ch14.htm

VC 21天 学习VC 的好东西

<PRE>1:  void CDbOdbcView::OnRecordNew() 
2:  {
3:      // TODO: Add your command handler code here
4:      // Get a pointer to the record set
5:      CRecordset* pSet = OnGetRecordset();
6:      // Make sure that any changes to the current record
7:      // have been saved
8:      if (pSet-&gt;CanUpdate() &amp;&amp; !pSet-&gt;IsDeleted())
9:      {
10:         pSet-&gt;Edit();
11:         if (!UpdateData())
12:             return;
13:
14:         pSet-&gt;Update();
15:     }
16:     // Get the ID for the new record
17:     long m_lNewID = m_pSet-&gt;GetMaxID() + 1;
18:     // Add the new record
19:     m_pSet-&gt;AddNew();
20:     // Set the ID in the new record
21:     m_pSet-&gt;m_AddressID = m_lNewID;
22:     // Save the new record
23:     m_pSet-&gt;Update();
24:     // Refresh the record set
25:     m_pSet-&gt;Requery();
26:     // Move to the new record
27:     m_pSet-&gt;MoveLast();
28:     // Update the form
29:     UpdateData(FALSE);
30: }
</PRE>
<P>Add a new toolbar button for the New Record menu, and then compile and run your
application. You should be able to add new records to the database, entering the
data you want into the records.</P>
<P>
<H3><A NAME="Heading9"></A>Deleting Records</H3>
<P>The only functionality remaining is the ability to delete the current record from
the database. You'll need to add another menu entry to trigger this action. Once
the action is triggered, you'll verify that the user really does want to delete the
current record and then call the Delete function to remove the record. Once the record
has been deleted, you'll call the MovePrev function to navigate to the previous record
in the set.</P>
<P>To add this functionality to your application, you'll need a menu entry that the
user can select to delete the current record from the database. Add a new menu entry
to the Record menu. Configure the new menu entry with the properties in Table 14.9.</P>
<P>
<H4>TABLE 14.9. 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">
			<PRE>Menu Entry</PRE>
		</TD>
		<TD ALIGN="LEFT">
			<PRE>ID</PRE>
		</TD>
		<TD ALIGN="LEFT">
			<PRE>IDM_RECORD_DELETE</PRE>
		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">&amp;Delete Record		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Prompt		</TD>
		<TD ALIGN="LEFT">Delete the current record\nDelete 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 14.6.</P>
<P>
<H4>LISTING 14.6. THE CDbOdbcView OnRecordDelete FUNCTION.</H4>
<PRE>1:  void CTestdb5View::OnRecordDelete() 
2:  {
3:      // TODO: Add your command handler code here
4:      // Make sure the user wants to delete this record
5:      if (MessageBox(&quot;Are you sure you want to delete this record?&quot;,
6:              &quot;Delete this record?&quot;, MB_YESNO | MB_ICONQUESTION) ==                   &Acirc;IDYES)
7:      {
8:          // Delete the record
9:          m_pSet-&gt;Delete();
10:         // Move to the previous record
11:         m_pSet-&gt;MovePrev();
12:         // Update the form
13:         UpdateData(FALSE);
14:     }
15: }
</PRE>
<P>Add another button to the toolbar and associate it with the IDM_RECORD_DELETE
menu ID so that the user can delete the current record without having to go to the
menu. If you compile and run your application at this point, you'll have a full-function
database application in which you can add, edit, and delete records, as shown in
Figure 14.10.</P>
<P><A HREF="javascript:popUp('14fig11.gif')"><B>FIGURE 14.10.</B></A><B> </B><I>The
completed application.</I></P>

<P><I></I>
<H2><A NAME="Heading10"></A>Summary</H2>
<P>Today, you learned how you can use the ODBC interface to build database applications
that can be easily run against any database you might need to use. You saw how the
CRecordset class provides you with a substantial amount of functionality so that
you can provide database functionality in your applications. You also saw how the
AppWizard provides you with a large amount of database functionality without your
typing a single line of code.</P>
<P>Tomorrow, you will learn about Microsoft's newest database access technology,
ActiveX Data Objects, and how this can be combined with the ODBC interface to make
your database access even easier.</P>
<P>
<H2><A NAME="Heading11"></A>Q&amp;A</H2>

<DL>
	<DT></DT>
	<DD><B>Q Why would I want to use the ODBC interface instead of the Data Access Objects?</B>
	<P>
	<DT><B></B></DT>
	<DD><B>A</B> The Data Access Objects (DAO) use the Microsoft Jet database engine
	to perform all of the database access. This adds at least a megabyte of overhead
	to your application, and if you're using a SQL-based database, the database is already
	doing all of the work that the Jet engine is doing for you. What's more, the Jet
	database engine uses the ODBC interface to access any SQL-based databases. As a result,
	unless you are using PC-based databases, such as Access, FoxPro, or Paradox, you
	get better performance from going directly to the ODBC interface yourself.
	<P>
	<DT></DT>
	<DD><B>Q How can I add different record sets in an MDI application?</B>
	<P>
	<DT><B></B></DT>
	<DD><B>A</B> You can add additional CRecordset-derived classes through the New Class
	Wizard in an MDI application project. You need to specify that the new class is an
	MFC class and that its base class is the CRecordset class. The New Class Wizard will
	have you specify the data source, just as the AppWizard had you do when creating
	the shell for today's application. Once you create the record set class, you can
	create a new view class the same way, specifying the base class as CRecordView. Once
	you click the OK button, the New Class Wizard asks you to specify which of the record
	set classes to use with the new record view class.
	<P>
</DL>

<H2><A NAME="Heading12"></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 and exercises are provided in Appendix
B, &quot;Answers.&quot;</P>
<P>
<H3><A NAME="Heading13"></A>Quiz</H3>

<DL>
	<DT></DT>
	<DD><B>1. </B>What does ODBC stand for?
	<P>
	<DT></DT>
	<DD><B>2. </B>What functions can you use to navigate the record set in a CRecordset
	object?
	<P>
	<DT></DT>
	<DD><B>3. </B>What view class should you use with an ODBC application?
	<P>
	<DT></DT>
	<DD><B>4. </B>What sequence of functions do you need to call to add a new record
	to a record set?
	<P>
	<DT></DT>
	<DD><B>5. </B>What function do you need to call before the fields in the CRecordset
	object can be updated with any changes?
	<P>
</DL>

<H3><A NAME="Heading14"></A>Exercise</H3>
<P>Add a menu entry and dialog to let the user indicate the record number to move
to, and then move to that record.</P>
<P>
<H2><A NAME="Heading15"></A>In Review</H2>
<P>Now that you've finished the second week, you should be getting very comfortable
working with Visual C++. You should be beginning to understand how you can use the
MFC class hierarchy to provide a substantial amount of existing functionality in
your applications. You should also be starting to understand how much supporting
infrastructure your applications start with when you use the Visual C++ wizards to
construct as much of your application as you can.</P>
<P>This is a good time to take a little break and try some of the things that you've
learned on your own. Build an MDI application, using a custom document type that
you've come up with yourself. See how you can save and restore the document, as well
as maintain it. Practicing on your own is key to cementing your understanding of
what you've learned in this book. This will help you identify any areas that you
might need to go back and read again, as well as those areas where you feel comfortable
enough to not review.</P>
<P>By this time, you should have a good understanding of the Document/View architecture
and how it can be used to maintain the separation of the data from the representation
of the data that is displayed for the user. You've used this model for both Single
Document Interface (SDI) and Multiple Document Interface (MDI) style applications,
and you've used it for reading and writing files to the disk drive. This model is
one of the main building blocks of MFC applications built with Visual C++. You should
know where to place any initialization information for a new set of data and where
to clean up when closing a set of data.</P>
<P>You should also have a good understanding of how the SDI and MDI application styles
are alike and how they differ from each other and from the dialog application style.
You should have a good idea of when an application you are building should use one
of these styles and when it should use a different style. You should be able to create
your own SDI and MDI applications, as you need to, without any significant problems.
If you've got any questions about either of these areas, you might want to take another
look at Days 10 and 11 to review how the Document/View architecture works in both
SDI and MDI style applications.</P>
<P>You should understand how, in SDI and MDI style applications, you can save and
restore complex data structures in files on the system hard drive. You should be
able to create mixed-type objects that you create and maintain in the document object
in your applications, be able to use the Serialize function with the CArchive object
to write the objects to a file, and then be able to restore the objects at a later
time. If you are having any trouble understanding how this works or are running into
any problems trying to implement this functionality in your own applications, review
Day 13.</P>
<P>Along with reading and writing files, you also have learned how you can design
and build toolbars for use in your SDI and MDI applications. At this point, you should
be completely comfortable with designing and creating your own toolbars and using
them in your applications. You should understand the importance of matching the toolbar
button ID to the ID of the menu for which the toolbar will be used as a substitute.
You should also have a basic understanding of creating and using your own customized
status bar elements in SDI and MDI applications. You should understand how you can
use the UPDATE_COMMAND_UI event message to evaluate and alter the status of menu,
toolbar, and status bar elements, relieving you of all the work of setting each of
these elements, and how to maintain their appearance and status yourself. If you
aren't clear on how you can do any of these things, you might want to go back over
Day 12 one more time.</P>
<P>You've seen how you can build a simple database application, pulling data from
a database through the ODBC interface. You should have a basic understanding of how
you can build database applications using this approach, how to maintain the data,
how to add new records, and how to delete records. You should know how all the database
interaction is directed through the record set class and how you can directly control
the data through this object. If you're not sure of some of this, you might want
to look back at Day 14 for a quick refresher.</P>
<P>You learned how easy it is to add ActiveX controls to your projects and how Visual
C++ builds C++ classes around the control, enabling you to interact with the control
as if it were just another C++ object. You should have a good grasp of how to add
any ActiveX control (armed with the documentation for the control) to your application
and interact with it in a seamless manner. You should be able to declare a variable
for the control, set the control's properties, call its methods, and react to its
events just as if it were a standard part of the Visual C++ development environment.
If you aren't sure how you can do some of this, you might want to go back and reread
Day 9.</P>
<P>Finally, you started this week by learning how to draw graphics on the windows
of your applications. You learned how to draw lines, circles, and squares, using
a variety of pens and brushes. You even learned how you can make a customized brush
from a bitmap. You learned how you can load a bitmap image from a file and display
it for the user to see. But most importantly, you learned about the device context
and how it is used to draw all these features on the windows of your applications.
You should be able to use these and other figure drawing device context methods to
draw any image you might want to draw on the window for the user to see and interact
with. If you are unsure about how you can do this, you probably want to look back
at Day 8 once more.</P>
<P>By this time, you have built up quite a set of programming skills with Visual
C++. You are probably ready to tackle most of the smaller programming tasks you might
encounter--and maybe even a few not-so-small tasks. At this point, you are well on
your way to becoming an accomplished Visual C++ programmer. That said--now is not
the time to stop because there's still more to be learned. There's only one more
week to go, so tallyho!</P>

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