ch06.htm

24小时学会vc++

Routines</B></FONT></H2>
<P>The DDV and DDX routines are helper functions that help manage data for dialog
boxes. DDV, or Dialog Data Validation, routines are used for data validation. DDX,
or Dialog Data Exchange, routines are used to exchange data to and from the controls
in a dialog box.


<BLOCKQUOTE>
	<P>
<HR>
<B> </B><FONT COLOR="#000077"><B>Time Saver:</B></FONT><B> </B>Although you can use
	the DDV and DDX routines in your dialog boxes directly, ClassWizard adds the code
	for you at the click of a button. Normally, you add the DDV and DDX routines with
	ClassWizard instead of trying to hand-code the necessary function calls. 
<HR>


</BLOCKQUOTE>

<H3><FONT COLOR="#000077"><B>Why Are DDV and DDX Routines Used?</B></FONT></H3>
<P>The DDV routines are useful when collecting data from an edit control. In general,
you have little control over how a user enters data in an edit control. A DDV enables
you to perform some simple validation based on range or string length.


<BLOCKQUOTE>
	<P>
<HR>
<B> </B><FONT COLOR="#000077"><B>Just a Minute:</B></FONT><B> </B>For example, if
	an edit control is used to collect an abbreviated state name, you want to limit the
	entered text to two characters. Using a DDV routine, it's easy to make sure that
	two characters have been entered. 
<HR>


</BLOCKQUOTE>

<P>DDX functions link member variables from the dialog box class to controls that
are contained in the dialog box. DDX routines enable data to be transferred to and
from the controls much easier than is otherwise possible. As discussed in Hour 4,
a dialog box is normally used something like this:</P>
<PRE><FONT COLOR="#0066FF"><TT>CMyDialog    dlgMine;</TT>
<TT>dlgMine.DoModal();</TT>
</FONT></PRE>
<P>In this example, the dialog box is created when <TT>DoModal</TT> is called, and
the function does not return until the user closes the dialog box. This presents
a problem if data must be passed to or from the dialog box. Because none of the controls
exist until the dialog box is created, using <TT>SetWindowText</TT>, <TT>GetWindowText</TT>,
or other functions to interact directly with controls contained in the dialog box
is not possible. After the dialog box has been dismissed, it is too late to use those
functions to collect user input.</P>
<P>When DDX routines are used to exchange information with a dialog box, the dialog
box can be used like this:</P>
<PRE><FONT COLOR="#0066FF"><TT>CMyDialog    dlgMine;</TT>
<TT>dlgMine.m_szTest = &quot;Hello World&quot;;</TT>
<TT>dlgMine.DoModal();</TT>
</FONT></PRE>
<P>The DDX routines enable you to have access to the dialog box's controls before
and after the dialog box has been created. This simplifies dialog box programming
because it is a much more flexible method than adding code in the <TT>InitDialog</TT>
member function.
<H3><FONT COLOR="#000077"><B>How Are DDV and DDX Routines Used?</B></FONT></H3>
<P>The easiest and most useful way to add DDV and DDX routines to your dialog box
class is by using ClassWizard. Member variables associated with dialog box controls
by value automatically use the DDV and DDX routines provided by MFC. For example,
<TT>CString</TT> member variables are often associated with edit controls. ClassWizard
adds source code to handle the exchange and validation of data in two places:

<UL>
	<LI>In the dialog box's constructor, source code is added to initialize the member
	variable.<BR>
	<BR>
	
	<LI>In the dialog box's <TT>DoDataExchange</TT> member function, ClassWizard adds
	DDV and DDX routines for each member variable associated with a control's value.
</UL>

<P><TT>DoDataExchange</TT> is a virtual function that is called to move data between
the control and the dialog box's member data. <TT>DoDataExchange</TT> takes a single
parameter, either <TT>TRUE</TT> or <TT>FALSE</TT>, with <TT>TRUE</TT> as the default
parameter. When <TT>DoDataExchange( FALSE )</TT> is called, data is moved from the
member variable to the control. When <TT>DoDataExchange( TRUE )</TT> is called, data
is copied from the control to the member variable.</P>
<P>When the dialog box is initially displayed during <TT>CDialog::OnInitDialog</TT>,
<TT>UpdateData(FALSE)</TT> is called to transfer data from the member variables to
the dialog box's controls. Later, during <TT>CDialog::OnOk</TT>, <TT>UpdateData()</TT>
is called to transfer data from the dialog box's controls to member variables.</P>
<P>As shown in Figure 6.2, <TT>DoDataExchange</TT> has a single parameter that controls
the direction that data, in this case <TT>m_szTest</TT>, is copied.</P>

<P><A HREF="02.htm"><B>Figure 6.2.</B></A> <I><BR>
DDV and DDX routines used to handle dialog box data.</I>
<H3><FONT COLOR="#000077"><B>Associating a Control's Value with a Member Variable</B></FONT></H3>
<P>You add member variables that are associated with a control's value almost exactly
the way you added control-type variables earlier in this hour. For example, to create
a member variable associated with the <TT>IDD_EDIT_TEST</TT> edit control, follow
these steps:

<DL>
	<DD>1. Open ClassWizard.<BR>
	<BR>
	2. Select the <TT>CDialog</TT>-derived class that manages the dialog box; in this
	case, <TT>CTestDlg</TT>.<BR>
	<BR>
	3. Select the Member Variables tab.
</DL>

<P>

<DL>
	<DD>4. Select the control ID representing the control associated with the new member
	variable; in this case, <TT>IDC_EDIT_TEST</TT>.<BR>
	<BR>
	5. Click the Add Variable button. An Add Member Variable dialog box appears. Enter
	the control's name, category, and variable type; then click OK. For this example,
	use the values from Table 6.3.
</DL>

<H4><FONT COLOR="#000077">Table 6.3. Values used to associate a CString member variable
with an edit control.</FONT></H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><B>Control ID</B></TD>
		<TD ALIGN="LEFT"><B>Variable Name</B></TD>
		<TD ALIGN="LEFT"><B>Category</B></TD>
		<TD ALIGN="LEFT"><B>Type</B></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT"><TT>IDC_EDIT_TEST</TT></TD>
		<TD ALIGN="LEFT"><TT>m_szTest</TT></TD>
		<TD ALIGN="LEFT">Value</TD>
		<TD ALIGN="LEFT"><TT>CString</TT></TD>
	</TR>
</TABLE>
</P>
<P>The preceding steps are exactly like the steps used to add a control-type variable
earlier in this hour, except that the control type is set to <TT>Value</TT>. A member
variable associated by value with an edit control can also be an <TT>int</TT>, <TT>UINT</TT>,
<TT>long</TT>, <TT>DWORD</TT>, <TT>float</TT>, <TT>double</TT>, or <TT>BYTE</TT>,
although it is most commonly a <TT>CString</TT>.</P>
<P>After closing the Add Member Variable dialog box, ClassWizard displays an edit
control that you can use to specify the type of validation to be performed on the
member variable. If a <TT>CString</TT> object is associated with an edit control,
the maximum string length can be specified. If a numeric variable is used, the allowed
range can be defined.
<H3><FONT COLOR="#000077"><B>Exchanging Edit-Control Information Using DDX Functions</B></FONT></H3>
<P>The member variables associated with dialog box controls by ClassWizard are added
to the dialog box class as public variables. This allows the member variables to
be easily accessed and used. For example, to use the <TT>m_szTest</TT> variable that
was added in the previous section, edit the <TT>CMainFrame::OnViewTest</TT> member
function so it looks like the function in Listing 6.5. Before compiling the project,
remove the following line, which was added to <TT>CTestDlg::OnInitDialog</TT> earlier:</P>
<PRE><FONT COLOR="#0066FF"><TT>m_editTest.SetWindowText( &quot;Default&quot; );</TT>
</FONT></PRE>
<H4><FONT COLOR="#000077">TYPE: Listing 6.5. Using member variables to exchange information
with an edit control.</FONT></H4>
<PRE><FONT COLOR="#0066FF"><TT>void CMainFrame::OnViewTest()</TT>
<TT>{</TT>
<TT>    CTestDlg    dlg;</TT>

<TT>    dlg.m_szTest = &quot;DDX Test&quot;;</TT>

<TT>    if( dlg.DoModal() == IDOK )</TT>
<TT>    {</TT>
<TT>        AfxMessageBox( dlg.m_szTest );</TT>
<TT>    }</TT>
<TT>    else</TT>
<TT>    {</TT>
<TT>        AfxMessageBox( &quot;Dialog cancelled&quot; );</TT>
<TT>    }</TT>
<TT>}</TT>
</FONT></PRE>
<P>Listing 6.5 sets the value of <TT>m_szTest</TT> before the dialog box is displayed
to the user. <TT>CDialog::OnInitDialog</TT> calls the <TT>CWnd::UpdateData</TT> function,
which calls <TT>UpdateData</TT>. Because <TT>UpdateData</TT> is a virtual function,
the proper version of the function is called--the version that is part of the <TT>CDialog</TT>-derived
class that handles the dialog box.</P>
<P>After the dialog box closes, the <TT>CMainFrame::OnViewTest</TT> function checks
the return value of <TT>DoModal</TT>. If <TT>IDOK</TT> was returned, the dialog box
was closed using the OK button, and the value of <TT>m_szTest</TT> is displayed.
<H2><FONT COLOR="#000077"><B>Summary</B></FONT></H2>
<P>In this hour, you learned about identifier scope and lifetime. You also learned
about the Windows edit control and how it is usually used in a dialog box. You saw
how to associate an edit control with a <TT>CEdit</TT> object using ClassWizard and
used data exchange and validation to pass parameters to and from dialog boxes.
<H2><FONT COLOR="#000077"><B>Q&amp;A</B></FONT></H2>

<DL>
	<DD><B>Q I would like to use the DDV routines for all my dialog box data, but what
	if I have a complex data type, such as a credit card number? How can I use the DDV
	DDX mechanism?</B><BR>
	<BR>
	<B>A</B> You have two options. You can perform the validation yourself when accepting
	user input by testing for valid data before the user is allowed to close the dialog
	box. You can also write your own custom DDV routine. Technical Note 26 describes
	how to write such a routine. You can find this technical note by searching for TN026
	in the Developer Studio online help.<BR>
	<BR>
	<B>Q Can I call <TT>UpdateData</TT> at any time? I would like to implement an Undo
	feature in my dialog box.</B><BR>
	<BR>
	<B>A</B> Normally, <TT>UpdateData</TT> is called when the dialog box is initialized
	and when the user closes the dialog box by clicking OK. However, if you must call
	<TT>UpdateData</TT> at other times, it's perfectly okay.
</DL>

<H2><FONT COLOR="#000077"><B>Workshop</B></FONT></H2>
<P>The Workshop is designed to help you anticipate possible questions, review what
you've learned, and begin thinking ahead to putting your knowledge into practice.
The answers to the quiz are in Appendix B, &quot;Quiz Answers.&quot;
<H3><FONT COLOR="#000077"><B>Quiz</B></FONT></H3>

<DL>
	<DD>1. What MFC class is used to manage edit controls?<BR>
	<BR>
	2. What is the difference between an MLE and an SLE?<BR>
	<BR>
	3. How are DDV and DDX routines used?<BR>
	<BR>
	4. What member function do you call to transfer data to and from your dialog box
	controls?<BR>
	<BR>
	5. What function is used to retrieve text from an edit control?<BR>
	<BR>
	6. What function is used to set text in an edit control?<BR>
	<BR>
	7. What property is used to hide user input in an edit control by replacing it with
	asterisks?<BR>
	<BR>
	8. What keystroke is used to paste text into an edit control?<BR>
	<BR>
	9. What keystroke is used to copy text from an edit control?<BR>
	<BR>
	10. What keystroke is used to cut text from an edit control?
</DL>

<H3><FONT COLOR="#000077"><B>Exercises</B></FONT></H3>

<DL>
	<DD>1. Set the maximum length for the text entered in the edit control in the Test
	project to five characters.<BR>
	<BR>
	2. Change the Test project so that the edit control is used to store an integer value
	instead of a string.<FONT COLOR="#000077"></FONT>
</DL>

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