CMPT 212 Object-Oriented Applications Design in C++ Spring 1998

Outputting Text to a Window

• The output statement

     cout << "Hello";
           

  will not work. It does not specify:
  • which window to display in,
  • where in the window to display,
  • which font to use,
  • which colour to use.

• The position of text output is specified by using X-Y coordinates relative to the top-left corner of the window, not to the screen.

• Suppose we want to display "Here" at the location where the user presses the left mouse button.
  • Here is the code for this:

       LRESULT CALLBACK WindowFunc(HWND   hwnd,
                                   UINT   message,
                                   WPARAM wParam,
                                   LPARAM lParam) {
          HDC  hdc;
          int  X;
          int  Y;
          char str[80];
    
          switch (message) {
    
             ...
    
             case WM_LBUTTONDOWN :
                hdc = GetDC(hwnd);
                strcpy(str, "Here");
                X = LOWORD(lParam);
                Y = HIWORD(lParam);
                TextOut(hdc, X, Y, str, strlen(str));
                ReleaseDC(hwnd, hdc);
                break;
    
             ...
    
             }
    
          return 0;
       }
     
                   

• The HDC variable is a handle to a device context.
  • A device context describes the output device:
    • printer vs. screen,
    • number of colours available,
    • font and colour to use,
    • clipping or no clipping,
    • and many others.

  • GetDC returns the default device context. It you want to change the font, colour, etc., you must change it after calling GetDC but before using the device context.

  • You must release the device context before returning from the callback function.

• TextOut does not erase text.
  • To erase text, you would have to write spaces at the location of the text.

  • But remember that if you are using a variable pitch font, then you may require more spaces than the number of characters you are trying to erase.

• 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.

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