ch14.htm

VC 21天 学习VC 的好东西

<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Object</I></TD>
		<TD ALIGN="LEFT"><I>Name</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_CBCARD		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_SendCard		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EADDR		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_Address		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_ECITY		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_City		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_ECOUNTRY		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_Country		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EEMAIL		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_EmailAddress		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EFAX		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_FaxNumber		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EFNAME		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_FirstName		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EHPHONE		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_HomePhone		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EID		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_AddressID		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_ELNAME		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_LastName		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_ENOTES		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_Notes		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_ESNAME		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_SpouseName		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_ESTATE		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_StateOrProvince		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EWEXT		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_WorkExtension		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EWPHONE		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_WorkPhone		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IDC_EZIP		</TD>
		<TD ALIGN="LEFT">m_pSet-&gt;m_PostalCode		</TD>
	</TR>
</TABLE>
</P>
<P>You probably noticed when it was time to attach a database field to the birthdate
control that the birthday field is missing from the list of database fields. If you
look at the record set class in the class view and expand its tree, you'll notice
that the birthdate field is included as one of the database fields, but it's not
available in the list of available columns for use with the controls. Double-click
on the birthdate field in the record set class to view its definition. You'll notice
that the m_Birthdate variable is declared as a CTime variable. This is the reason
that it's not available in the list of database fields that can be attached to controls.
There isn't a macro or function you can call for exchanging data between a control
and a CTime variable. This is also a problem because the CTime variable type cannot
handle dates before December 31, 1969. To use this database field, you'll need to
change its definition from a CTime to a COleDateTime variable type, as in line 17
in Listing 14.1. Once you change the variable type of this database field, you will
be able to attach it to the IDC_EDOB control.</P>
<P>
<H4>LISTING 14.1. THE DATABASE FIELD VARIABLE DECLARATIONS.</H4>
<PRE>1:  // Field/Param Data
2:      //{{AFX_FIELD(CTestdb5Set, CRecordset)
3:      long    m_AddressID;
4:      CString    m_FirstName;
5:      CString    m_LastName;
6:      CString    m_SpouseName;
7:      CString    m_Address;
8:      CString    m_City;
9:      CString    m_StateOrProvince;
10:     CString    m_PostalCode;
11:     CString    m_Country;
12:     CString    m_EmailAddress;
13:     CString    m_HomePhone;
14:     CString    m_WorkPhone;
15:     CString    m_WorkExtension;
16:     CString    m_FaxNumber;
17:     COleDateTime    m_Birthdate;
18:     BOOL    m_SendCard;
19:     CString    m_Notes;
20: //}}AFX_FIELD</PRE>


<BLOCKQUOTE>
	<P>
<HR>
<STRONG>NOTE:</STRONG> Normally, you do not want to edit the portions of code in your applications
	that are created and maintained by the various wizards. The change I outline here
	is one of the few exceptions to this rule. This obstacle could possibly be considered
	a bug in the Visual C++ AppWizard, although it's technically not a bug. You can convert
	the date/time database field to sev- eral variable types when creating a class variable
	to represent that field. CTime is one of these variable types; COleDateTime is another.
	Because these are both equally valid choices, and the functions that populate this
	variable can work with either, making this change is possible without dire consequences.
<HR>


</BLOCKQUOTE>

<P>Once you make the change to the variable type for the m_Birthdate variable in
the record set class (CDbOdbcSet), and attach this database field to the Birthdate
control on the form, you might think that you are ready to compile and run your application.
Unfortunately, your application will not compile. You'll get a compiler error stating
that the DDX_FieldText cannot convert the COleDateTime variable type. What you need
to do is add the code to perform this conversion yourself. Return to the Class Wizard
and delete the variable that you added to the IDC_EDOB control. Add a new variable
to this control. Specify that the variable is type COleDateTime, and give the variable
a name such as m_oledtDOB. Pull up the DoDataExchange function in the view class,
CDbOdbcView, into the editor, and add lines 4 through 6 and lines 26 through 28 to
the function, as shown in Listing 14.2.</P>
<P>
<H4>LISTING 14.2. THE CDbOdbcView DoDataExchange FUNCTION.</H4>
<PRE>1:  void CDbOdbcView::DoDataExchange(CDataExchange* pDX)
2:  {
3:      CRecordView::DoDataExchange(pDX);
4:      // Copy the DOB from the record set to the view variable
5:      if (pDX-&gt;m_bSaveAndValidate == FALSE)
6:          m_oledtDOB = m_pSet-&gt;m_Birthdate;
7:      //{{AFX_DATA_MAP(CTestdb5View)
8:      DDX_FieldText(pDX, IDC_EID, m_pSet-&gt;m_AddressID, m_pSet);
9:      DDX_FieldText(pDX, IDC_EFNAME, m_pSet-&gt;m_FirstName, m_pSet);
10:     DDX_FieldText(pDX, IDC_ELNAME, m_pSet-&gt;m_LastName, m_pSet);
11:     DDX_FieldText(pDX, IDC_ESNAME, m_pSet-&gt;m_SpouseName, m_pSet);
12:     DDX_FieldText(pDX, IDC_ESTATE, m_pSet-&gt;m_StateOrProvince, m_pSet);
13:     DDX_FieldText(pDX, IDC_ECITY, m_pSet-&gt;m_City, m_pSet);
14:     DDX_FieldText(pDX, IDC_EADDR, m_pSet-&gt;m_Address, m_pSet);
15:     DDX_FieldCheck(pDX, IDC_CBCARD, m_pSet-&gt;m_SendCard, m_pSet);
16:     DDX_FieldText(pDX, IDC_ECOUNTRY, m_pSet-&gt;m_Country, m_pSet);
17:     DDX_FieldText(pDX, IDC_EEMAIL, m_pSet-&gt;m_EmailAddress, m_pSet);
18:     DDX_FieldText(pDX, IDC_EFAX, m_pSet-&gt;m_FaxNumber, m_pSet);
19:     DDX_FieldText(pDX, IDC_EHPHONE, m_pSet-&gt;m_HomePhone, m_pSet);
20:     DDX_FieldText(pDX, IDC_ENOTES, m_pSet-&gt;m_Notes, m_pSet);
21:     DDX_FieldText(pDX, IDC_EWEXT, m_pSet-&gt;m_WorkExtension, m_pSet);
22:     DDX_FieldText(pDX, IDC_EWPHONE, m_pSet-&gt;m_WorkPhone, m_pSet);
23:     DDX_FieldText(pDX, IDC_EZIP, m_pSet-&gt;m_PostalCode, m_pSet);
24:     DDX_Text(pDX, IDC_EDOB, m_oledtDOB);
25:     //}}AFX_DATA_MAP
26:     // Copy the DOB variable back from the view variable to the record         &Acirc;set
27:     if (pDX-&gt;m_bSaveAndValidate == TRUE)
28:         m_pSet-&gt;m_Birthdate = m_oledtDOB;
29: }
</PRE>
<P>In addition to the above change, you have to remove the initialization of the
m_Birthdate variable in the set class. This is also code that was added by the AppWizard,
and once again you have to break the rules by modifying the code that you are never
supposed to touch. To make this change, you can take the simple approach by commenting
out the initialization of this variable in the set class constructor, in line 19
of Listing 14.3.</P>
<P>
<H4>LISTING 14.3. THE CDbOdbcSet CONSTRUCTOR.</H4>
<PRE>1: CDbOdbcSet::CDbOdbcSet(CDatabase* pdb)
2:      : CRecordset(pdb)
3:  {
4:      //{{AFX_FIELD_INIT(CTestdb5Set)
5:      m_AddressID = 0;
6:      m_FirstName = _T(&quot;&quot;);
7:      m_LastName = _T(&quot;&quot;);
8:      m_SpouseName = _T(&quot;&quot;);
9:      m_Address = _T(&quot;&quot;);
10:     m_City = _T(&quot;&quot;);
11:     m_StateOrProvince = _T(&quot;&quot;);
12:     m_PostalCode = _T(&quot;&quot;);
13:     m_Country = _T(&quot;&quot;);
14:     m_EmailAddress = _T(&quot;&quot;);
15:     m_HomePhone = _T(&quot;&quot;);
16:     m_WorkPhone = _T(&quot;&quot;);
17:     m_WorkExtension = _T(&quot;&quot;);
18:     m_FaxNumber = _T(&quot;&quot;);
19:     //m_Birthdate = 0;
20:     m_SendCard = FALSE;
21:     m_Notes = _T(&quot;&quot;);
22:     m_nFields = 17;
23:     //}}AFX_FIELD_INIT
24:     m_nDefaultType = dynaset;
25: }
</PRE>
<P>Now compile and run your application once again. You'll find that you have a fully
functioning database application that retrieves a set of records from the database
and allows you to scroll through them and make changes to the data, as shown in Figure
14.9.</P>
<P>
<H3><A NAME="Heading8"></A>Adding New Records</H3>
<P>You've already created a fully functioning database application without writing
a single line of code. However, a few functions are missing. Most database applications
let the user add new records to the database. To add a new record to the database,
you'll want to figure out what the next ID number should be, so you'll scroll to
the last record in the set to get the ID and then increment it by one. Next, you'll
call the AddNew function to add a new record, set the ID field to the new ID you
calculated, and then call the Update function to save the new record. Finally, you'll
call the Requery function to refresh the set of records and then scroll to the last
record in the set to let the user enter data into the new record.</P>
<P><A HREF="javascript:popUp('14fig10.gif')"><B>FIGURE 14.9.</B></A><B> </B><I>The
running appli- cation.</I></P>


<BLOCKQUOTE>
	<P>
<HR>
<STRONG>TIP:</STRONG> Because the ID field in the database in defined as an AutoIncrement
	field, you do not normally specify your own ID for the field. However, because the
	record set is creating a new record with the ID field, you need to assign a valid
	ID to the record or you won't be able to add it to the database. The method used
	in this application will not work with any multiuser database because each person
	would generate the same IDs for new records. In this situation, a centralized method
	for generating new IDs, such as a counter field in the database, is a better solution.
	The other option is to create a SQL statement to insert a new record into the database
	that was missing the ID field. This allows the auto-increment functionality to work
	correctly.
<HR>


</BLOCKQUOTE>

<P>To add this functionality to your application, start by adding a function to your
record set class to determine the next ID number to be used. Add a member function
to the record set class, CDbOdbcSet. Specify the function type as long, the function
declaration as GetMaxID, and the access as public. Edit the function, adding the
code in Listing 14.4.</P>
<P>
<H4>LISTING 14.4. THE CDbOdbcSet GetMaxID FUNCTION.</H4>
<PRE>1: long CDbOdbcSet::GetMaxID()
2: {
3:     // Move to the last record
4:     MoveLast();
5:     // return the ID of this record
6:     return m_AddressID;
7: }
</PRE>
<P>Next, you'll need a menu entry that the user can select to add a new record to
the database. Add a new menu entry to the Record menu. Configure the new menu entry
with the properties in Table 14.8.</P>
<P>
<H4>TABLE 14.8. MENU PROPERTY SETTINGS.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Object</I></TD>
		<TD ALIGN="LEFT"><I>Property</I></TD>
		<TD ALIGN="LEFT"><I>Setting</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Menu Entry		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDM_RECORD_NEW		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">N&amp;ew Record		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Prompt		</TD>
		<TD ALIGN="LEFT">Add a new record\nNew Record		</TD>
	</TR>
</TABLE>
</P>
<P>Using the Class Wizard, add an event-handler function for the COMMAND event message
for this menu to the view class, CDbOdbcView. Edit this function, adding the code
in Listing 14.5.</P>
<P>
<H4>LISTING 14.5. THE CDbOdbcView OnRecordNew FUNCTION.</H4>