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Saving and Restoring Work--File Access

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DOCUMENTE SIMILARE

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Character handling - Strings, Untyped pointers
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Statements: Selection Statements
Declarations, Definitions and Accessibility - Storage class specifiers
Input and Output: Standard Input and Output


Saving and Restoring Work--File Access

* Serialization




* The CArchive and CFile Classes

* The Serialize Function

* Making Objects Serializable

* Listing 13.3. Including the IMPLEMENT_SERIAL macro in the class implementation.

* Implementing a Serializable Class

* Creating a Serialized Application

* Creating a Serializable Class

* Building Support in the Document Class

* Adding Navigating and Editing Support in the View Class

* Summary

* Q&A

* Workshop

* Quiz

Most applications provide the user the option of saving what has been created. The creation can be a word-processing document, a spreadsheet, a drawing, or a set of data records. Today, you will explore how Visual C++ provides you with the means to implement this functionality easily. Today, you will learn

* How Visual C++ uses C++ streams to save information about your application

* How to store your application data in binary files

* How to make your application objects serializable

* How you can store variables of differing data types into a single file

Serialization

There are two parts of serialization. When application data is stored on the system drive in the form of a file, it is called serialization. When the application state is restored from the file, it is called deserialization. The combination of these two parts makes up the serialization of application objects in Visual C++.

The CArchive and CFile Classes

Serialization in Visual C++ applications is accomplished through the CArchive class. The CArchive class is designed to act as an input/output (I/O) stream for a CFile object, as shown in Figure 13.1. It uses C++ streams to enable efficient data flow to and from the file that is the storage of the application data. The CArchive class cannot exist without a CFile class object to which it is attached.

FIGURE 13.1. The CArchive class stores application data in a CFile object.

The CArchive class can store data in a number of types of files, all of which are descendants of the CFile class. By default, the AppWizard includes all the functionality to create and open regular CFile objects for use with CArchive. If you want or need to work with one of these other file types, you might need to add additional code to your application to enable the use of these different file types.

The Serialize Function

The CArchive class is used in the Serialize function on the document and data objects in Visual C++ applications. When an application is reading or writing a file, the document object's Serialize function is called, passing the CArchive object that is used to write to or read from the file. In the Serialize function, the typical logic to follow is to determine whether the archive is being written to or read from by calling the CArchive IsStoring or IsLoading functions. The return value from either of these two functions determines if your application needs to be writing to or reading from the CArchive class's I/O stream. A typical Serialize function in the view class looks like Listing 13.1.

LISTING 13.1. A TYPICAL Serialize FUNCTION.

1: void CAppDoc::Serialize(CArchive& ar)

2:

9: else

You can place a Serialize function in any classes you create so that you can call their Serialize function from the document Serialize function. If you place your custom objects into an object array, such as the CObArray that you used in your drawing application for the past three days, you can call the array's Serialize function from the document's Serialize function. The object array will, in turn, call the Serialize function of any objects that have been stored in the array.

Making Objects Serializable

When you created the CLine class on Day 10, 'Creating Single Document Interface Applications,' you had to add two macros before you could save and restore your drawings. These two macros, DECLARE_SERIAL and IMPLEMENT_SERIAL, include functionality in your classes that are necessary for the Serialize function to work correctly.

Including the DECLARE_SERIAL Macro

You must include the DECLARE_SERIAL macro in your class declaration, as shown in Listing 13.2. The DECLARE_SERIAL macro takes a single argument, the class name. This macro automatically adds to your class some standard function and operator declarations that are necessary for serialization to work correctly.

LISTING 13.2. INCLUDING THE DECLARE_SERIAL MACRO IN THE CLASS DECLARATION.

1: class CMyClass : public CObject

Including the IMPLEMENT_SERIAL Macro

You need to add the IMPLEMENT_SERIAL macro to the implementation of your class. This macro needs to appear outside any other class functions because it adds the code for the class functions that were declared with the DECLARE_SERIAL macro.

The IMPLEMENT_SERIAL macro takes three arguments. The first argument is the class name, as in the DECLARE_SERIAL macro. The second argument is the name of the base class, from which your class is inherited. The third argument is a version number that can be used to determine whether a file is the correct version for reading into your application. The version number, which must be a positive number, should be incremented each time the serialization method of the class is changed in any way that alters the data being written to or read from a file. A typical usage of the IMPLEMENT_SERIAL macro is provided in Listing 13.3.

Listing 13.3. Including the IMPLEMENT_SERIAL macro in the class implementation.

1: // MyClass.cpp: implementation of the CMyClass class.

2: //

3: //////////////////////////////////////////////////////////////////////

4:

5: #include 'stdafx.h'

6: #include 'MyClass.h'

7:

8: #ifdef _DEBUG

9: #undef THIS_FILE

10: static char THIS_FILE[]=__FILE__;

11: #define new DEBUG_NEW

12: #endif

14: IMPLEMENT_SERIAL (CMyClass, CObject, 1)

16: // Construction/Destruction

19: CMyClass::CMyClass()

23: CMyClass::~CMyClass()

Defining the Serialize Function

Along with the two macros, you need to include a Serialize function in your class. This function should be declared as a void function with a single argument (CArchive &ar), public access, and the virtual check box selected--producing the function declaration in Listing 13.2. When you implement the Serialize function for your class, you typically use the same approach as that used in the document class, shown in Listing 13.1, where you check to determine whether the file is being written to or read from.

Implementing a Serializable Class

When you begin designing a new application, one of the first things you need to design is how to store the data in the document class that your application will create and operate on. If you are creating a data-oriented application that collects sets of data from the user, much like a contact database application, how are you going to hold that data in the application memory? What if you are building a word processor application--how are you going to hold the document being written in the application memory? Or a spreadsheet? Or a painting program? Oryou get the idea.

Once you determine how you are going to design the data structures on which your application will operate, then you can determine how best to serialize your application and classes. If you are going to hold all data directly in the document class, all you need to worry about is writing the data to and reading the data from the CArchive object in the document's Serialize function. If you are going to create your own class to hold your application data, you need to add the serialization functionality to your data classes so that they can save and restore themselves.

In the application that you are going to build today, you will create a simple, flat-file database application that illustrates how you can combine a mixture of data types into a single data stream in the application serialization. Your application will display a few fields of data, some of which are variable-length strings, and others that are integer or boolean, and will save and restore them in a single data stream to and from the CArchive object.

Creating a Serialized Application

You can create your own classes, which can also be serialized, for use in an SDI or MDI application. In short, any application that works with any sort of data, whether a database or a document, can be serialized. Now you will create a simple, flat-file database application that you will serialize.

NOTE: A flat-file database is one of the original types of databases. It is a simple file-based database, with the records sequentially appended to the end of the previous record. It has none of the fancy relational functionality that is standard in most databases today. The database that you will build today is closer to an old dBASE or Paradox database, without any indexes, than to databases such as Access or SQL Server.

Creating the Application Shell

To get your application started, create a new AppWizard application. Give your application a name, such as Serialize, and click OK to start the AppWizard.

In the AppWizard, select to create a single document style application using the Document/View architecture. You can choose to include support for ActiveX controls in the third AppWizard step, although it's not really necessary for the example that you will build.

In the fourth step, be sure to specify the file extension for the files that your application will create and read. An example of a file extension that you might want to use is ser for serialize or fdb for flat-file database.

In the sixth AppWizard step, you need to specify which base class to use for the application view class. For a description of the different base classes available for inheriting the view class from, refer to Day 10 in the section 'The Document/View Architecture.' For the sample application you are building, because it will be a database application, you'll find it easiest to use CFormView as the base class from which your view class will be inherited. This enables you to use the dialog designer for your application view.

Once you finish making your way through the AppWizard and let the AppWizard create your application shell, you will see a large window canvas in the dialog designer as if you had created a dialog-style application, only without the OK and Cancel buttons, as shown in Figure 13.2.

FIGURE 13.2. The window designer for an SDI appli-cation.

Designing Your Application Window

After you create an SDI or MDI application where the view class is based on the CFormView class, you need to design your application view. Designing the view is much like designing the window layout for a dialog window, but you don't need to worry about including any buttons to close the window while either saving or canceling the work done by the user. With an SDI or MDI application, the functionality to save and exit the window is traditionally located on the application menus or on the toolbar. As a result, you need to include only the controls for the function that your application window will perform.

NOTE: If you are building dialog-style applications, the AppWizard doesn't provide any serialization code in your application shell. If you need to serialize a dialog-style application, you'll need to add all this code yourself.

For the sample application that you are building today, lay out controls on the window canvas as shown in Figure 13.3 using the control properties listed in Table 13.1.

FIGURE 13.3. The sample application window layout.

TABLE 13.1. CONTROL PROPERTY SETTINGS.

Object  Property Setting

Static Text

ID

IDC_STATIC

Caption &Name:

Edit Box ID IDC_ENAME

Static Text ID IDC_STATIC

Caption &Age

Edit Box ID IDC_EAGE

Static Text ID IDC_STATIC

Caption Marital Status:

Radio Button ID IDC_RSINGLE

Caption &Single

Group Checked

Radio Button ID IDC_RMARRIED

Caption &Married

Radio Button ID IDC_RDIVORCED

Caption &Divorced

Radio Button ID IDC_RWIDOW



Caption &Widowed

Check Box ID IDC_CBEMPLOYED

Caption &Employed

Button ID IDC_BFIRST

Caption &First

Button ID IDC_BPREV

Caption &Previous

Button ID IDC_BNEXT

Caption Nex&t

Button ID IDC_BLAST

Caption &Last

Static Text ID IDC_SPOSITION

Caption Record 0 of 0

When you were developing dialog-style applications or windows, you attached variables to the controls on the window in the dialog class. However, with an SDI or MDI application, which class do you create the variables in? Because the UpdateData function is a member of the CWnd class, and the view class is descended from the CWnd class, although the document is not, then the view class is the most logical place to add the variables that you will attach to the controls you placed on the window.

To attach variables to the controls in your sample application, open the Class Wizard and add variables to the controls, specifying that the place to add them is the view class (in this case, CSerializeView). For the sample application, add the variables in Table 13.2 to the controls specified.

TABLE 13.2. CONTROL VARIABLES.

Object  Name Category Type

IDC_CBEMPLOYED m_bEmployed Value BOOL

IDC_EAGE m_iAge Value int

IDC_ENAME m_sName Value CString

IDC_RSINGLE m_iMaritalStatus Value int

IDC_SPOSITION m_sPosition Value CString

If you examine the source code for the view class, you will notice that there is no OnDraw function. If you are using the CFormView ancestor class for your SDI or MDI application, you don't need to worry about the OnDraw function. Instead, you treat the view class very much as you would the dialog class in a dialog window or dialog-style application. The primary difference is that the data that you need to use to populate the controls on the window are not in the view class, but in the document class. As a result, you need to build the interaction between these two classes to pass the data for the controls back and forth.

Creating a Serializable Class

When you create a form-based application, it is assumed that your application will hold multiple records in the form and that the user will be able to scroll through the records to make changes. The user will be able to add additional records or even remove records from the record set. The challenge at this point in building this application is how you represent this set of records, supporting all the necessary functionality.

One approach is to create a class that would encapsulate each record, and then hold these records in an array, much as you did with the drawing application that you created and enhanced over the past few days. This class would need to descend from the CObject class and would need to contain variables for all the control variables that you added to the view class, along with methods to read and write all of these variables. Along with adding the methods to set and read all of the variables, you need to make the class serializable by adding the Serialize function to the class, as well as the two macros that complete the serialization of the class.

Creating the Basic Class

As you may remember from Day 10, when you want to create a new class, you can select the project in the Class View tab of the workspace pane, right-click the mouse button, and select New Class from the context menu. This opens the New Class dialog.

In the New Class dialog, you specify the type of class, whether it's an MFC class, and generic class, or a form class. To create a class that can contain one record's data, you most likely want to create a generic class. You'll learn more about how to determine which of these types of classes to create on Day 16, 'Creating Your Own Classes and Modules.' The other things that you need to do are give your class a name and specify the base class from which it will be inherited.

For your sample application, because the form that you created has information about a person, you might want to call your class something like CPerson. To be able to hold your class in the object array, you need to give it CObject as the base class. Just like on Day 10, the New Class dialog will claim that it cannot find the header with the base class in it and that you need to add this. Well, it's already included, so you can ignore this message. (On Day 16, you'll learn when you need to pay attention to this message.)

Once you create your new class, you'll need to add the variables for holding the data elements that will be displayed on the screen for the user. Following good object-oriented design, these variables will all be declared as private variables, where they cannot be directly manipulated by other classes. The variable types should match the variable types of the variables that are attached to the window controls in the view class.

With the sample application you are creating, you need to add the variables in Table 13.3.

TABLE 13.3. CLASS VARIABLES FOR THE CPerson CLASS.

Name

Type

m_bEmployed BOOL

m_iAge int

m_sName CString

m_iMaritalStatus int

NOTE: An inline function is a short C++ function in which, when the application is being compiled, the function body is copied in place of the function call. As a result, when the compiled application is running, the function code is executed without having to make a context jump to the function and then jump back once the function has completed. This reduces the overhead in the running application, increasing the execution speed slightly, but also makes the resulting executable application slightly larger. The more places the inline function is called, the larger the application will eventually get. For more information on inline functions, consult Appendix A, 'C++ Review.'

Adding Methods for Reading and Writing Variables

Once you create your class, you need to provide a means for reading and writing to the variables in the class. One of the easiest ways to provide this functionality is to add inline functions to the class definition. You create a set of inline functions to set each of the variables and then make another set for retrieving the current value of each variable.

If you want to implement the Get and Set variable functions for your CPerson class in the sample application that you are building, edit the Person.h header file, adding the lines in Listing 13.4.

LISTING 13.4. THE Get AND Set INLINE FUNCTION DECLARATIONS.

1: class CPerson : public CObject

2:

6: void SetMaritalStat(int iStat)

7: void SetAge(int iAge)

8: void SetName(CString sName)

9: // Functions for getting the current settings of the variables

BOOL GetEmployed()

int GetMaritalStatus()

int GetAge()

CString GetName()

CPerson();

virtual ~CPerson();

17: private:

BOOL m_bEmployed;

int m_iMaritalStatus;

int m_iAge;

CString m_sName;

After you have the methods for setting and retrieving the values of the variables in your custom class, you'll probably want to make sure that the variables are initialized when the class is first created. You can do this in the class constructor by setting each of the variables to a default value. For instance, in your sample application, you add the code in Listing 13.5 to the constructor of the CPerson class.

LISTING 13.5. THE CPerson CONSTRUCTOR.

1: CPerson::CPerson()

Serializing the Class

After you have your custom class with all variables defined and initialized, you need to make the class serializable. Making your class serializable involves three steps. The first step is adding the Serialize function to the class. This function writes the variable values to, and reads them back from, the CArchive object using C++ streams. The other two steps consist of adding the DECLARE_SERIAL and IMPLEMENT_SERIAL macros. Once you add these elements, your custom class will be serializable and ready for your application.

To add the Serialize function to your custom class, add a member function through the Class View tab in the workspace pane. Specify the function type as void, the function declaration as Serialize(CArchive &ar), and the access as public and check the Virtual check box. This should add the Serialize function and place you in the editor, ready to flesh out the function code.

In the Serialize function, the first thing you want to do is to call the ancestor's Serialize function. When you call the ancestor's function first, any foundation information that has been saved is restored first, providing the necessary support for your class before the variables in your class are restored. Once you call the ancestor function, you need to determine whether you need to read or write the class variables. You can do this by calling CArchive's IsStoring method. This function returns TRUE if the archive is being written to and FALSE if it's being read from. If the IsStoring function returns TRUE, you can use C++ I/O streams to write all your class variables to the archive. If the function returns FALSE, you can use C++ streams to read from the archive. In both cases, you must be certain to order the variables in the same order for both reading and writing. If you need more information about C++ streams, see Appendix A.

An example of a typical Serialize function for your sample custom class is shown in Listing 13.6. Notice that the CPerson variables are in the same order when writing to and reading from the archive.

LISTING 13.6. THE CPerson.Serialize FUNCTION.

1: void CPerson::Serialize(CArchive &ar)

2:

Once you have the Serialize function in place, you need to add the macros to your custom class. The first macro, DECLARE_SERIAL, needs to go in the class header and is passed the class name as its only argument.

For example, to add the DECLARE_SERIAL macro to the custom CPerson class in your sample application, you add the macro just below the start of the class declaration, where it will receive the default access for the class. You specify the class name, CPerson, as the only argument to the macro, as in Listing 13.7.

NOTE: The default access permission for functions and variables in C++ classes is public. All functions and variables that are declared before the first access declaration are public by default. You could easily add all of the public class functions and variables in this area of the class declaration, but explicitly declaring the access permission for all functions and variables is better practice--because that way, there is little to no confusion about the visibility of any of the class functions or variables.

NOTE: Most C++ functions need a semicolon at the end of the line of code. The two serialization macros do not, due to the C preprocessor, which replaces each of the macros with all of the code before compiling the application. It doesn't hurt to place the semicolons there; they are simply ignored.

LISTING 13.7. THE SERIALIZED CPerson CLASS DECLARATION.

1: class CPerson : public CObject

2:

7: void SetMaritalStat(int iStat)

8: void SetAge(int iAge)

9: void SetName(CString sName)

// Functions for getting the current settings of the variables

BOOL GetEmployed()

int GetMaritalStatus()

int GetAge()

CString GetName()

CPerson();

virtual ~CPerson();

18: private:

BOOL m_bEmployed;

int m_iMaritalStatus;

int m_iAge;



CString m_sName;

NOTE: In practice, if you read a file that was written using a previous version of the Serialize function in your class, your application will raise an exception, which you can then catch using standard C++ exception-handling techniques. This allows you to add code to your application to recognize and convert files created with earlier versions of your application. For information on C++ exception handling, see Appendix A.

To complete the serialization of your custom class, you need to add the IMPLEMENT_ SERIAL macro to the class definition. The best place to add this macro is before the constructor definition in the CPP file containing the class source code. This macro takes three arguments: the custom class name, the base class name, and the version number. If you make any changes to the Serialize function, you should increment the version number argument to the IMPLEMENT_SERIAL macro. This version number indicates when a file was written using a previous version of the Serialize function and thus may not be readable by the current version of the application.

To add the IMPLEMENT_SERIAL macro to your sample application, add it into the Person.cpp file just before the CPerson class constructor. Pass CPerson as the first argument (the class name), CObject as the second argument (the base class), and 1 as the version number, as in Listing 13.8.

LISTING 13.8. THE IMPLEMENT_SERIAL MACRO IN THE CPerson CODE.

1: // Person.cpp: implementation of the CPerson class.

2: //

3: //////////////////////////////////////////////////////////////////////

4:

5: #include 'stdafx.h'

6: #include 'Serialize.h'

7: #include 'Person.h'

8:

9: #ifdef _DEBUG

10: #undef THIS_FILE

11: static char THIS_FILE[]=__FILE__;

12: #define new DEBUG_NEW

13: #endif

15: IMPLEMENT_SERIAL (CPerson, CObject, 1)

17: // Construction/Destruction

20: CPerson::CPerson()

Building Support in the Document Class

When you build a form-based application, where the form on the window is the primary place for the user to interact with the application, there is an unstated assumption that your application will allow the user to work with a number of records. This means that you need to include support for holding and navigating these records. The support for holding the records can be as simple as adding an object array as a variable to the document class, as you did back on Day 10. This allows you to add additional record objects as needed. The navigation could be a number of functions for retrieving the first, last, next, or previous record objects. Finally, you need informational functionality so that you can determine what record in the set the user is currently editing.

To hold and support this functionality, the document class will probably need two variables, the object array and the current record number in the array. These two variables will provide the necessary support for holding and navigating the record set.

For your example, add the two variables for supporting the record set of CPerson objects as listed in Table 13.4. Specify private access for both variables.

TABLE 13.4. DOCUMENT CLASS VARIABLES.

Name

Type

m_iCurPosition int

m_oaPeople CObArray

The other thing that you need to do to the document class to provide support for the record objects is make sure that the document knows about and understands the record object that it will be holding. You do this by including the custom class header file before the header file for the document class is included in the document class source code file. Because the document class needs to trigger actions in the view class, it's a good idea to also include the header file for the view class in the document class.

To include these header files in your sample application, open the source-code file for the document class and add the two #include statements as shown in Listing 13.9.

LISTING 13.9. INCLUDING THE CUSTOM AND VIEW CLASSES IN THE DOCUMENT CLASS IMPLEMENTATION.

1: // SerializeDoc.cpp : implementation of the CSerializeDoc class

2: //

3:

4: #include 'stdafx.h'

5: #include 'Serialize.h'

6:

7: #include 'Person.h'

8: #include 'SerializeDoc.h'

9: #include 'SerializeView.h'

11: #ifdef _DEBUG

12: #define new DEBUG_NEW

13: #undef THIS_FILE

14: static char THIS_FILE[] = __FILE__;

15: #endif

18: // CSerializeDoc

Adding New Records

Before you can navigate the record set, you need to be able to add new records to the object array. If you add a private function for adding new records, you can add new records to the set dynamically as new records are needed. Because new records should be presenting the user with blank or empty data fields, you don't need to set any of the record variables when adding a new record to the object array, so you can use the default constructor.

Following the same logic that you used to add new line records on Day 10, you should add a new person record to the object array in your document class in today's sample application. Once you add a new record, you can return a pointer to the new record so that the view class can directly update the variables in the record object.

Once the new record is added, you will want to set the current record position marker to the new record in the array. This way, the current record number can easily be determined by checking the position counter.

If there are any problems in creating the new person record object, let the user know that the application has run out of available memory and delete the allocated object, just as you did on Day 10.

To add this functionality to your sample application, add a new member function to the document class. Specify the type as a pointer to your custom class. If you named your custom class CPerson, the function type is CPerson*. This function needs no arguments. Give the function a name that reflects what it does, such as AddNewRecord. Specify the access for this function as private because it will only be accessed from other functions within the document class. You can edit the resulting function, adding the code in Listing 13.10.

LISTING 13.10. THE CSerializeDoc.AddNewRecord FUNCTION.

1: CPerson * CSerializeDoc::AddNewRecord()

2:

// Did we run into a memory exception?

catch (CMemoryException* perr)

// Delete the exception object

perr->Delete();

}

return pPerson;

Getting the Current Position

To aid the user in navigating the record set, it's always helpful to provide a guide about where the user is in the record set. To provide this information, you need to be able to get the current record number and the total number of records from the document to display for the user.

The functions to provide this information are both fairly simple. For the total number of records in the object array, all you need to do is get the size of the array and return that to the caller.

For your sample application, add a new member function to the document class. Specify the function type as int, the function name as GetTotalRecords, and the access as public. Once you add the function, edit it using the code in Listing 13.11.

LISTING 13.11. THE CSerializeDoc.GetTotalRecords FUNCTION.

1: int CSerializeDoc::GetTotalRecords()

Getting the current record number is almost just as simple. If you are maintaining a position counter in the document class, this variable contains the record number that the user is currently editing. As a result, all you need to do is return the value of this variable to the calling routine. Because the object array begins with position 0, you probably need to add 1 to the current position before returning to display for the user.

To add this function to your sample application, add another new member function to the document class. Specify the type as int, the function name as GetCurRecordNbr, and the access as public. Edit the function using the code in Listing 13.12.

LISTING 13.12. THE CSerializeDoc.GetCurRecordNbr FUNCTION.

1: int CSerializeDoc::GetCurRecordNbr()

Navigating the Record Set

To make your application really useful, you will need to provide the user with some way of navigating the record set. A base set of functionality for performing this navigation is a set of functions in the document class to get pointers to specific records in the record set. First is a function to get a pointer to the current record. Next are functions to get pointers to the first and last records in the set. Finally, you need functions to get the previous record in the set and the next record in the set. If the user is already editing the last record in the set and attempts to move to the next record, you can automatically add a new record to the set and provide the user with this new, blank record.

To add all this functionality, start with the function to return the current record. This function needs to check the value in the position marker to make sure that the current record is a valid array position. Once it has made sure that the current position is valid, the function can return a pointer to the current record in the array.

To add this function to your sample application, add a new member function to the document class. Specify the function type as CPerson* (a pointer to the custom class), the function name as GetCurRecord, and the access as public. Edit the function, adding the code in Listing 13.13.

LISTING 13.13. THE CSerializeDoc.GetCurRecord FUNCTION.

1: CPerson* CSerializeDoc::GetCurRecord()

2:

The next function you might want to tackle is the function to return the first record in the array. In this function, you need to first check to make sure that the array has records. If there are records in the array, set the current position marker to 0 and return a pointer to the first record in the array.

To add this function to your sample application, add a new member function to the document class. Specify the function type as CPerson* (a pointer to the custom class), the function name as GetFirstRecord, and the access as public. Edit the function, adding the code in Listing 13.14.

LISTING 13.14. THE CSerializeDoc.GetFirstRecord FUNCTION.

1: CPerson* CSerializeDoc::GetFirstRecord()

2:

else

// No records, return NULL

return NULL;

For the function to navigate to the next record in the set, you need to increment the current position marker and then check to see if you are past the end of the array. If you are not past the end of the array, you need to return a pointer to the current record in the array. If you are past the end of the array, you need to add a new record to the end of the array.

To add this function to your sample application, add a new member function to the document class. Specify the function type as CPerson* (a pointer to the custom class), the function name as GetNextRecord, and the access as public. Edit the function, adding the code in Listing 13.15.

LISTING 13.15. THE CSerializeDoc.GetNextRecord FUNCTION.

1: CPerson * CSerializeDoc::GetNextRecord()

2:

For the function to navigate to the previous record in the array, you need to make several checks. First, you need to verify that the array has records. If there are records in the array, you need to decrement the current position marker. If the marker is less than zero, you need to set the current position marker to equal zero, pointing at the first record in the array. Once you've made it through all of this, you can return a pointer to the current record in the array.

To add this function to your sample application, add a new member function to the document class. Specify the function type as CPerson* (a pointer to the custom class), the function name as GetPrevRecord, and the access as public. Edit the function, adding the code in Listing 13.16.

LISTING 13.16. THE CSerializeDoc.GetPrevRecord FUNCTION.

1: CPerson * CSerializeDoc::GetPrevRecord()

2:

else

// No records, return NULL

return NULL;

For the function that navigates to the last record in the array, you still need to check to make sure that there are records in the array. If the array does have records, you can get the current size of the array and set the current position marker to one less than the number of records in the array. This is actually the last record in the array because the first record in the array is record 0. Once you set the current position marker, you can return a pointer to the last record in the array.

To add this function to your sample application, add a new member function to the document class. Specify the function type as CPerson* (a pointer to the custom class), the function name as GetLastRecord, and the access as public. Edit the function, adding the code in Listing 13.17.

LISTING 13.17. THE CSerializeDoc.GetLastRecord FUNCTION.

1: CPerson * CSerializeDoc::GetLastRecord()

2:

else

// No records, return NULL

return NULL;

Serializing the Record Set

When filling in the Serialize functionality in the document class, there's little to do other than pass the CArchive object to the object array's Serialize function, just as you did on Day 10.

When reading data from the archive, the object array will query the CArchive object to determine what object type it needs to create and how many it needs to create. The object array will then create each object in the array and call its Serialize function, passing the CArchive object to each in turn. This enables the objects in the object array to read their own variable values from the CArchive object in the same order that they were written.

When writing data to the file archive, the object array will call each object's Serialize function in order, passing the CArchive object (just as when reading from the archive). This allows each object in the array to write its own variables into the archive as necessary.

For the sample application, edit the document class's Serialize function to pass the CArchive object to the object array's Serialize function, as in Listing 13.18.

Listing 13.18. THE CSerializeDoc.Serialize FUNCTION.

1: void CSerializeDoc::Serialize(CArchive& ar)

Cleaning Up

Now you need to add the code to clean up the document once the document is closed or a new document is opened. This consists of looping through all objects in the object array and deleting each and every one. Once all the objects are deleted, the object array can be reset when you call its RemoveAll function.

To implement this functionality in your sample application, add an event-handler function to the document class on the DeleteContents event message using the Class Wizard. When editing the function, add the code in Listing 13.19.

LISTING 13.19. THE CSerializeDoc.DeleteContents FUNCTION.



1: void CSerializeDoc::DeleteContents()

2:

///////////////////////

// MY CODE ENDS HERE

///////////////////////

CDocument::DeleteContents();

NOTE: One thing to keep in mind when writing this code is that you need to cast the pointer to the view as a pointer of the class of your view object. The GetNextView function returns a pointer of type CView, so you will not be able to call any of your additions to the view class until you cast the pointer to your view class. Casting the pointer tells the compiler that the pointer is really a pointer to your view object class and thus does contain all the functions that you have added. If you don't cast the pointer, the compiler will assume that the view object does not contain any of the functions that you have added and will not allow you to compile your application.

Opening a New Document

When a new document is started, you need to present the user with an empty form, ready for new information. To make that empty record ready to accept new information, you need to add a new record into the object array, which is otherwise empty. This results in only one record in the object array. Once the new record is added to the array, you must modify the view to show that a new record exists; otherwise, the view will continue to display the last record edited from the previous record set (and the user will probably wonder why your application didn't start a new record set).

To implement this functionality, you will need to edit the OnNewDocument function in your document class. This function is already in the document class, so you do not need to add it through the Class Wizard. The first thing that you do in this function is add a new record to the object array. Once the new record is added, you need to get a pointer to the view object. You use the GetFirstViewPosition function to get the position of the view object. Using the position returned for the view object, you can use the GetNextView function to retrieve a pointer to the view object. Once you have a valid pointer, you can use it to call a function that you will create in the view class to tell the view to refresh the current record information being displayed in the form.

Locate the OnNewDocument function in the document class source code, and add the code in Listing 13.20. Before you will be able to compile your application, you will need to add the NewDataSet function to the view class.

LISTING 13.20. THE CSerializeDoc.OnNewDocument FUNCTION.

1: BOOL CSerializeDoc::OnNewDocument()

2:

When opening an existing data set, you don't need to add any new records, but you still need to let the view object know that it needs to refresh the record being displayed for the user. As a result, you can add the same code to the OnOpenDocument function as you added to the OnNewDocument, only leaving out the first part where you added a new record to the object array.

Add an event-handler function to the document class for the OnOpenDocument event using the Class Wizard. Once you add the function, edit it adding the code in Listing 13.21.

LISTING 13.21. THE CSerializeDoc.OnOpenDocument FUNCTION.

1: BOOL CSerializeDoc::OnOpenDocument(LPCTSTR lpszPathName)

2:

Adding Navigating and Editing Support in the View Class

Now that you've added support for the record set to your document class, you need to add the functionality into the view class to navigate, display, and update the records. When you first designed your view class, you placed a number of controls on the window for viewing and editing the various data elements in each record. You also included controls for navigating the record set. Now you need to attach functionality to those controls to perform the record navigation and to update the record with any data changes the user makes.

Because of the amount of direct interaction that the form will have with the record object--reading variable values from the record and writing new values to the record--it makes sense that you want to add a record pointer to the view class as a private variable. For your example, add a new member variable to the view class, specify the type as CPerson*, give it a name such as m_pCurPerson, and specify the access as private. Next, edit the view source code file and include the header file for the person class, as in Listing 13.22.

LISTING 13.22. INCLUDING THE CUSTOM OBJECT HEADER IN THE VIEW CLASS SOURCE CODE.

1: // SerializeView.cpp : implementation of the CSerializeView class

2: //

3:

4: #include 'stdafx.h'

5: #include 'Serialize.h'

6:

7: #include 'Person.h'

8: #include 'SerializeDoc.h'

9: #include 'SerializeView.h'

11: #ifdef _DEBUG

Displaying the Current Record

The first functionality that you will want to add to the view class is the functionality to display the current record. Because this functionality will be used in several different places within the view class, it makes the most sense to create a separate function to perform this duty. In this function, you get the current values of all the variables in the record object and place those values in the view class variables that are attached to the controls on the window. The other thing that you want to do is get the current record number and the total number of records in the set and display those for the user so that the user knows his or her relative position within the record set.

In your sample application, add a new member function, specify the function type as void, give the function a name that makes sense, such as PopulateView, and specify the access as private. In the function, get a pointer to the document object. Once you have a valid pointer to the document, format the position text display with the current record number and the total number of records in the set, using the GetCurRecordNbr and GetTotalRecords functions that you added to the document class earlier. Next, if you have a valid pointer to a record object, set all the view variables to the values of their respective fields in the record object. Once you set the values of all of the view class variables, update the window with the variable values, as shown in Listing 13.23.

LISTING 13.23. THE CSerializeView.PopulateView FUNCTION.

1: void CSerializeView::PopulateView()

2:

// Do we have a valid record object?

if (m_pCurPerson)

// Update the display

UpdateData(FALSE);

Navigating the Record Set

If you added navigation buttons to your window when you were designing the form, then adding navigation functionality is a simple matter of adding event-handler functions for each of these navigation buttons and calling the appropriate navigation function in the document. Once the document navigates to the appropriate record in the set, you need to call the function you just created to display the current record. If the document navigation functions are returning pointers to the new current record object, you should capture that pointer before calling the function to display the current record.

To add this functionality to your sample application, add an event handler to the clicked event for the First button using the Class Wizard. In the function, get a pointer to the document object. Once you have a valid pointer to the document, call the document object's GetFirstRecord function, capturing the returned object pointer in the view CPerson pointer variable. If you receive a valid pointer, call the PopulateView function to display the record data, as in Listing 13.24.

LISTING 13.24. THE CSerializeView.OnBfirst FUNCTION.

1: void CSerializeView::OnBfirst()

2:

}

For the Last button, perform the same steps as for the First button, but call the document object's GetLastRecord function, as in Listing 13.25.

LISTING 13.25. THE CSerializeView.OnBlast FUNCTION.

1: void CSerializeView::OnBlast()

2:

}

For the Previous and Next buttons, repeat the same steps again, but call the document object's GetPrevRecord and GetNextRecord functions. This final step provides your application with all the navigation functionality necessary to move through the record set. Also, because calling the document's GetNextRecord on the last record in the set automatically adds a new record to the set, you also have the ability to add new records to the set as needed.

Saving Edits and Changes

When the user enters changes to the data in the controls on the screen, these changes somehow need to make their way into the current record in the document. If you are maintaining a pointer in the view object to the current record object, you can call the record object's various set value functions, passing in the new value, to set the value in the record object.

To implement this in your sample application, add an event handler to the CLICKED event for the Employed check box using the Class Wizard. In the function that you created, first call the UpdateData to copy the values from the form to the view variables. Check to make sure that you have a valid pointer to the current record object, and then call the appropriate Set function on the record object (in this case, the SetEmployed function as in Listing 13.26).

LISTING 13.26. THE CSerializeView.OnCbemployed FUNCTION.

1: void CSerializeView::OnCbemployed()

2:

Repeat these same steps for the other controls, calling the appropriate record object functions. For the Name and Age edit boxes, you add an event handler on the EN_CHANGE event and call the SetName and SetAge functions. For the marital status radio buttons, add an event handler for the BN_CLICKED event and call the same event-handler function for all four radio buttons. In this function, you call the SetMaritalStat function in the record object.

Displaying a New Record Set

The last functionality that you need to add is the function to reset the view whenever a new record set is started or opened so that the user doesn't continue to see the old record set. You will call the event handler for the First button, forcing the view to display the first record in the new set of records.

To implement this functionality in your sample application, add a new member function to the view class. Specify the function type as void, give the function the name that you were calling from the document object (NewDataSet), and specify the access as public (so that it can be called from the document class). In the function, call the First button event handler, as in Listing 13.27.

LISTING 13.27. THE CSerializeView.NewDataSet FUNCTION.

1: void CSerialize1View::NewDataSet()

Wrapping Up the Project

Before you can compile and run your application, you need to include the header file for your custom class in the main application source-code file. This file is named the same as your project with the CPP extension. Your custom class header file should be included before the header files for either the document or view classes. For your sample application, you edit the Serialize.cpp file, adding line 8 in Listing 13.28.

FIGURE 13.4. The running serialization application.

LISTING 13.28. INCLUDING THE RECORD CLASS HEADER IN THE MAIN SOURCE FILE.

1: // Serialize.cpp : Defines the class behaviors for the application.

2: //

3:

4: #include 'stdafx.h'

5: #include 'Serialize.h'

6:

7: #include 'MainFrm.h'

8: #include 'Person.h'

9: #include 'SerializeDoc.h'

10: #include 'SerializeView.h'

12: #ifdef _DEBUG

At this point, you can add, edit, save, and restore sets of records with your application. If you compile and run your application, you can create records of yourself and all your family members, your friends, and anyone else you want to include in this application. If you save the record set you create and then reopen the record set the next time that you run your sample application, you should find that the records are restored back to the state that you originally entered them, as in Figure 13.4.

Summary

Today, you learned quite a bit. You learned how serialization works and what it does. You learned how to make a custom class serializable and why and how to use the two macros that are necessary to serialize a class. You also learned how to design and build a form-based SDI application, maintaining a set of records in a flat-file database for use in the application. You learned how to use serialization to create and maintain the flat-file database and how to construct the functionality in the document and view classes to provide navigating and editing capabilities on these record sets.

Q&A

Q If I make any changes to one of the records in my record set after I save the record set and then I close the application, or open a different set of records, my application doesn't ask if I want to save my changes. How do I get it to ask me? How do I get my application to prompt for saving when data has been changed?

A One function call in the AddNewRecord function in the document object is the key to this problem. After adding a new record to the object array, you call the SetModifiedFlag function. This function marks the document as 'dirty.' When you save the record set, the document is automatically set to a 'clean' state (unless the application is unable to save the record set for any reason). What you need to do when saving the edits is set the document to the 'dirty' state so that the application knows that the document has unsaved changes.

You can fix this by adding some code to each of your data control event handlers. Once you save the new value to the current record, get a pointer to the document object and call the document's SetModifiedFlag function, as in Listing 13.29. If you make this same addition to all the data change event handlers, your application will ask you whether to save the changes you made since the last time the record set was saved.

LISTING 13.29. THE MODIFIED CSerializeView.OnCbemployed FUNCTION.

1: void CSerializeView::OnCbemployed()

2:

Q Why do I need to change the version number in the IMPLEMENT_SERIAL macro if I change the Serialize function in the record custom class?

A Whether you need to increment the version number depends on the type of change you make. For instance, if you add a calculated field in the record class and you add the code to calculate this new variable from the values you read in the variables from the CArchive object, then you don't really need to increment the version number because the variables and order of the variables that you are writing to and reading from the archive did not change. However, if you add a new field to the record class and add the new field into the I/O stream being written to and read from the CArchive object, then what you are writing to and reading from the archive will have changed, and you do need to increment the version number. If you don't increment the version number, reading files created using the previous version of your application will result in an 'Unexpected file format' message instead of the file being read. Once you increment the version number and you read a file written with the old version number, you get the same message, but you have the option of writing your own code to handle the exception and redirecting the archive to a conversion routine to convert the file to the new file format.

FIGURE 13.5. The running serialization application with the person's sex.

Workshop

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, 'Answers.'

Quiz

1. What two macros do you have to add to a class to make it serializable?

2. How can you determine whether the CArchive object is reading from or writing to the archive file?

3. What arguments do you need to pass to the IMPLEMENT_SERIAL macro?

4. What class do you need to inherit the view class from to be able to use the dialog designer to create a form for the main window in an SDI or MDI application?

5. What type of file does the CArchive write to by default?

Exercise

Add a couple of radio buttons to the form to specify the person's sex, as shown in Figure 13.5. Incorporate this change into the CPerson class to make the field persistent.






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