answer: Both MS-Windows and your application may request that a window be updated, but it is the responsibility of your application to update the window.

• The process of updating the contents of a window is called painting.

• A WM_PAINT message is sent to your application when it needs to paint a window.
  • MS-Windows may send this message when the window is resized to a bigger size, when the window is opened from an icon, when another window is moved away from in front of it, and so on.

  • Your application may send this message when the data has changed and the window needs to be painted to show the change.

• The following code shows how your application could respond to a WM_PAINT message:

     HDC         hdc;
     PAINTSTRUCT paintstruct;
     ...
     switch (message) {
        ...
        case WM_PAINT:
  	 hdc = BeginPaint(hwnd, &paintstruct);
  	 TextOut(hdc, 1, 1, str, strlen(str));
  	 EndPaint(hwnd, &paintstruct);
  	 break;
        ...
       }
  	 

  • Like GetDC, BeginPaint returns a HDC. WHen responding to a WM_PAINT message, you must use BeginPaint instead of GetDC.

  • The PAINTSTRUCT structure describes how MS-Windows will paint the window.

• Your application sends a WM_PAINT message to a window by using the InvalidateRect function.

     InvalidateRec(hwnd, NULL, 1);
  	 

  • The first parameter is the handle of the window that is to receive the WM_PAINT message.

  • The second parameter is a pointer to a RECT structure which describes the rectangle within the window that should be painted. Passing NULL requests that the entire window be painted.

  • The third parameter specifies whether or not the background should be erased before painting.

• Your application will typically need to store its data in variables accessible by different functions (eg. module variables) because the functions which update the data cannot pass the data to the function which uses the data to paint the window.

Message Boxes

• Message boxes are windows that display a message and wait for an acknowledgement.

• Message boxes normally have buttons which allow the user to respond in certain ways:
  • Abort, Retry, Ignore
  • Yes, No
  • OK
  • OK, Cancel
  • and others.

• Message boxes are created and displayed using the function

     int MessageBox(HWND, LPCSTR, LPCSTR, UINT)
  	 

  where
  • HWND is the handle of the parent window,
  • the first LPCSTR is a pointer to the message string,
  • the second LPCSTR is a pointer to the caption string,
  • UINT specifies the types of buttons and icons to display,
  • the return value specifies how the user responded.

• Example: Here is some code that will product this message box:
  

     int response;
  
     response = MessageBox(hwnd, "What now?", "Question", 
                           MB_OKCANCEL | MB_ICONQUESTION);
  
     switch (response) {
  
        case IDOK:
  	 ...   // user pressed "OK" button.
  	 break;
  
        case IDCANCEL:
  	 ...   // user pressed "Cancel" button.
  	 break;
  
        }
  	 

Resource Files

• Resources are items which are used in your program, but are defined outside of your program.

• Resources include:
  • menus,
  • icons,
  • dialog boxes,
  • bitmapped graphics.

• Resources are defined in a .rc file using a scripting language.
  • They are compiled into a .res file using a resource compiler (typically rc.exe).

  • The .res file is then linked into your .exe file by the linker.

• You may edit the .rc resource file by hand using a text editor, or by a resource editor such as the Borland Resource Editor.

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