ch06.htm

VC 21天 学习VC 的好东西

<PRE> 1: void CMenusDlg::OnHelpAbout() 
 2: {
 3:     // TODO: Add your command handler code here
 4: 
 5:     ///////////////////////
 6:     // MY CODE STARTS HERE
 7:     ///////////////////////
 8: 
 9:     // Declare an instance of the About window
10:     CAboutDlg dlgAbout;
11: 
12:     // Show the About window
13:     dlgAbout.DoModal();
14: 
15:     ///////////////////////
16:     // MY CODE ENDS HERE
17:     ///////////////////////
</PRE>

<PRE>18: }</PRE>
<P>You attached the File | Exit menu entry to an existing function that closes the
application. On the File | Hello, you added a new function that called the MessageBox
function to display a simple message to the user. With Help | About, you added another
function that declared an instance of the About dialog window and called its DoModal
method.</P>
<P>If you compile and run your application, you find that all the menu entries are
working. If you select Help | About, as shown in Figure 6.8, you see the application
About dialog (see Figure 6.9). If you select File | Hello, you see a Hello there
message box, as shown in Figure 6.10. And if you select File | Exit, your application
closes.</P>
<P><A HREF="javascript:popUp('06fig07.gif')"><B>FIGURE 6.8.</B></A><B> </B><I>The
Help | About menu entry.</I></P>

<P><A HREF="javascript:popUp('06fig08.gif')"><B>FIGURE 6.9.</B></A><B> </B><I>The
About dialog.</I></P>

<P><A HREF="javascript:popUp('06fig09.gif')"><B>FIGURE 6.10.</B></A><B> </B><I>The
Hello there message box.</I></P>

<P><I></I>
<H2><A NAME="Heading11"></A>Creating Pop-Up Menus</H2>
<P>Most Windows applications have what are called either pop-up or context menus,
which are triggered by the user right-clicking an object. These are called <I>pop-up
menus</I> because they pop up in the middle of the application area, not attached
to a menu bar, the window frame, or anything else on the computer screen (not counting
the mouse pointer). These menus are often referred to as <I>context menus</I> because
the contents of a menu depend on the context in which it is opened; the elements
available on the menu depend on what objects are currently selected in the application
or what the mouse pointer is positioned over.</P>
<P>To provide a pop-up menu in your application, you have two approaches available.
You can either design a menu specifically for use as a pop-up menu, or you can use
one of the pull-down menus from the primary menu that you have already designed.
If you design a menu specifically for use as a pop-up menu, you will need to skip
the top-level, menu bar element by placing a space or some other text in the caption,
knowing that it will not be seen. You will see how this works when you build a custom
menu specifically for use as a pop-up menu on Day 11, &quot;Creating Multiple Document
Interface Applications,&quot; in the section &quot;Adding a Context Menu.&quot;</P>
<P>Every drop-down portion of a menu can also be used as a pop-up menu. To use it
in this way, you must get a handle to the submenu (the drop-down menu) and then call
the TrackPopupMenu function on the submenu. The rest of the pop-up menu functionality
is already covered in the other menu building and coding that you have already done.
To add a pop-up menu to your application, follow these steps:</P>
<P>

<DL>
	<DT></DT>
	<DD><B>1. </B>Using the Class Wizard, add a function for the WM_CONTEXTMENU event
	message in your dialog window.
	<P>
</DL>



<BLOCKQUOTE>
	<P>
<HR>
<STRONG>NOTE:</STRONG> There are two dialog event messages that you can use to trigger your
	context menu. The event that you'd expect to use is the WM_RBUTTONDOWN event, which
	is triggered by the user right-clicking. The other event that can (and should) be
	used is the WM_CONTEXTMENU event, which is intended for use specifically to trigger
	a context menu. This event is triggered by a couple user actions: One of these is
	the release of the right mouse button, and another is the pressing of the context
	menu button on one of the newer Windows-enabled keyboards.
<HR>


</BLOCKQUOTE>


<DL>
	<DT><B></B></DT>
	<DD><B>2. </B>Edit the function, adding the code in Listing 6.3.
	<P>
</DL>

<H4>LISTING 6.3. THE ONCONTEXTMENU FUNCTION.</H4>
<PRE> 1: void CMenusDlg:: OnContextMenu(CWnd* pWnd, CPoint point) 
 2: {
 3:     // TODO: Add your message handler code here
 4:     
 5:     ///////////////////////
 6:     // MY CODE STARTS HERE
 7:     ///////////////////////
 8: 
 9:     // Declare local variables
10:     CMenu *m_lMenu;     // A pointer to the menu
11:     CPoint m_pPoint;    // A copy of the mouse position
12: 
13:     // Copy the mouse position to a local variable
14:     m_pPoint = point;
15:     // Convert the position to a screen position
16:     ClientToScreen(&amp;m_pPoint);
17:     // Get a pointer to the window menu
18:     m_lMenu - GetMenu();
19:     // Get a pointer to the first submenu
20:     m_lMenu = m_lMenu-&gt;GetSubMenu(0);
21:     // Show the Popup Menu
22:     m_lMenu-&gt;TrackPopupMenu(TPM_CENTERALIGN + TPM_LEFTBUTTON,
23:         m_pPoint.x, m_pPoint.y, this, NULL);
24: 
25:     ///////////////////////
26:     // MY CODE ENDS HERE
27:     ///////////////////////
</PRE>

<PRE>28: }</PRE>
<P>In Listing 6.3, the first thing that you did was make a copy of the mouse position.
This mouse position is a relative position within the window area. It must be converted
to an absolute position on the entire screen area for displaying the pop-up menu.
If you don't convert the position coordinates, you can't predict where your pop-up
menu will appear.</P>
<P>After you convert the position to an absolute position, you get a pointer to the
window menu. This pointer should always be a local pointer within the function where
you are going to use it because the location of the menu might change as the application
runs. From the menu pointer, you next get a pointer to the first drop-down menu (submenu
numbering begins with 0, like just about everything else in C/C++). After you have
a pointer to the submenu, you can treat it as a regular CMenu class instance.</P>
<P>The final piece in this puzzle is the call to the CMenu member function TrackPopupMenu.
This function takes five arguments and uses them to determine where and how to show
the pop-up menu. The first argument is a combination of two flags. The first flag,
TPM_CENTERALIGN, centers the pop-up menu on the mouse point. You can also use TPM_LEFTALIGN
or TPM_RIGHTALIGN instead. These flags line up the left or right edge of the pop-up
menu with the mouse position. The second part of this flag combination is TPM_LEFTBUTTON,
which makes the pop-up menu trigger from the left mouse button. You can also use
TPM_RIGHTBUTTON to make the menu trigger from the right mouse button.</P>
<P>The second and third arguments to the TrackPopupMenu function specify the screen
position for the pop-up menu. This is the absolute position on the screen, not a
relative position within the window area. The fourth argument is a pointer to the
window that receives the menu command messages. The final argument is a rectangle
that the user can click without closing the pop-up menu. By passing NULL, you specify
that if the user clicks outside the pop-up menu, the menu closes. This code enables
you to include a pop-up menu in your application, as shown in Figure 6.11.</P>
<P><A HREF="javascript:popUp('06fig10.gif')"><B>FIGURE 6.11.</B></A><B> </B><I>The
pop-up menu in action.</I></P>

<P><I></I>
<H2><A NAME="Heading12"></A>Creating a Menu with Accelerators</H2>
<P>One of the original keyboard shortcuts for selecting menu entries were accelerator
keys. As mentioned earlier in the chapter, accelerator keys are specific key combinations,
usually the Ctrl key combined with another key, or function keys, that are unique
within the entire application. Each of these key combinations triggers one menu event
function.</P>
<P>The way that accelerator keys work is similar to the way menus work. They are
also an application resource that is defined in a table in the resource tab of the
workspace pane. Each table entry has an object ID and a key code combination. After
you define the accelerators, you can attach functionality to the object IDs. You
can also assign accelerator entries the same object ID as the corresponding menu
entry so that you have to define only a single entry in the application message map.</P>
<P>After you define all your accelerator keys, you can add the key combination to
the menu entry so that the user will know about the accelerator key combination.
Add \t to the end of the menu entry caption, followed by the key combination. The
\t is replaced in the menu display by a tab, which separates the menu caption from
the accelerator key combination.</P>
<P>Unfortunately, accelerator keys don't work in dialog-style windows, so you cannot
add them to today's application. You will learn how to attach accelerator keys to
menus in a few days when you learn about single and multi-document interface style
applications.</P>
<P>
<H2><A NAME="Heading13"></A>Summary</H2>
<P>Today you learned about menus in Visual C++ applications. You learned how to use
the tools in Visual C++ to create a menu for use in your application and then how
to attach the menu to a window in your application. After you had the menu attached
to your window, you learned how to attach functionality to the various menu entries.
Later in the day, you learned how you can use a portion of your menu as a pop-up,
or context, menu. Finally, you learned how accelerator keys are added to most applications.</P>
<P>
<H2><A NAME="Heading14"></A>Q&amp;A</H2>

<DL>
	<DT></DT>
	<DD><B>Q Do I have to name my menu items the same names everyone else uses? For example,
	a lot of applications use File and Help. Can I name my menus something else?</B>
	<P>
	<DT><B></B></DT>
	<DD><B>A</B> You can name your top-level menus anything you want. However, there
	are ac-cepted menu name conventions that place all file-oriented functionality under
	a menu labeled File and all help-related functionality under a menu labeled Help.
	If you have a menu with entries such as Broccoli, Corn, and Carrots, you will probably
	want to call the menu Vegetables, although an equally valid label would be Food or
	Plants. In general, if you want to make your application easy for your users to learn,
	you will want to use menu labels that make sense for the entries o n the pull-down
	portion of the menu.
	<P>
	<DT></DT>
	<DD><B>Q Why can't I specify a single character as an accelerator key?</B>
	<P>
	<DT><B></B></DT>
	<DD><B>A</B> The single character would trigger the WM_KEY messages, not the menu
	messages. When the designers of Windows were deciding how accelerator keys would
	work, they decided that single-character keys would most likely be input to the active
	application. If they had allowed single-character accelerators, Windows wouldn't
	be able to determine whether the character was input or a shortcut. By requiring
	a key combination (with the exception of function keys), the designers ensured that
	Windows won't have to make this determination.
	<P>
</DL>

<H2><A NAME="Heading15"></A>Workshop</H2>
<P>The Workshop provides quiz questions to help you solidify your understanding of
the material covered and exercises to provide you with experience in using what you've
learned. The answers to the quiz questions are provided in Appendix B, &quot;Answers.&quot;</P>
<P>
<H3><A NAME="Heading16"></A>Quiz</H3>

<DL>
	<DT></DT>
	<DD><B>1. </B>What event message does a menu selection send to the window message
	queue?
	<P>
	<DT></DT>
	<DD><B>2. </B>How do you attach a menu to a dialog window?
	<P>
	<DT></DT>
	<DD><B>3. </B>Which existing class do you specify for handling event messages for
	the menu?
	<P>
	<DT></DT>
	<DD><B>4. </B>What event message should a pop-up menu be triggered by?
	<P>
</DL>

<H3><A NAME="Heading17"></A>Exercises</H3>

<DL>
	<DT></DT>
	<DD><B>1. </B>Add a button to the main window and have it call the same function
	as the Hello menu entry.
	<P>
	<DT></DT>
	<DD><B>2. </B>Add a pop-up menu to your application that uses the Help drop-down
	menu as the pop-up menu.
</DL>

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