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Versionable Object’s Serialization in MDI applications

This article represents a follow-up of the last article, “Versionable Object’s Serialization using MFC in non Document View applications”. In that article I presented to you a way to solve incompatibility issues between different file versions of the same application, based on MFC serialization into a dialog base application.
But, the dialog base applications are not the best way to use and apply MFC serialization.
Applications base on document view architecture (MDI or SDI) are the best solution when we want to develop MFC application with serialization support.
Document View MFC architecture offers support for automatic save and load document to/from a file on a storage area, using a serialization mechanism. MDI (Multiple Document Interface) and SDI (Single Document Interface) application offers default serialization basic mechanism.

SerAddressBookMDI Main Window

The serialization is customizable. It’s important to define the right binary elements format, file version and element count. Finally, we have to complete the serialize method.
Into a document view application some document class’s methods are mapped over New, Open, Save and Save As items, available in File menu. The application’s user can use these commands in order to create or open files, tracking document status changes and serialize data to/from a file.
MDI applications create a CDocument derived class instance for each open document. SDI applications reuse the same single CDocument derived class instance for each open file.
In a MDI application CDocument class and the classes derived from this are responsible with internal objects serialization control. This class tracks each change that appears in our document. In this way, our application knows that some changes have been made when we accidentally want to close the application, without saving last changed data.
When a document is loaded, a CArchive instance is created for reading file internal data. When we create a new document, a CArchive store instance is created and this instance is used for store to a file process.
CArchive routines are strongly optimized in order to provide a viable store/load mechanism, even if we are serializing a huge number of small items.

In my demo application, I used the same idea as in my last article: an address book with two versions.

In current application the serialization process it’s very different then the old application. The serialization process is realized by a CDocument class instance that interaction with the rest application classes. CAddressBook class place was taken by the document class CSerAddressBookMDIDoc.

In a real application it’s recommended to use unique identifier (UID) in order to “detect” the right object. For simplicity, in my demo application this unique identifier was defined “name” attribute. For instance, I’m using it for a contact update process.

Document class – CSerAddressBookMDIDoc

The interface of document class looks like this:

As you can see, this time I’m using DECLARE_DYNCREATE() macro. This macro allows dynamic document objects runtime creation (a MDI application’s requirement).
In this class I reused some CAddressBook’s methods. These methods handle objects from m_cContactsList list.
ContactList is an alias for our Contacts MFC list:

The serialization data method of this document class is listed down:

This method read (load) or write (store) serialized Contact class’s object using a CArchive object at runtime. If the code flow runs over true branch all information is saving from our list to our file. If code flow choose else branch it means that we are loading an existing file and all file data is loaded in our list.

In order to store data in a file, initially I save file version (m_nFileVersionSchema) and items count. Then I iterate over all m_cContactsList items (Contact type item) and I serialize this data in order to store in my new file.

If I want to load data from a file, I am reading file version, I clean my list, I get Contact stored items count and as long as this count variable value is positive I serialized with load flag all Contact file data.
All serialized Contact entities go to m_cContactsList list. Each time we want to display our files data we have to iterate over this list.

Internal serialized class – Contact

As you have seen, in CSerAddressBookMDIDoc::Serialize() method, for both situations (store/load) a new Contact instance is created and this object is passed to Contact::Serialize() for load/store operation.
The serializing Contact items method looks like this:

If I want to store my data to a file, I obtain a runtime pointer to my serialized class, in order to set my file version schema. Then, depending on file version I serialize right object data and finally I reassign initial version schema value of my runtime class.

If I am loading a file, I call Contact::Serialize() method, I get file version schema and depending on schema value, I add right data to my document class.

View class – CSerAddressBookMDIView
This class is responsible with document class content (loaded file’s data) graphical representation. In my demo application, the view class is derived from CListView and has REPORT flag set in order to display data into a grid like.

Main responsible method with client window’s list control population is CSerAddressBookMDIView::PopulateList() and is listed down:

First, we obtain a pointer to our current document. Then, we call the method that inserts right list control columns, depending on file version (CreateViews() method).
We obtain a CListView pointer and an object reference to fist contact from contacts list. Then, as long as we have elements, we iterate over list’s elements (in a while() loop) and insert data to our list control.

PopulateList() method is called from overwrite CSerAddressBookMDIView::OnUpdate() method. OnUpdate() method is called by MFC framework as long as a document is changed.
The original OnUpdate() method is called by CDocument::UpdateAllViews() and is implemented in CView class.

In order to add/remove/update records from our documents I created a special dialog, launched from my Menu menu.
Display modal dialog method is listed down:

Because I have to interact from my dialog window with contacts list of current document, I have to pass the pointer of my document class ( dlg.SetAddressDocument(pDoc) ) to my dialog window class. If the dialog is closed using Exit button (IDOK id) then the view is refilled, using PopulateList() call.

CManipulateDataDlg class

This class is responsible with the management of document contact list items. The difference between this dialog class and the dialog class of last article is that this class is not responsible with load/store process. This role was taken by document view architecture.

Dialog’s control list population method looks like this:

Each time we clean the contact list we obtain a reference to the beginning of document contacts list. Depending on file version schema (1 or 2), dialog’s controls are customized. Then, we iterate over contact list elements (ContactList) and I insert data into my control list.

MDI support for many file extension

Default MDI applications come with only one file support and only one file extension file format.
Sometimes, our applications need to support different file format and more file extensions. In my demo application is necessary to support two file format and two file version (version 1 (*.sab1) and version 2 (*.sab2)).
Same time, the application must support old file format conversion to new file format and vice versa.
You can find multi file support detailed information for document view MFC application to Microsoft KB 141921. Other useful reference you can find here.
Starting from these references my application support two file format. I figure some important changes that I made into my initialize method, CSerAddressBookMDIApp::InitInstance.

First point that I should mention, after LoadStdProfileSettings() (function written by MFC wizard) call, is the initialization of m_pDocManager attribute (pointer to CDocManager class, used for document template management) with a new object pointer to CMultiDocManager (class defined be me according with Microsoft Knowledge Base 141921). CMultiDocManager class overwrites some methods from CDocManager: CreateNewDocument(), DoPromptFileName(), OnFileNew().

Then, besides default document application template (with resource ID IDR_SerAddressBookTYPE), I create two new templates for my two different files format.
All templates are added into my document template list (AddDocTemplate()). Last significant change from InitInstance() means the right frame window (IDR_SerAddressBookTYPE – contains Save and Save As options).

Conclusion:
Multiple Document Interface (MDI) architecture is the best for this kind of data container application. MFC framework offers stable and complete support for objects serialization: storing and loading process.
Many of Microsoft Office applications are based on this architecture.

Download demo application: SerAddressBookMDI (Visual C++ 2005 project)

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Versionable Object’s Serialization using MFC in non Document View applications

Most existing applications operate with data that must be stored and loaded in different times and different locations. The data is stored in text or binary files with a well defined format.
The Problem
Initially, in the first version 1.0, an application operates with data structures that can be stored and loaded. But, next version (2.0) these data structures suffers changes. Some structure’s attributes are added and other could be removed. These things change files format and structure when a new file version is saved.
Question: What happens when you are using an application version 2.0 and you are trying to load old files format (version 1.0)?
Answer: Most cases should cause incompatibility troubles between the current new application and files in old format. This could throw exceptions and the application could have undefined behavior.
That’s why the application must be written in order to be able to open both file versions.
Solution
In order to solve the compatibility issue exist many solutions, more or less professional. The recommended solution is the serialization.
The serialization is a write / read object process to/from a persistent storage. Serialization it’s a good choice in order to maintain a good data structure. Many different frameworks offer serialization support. One of these is Microsoft Foundation Classes – MFC.

If we want to use MFC serialization support, we can use a CArchive instance. This object, combined with a CFile instance provides a strong mechanism for objects serialization.

Because, in different applications version, the file suffers significant structure changes we have to use MFC’s serialization concept called Versionable Schema.
Versionable Schema means the using of CArchive class methods GetObjectSchema() and SetObjectSchema() and a constant VERSIONABLE_SCHEMA (that you can find it in afx.h file and has 0x80000000 value) combined with a OR LOGIC operator and the last application version number as a parameter of IMPLEMENT_SERIAL macro.
The GetObjectSchema() method is used in order to detect stored objects version from a file that is loaded in our application. The complement of this method, SetObjectSchema() method, allow us to save the objects version.
Different by the C++ I/O standard streams, the CArchive class is special designed only for objects serialization in binary files.

In order to serialize a class’s objects we have to follow next steps:
1. The class that we want to serialize has to be derived from the abstract class CObject (or other classes derived from CObject).
2. Overwrite CObject’s Serialize() method.
3. Use DECLARE_SERIAL macro in your class declaration.
4. The serializable class has to have a default constructor, without arguments.
5. Use IMPLEMENT_SERIAL macro in the implementation file of serializable class.

More information you can find it here and in links of this page.

But, from these steps until to a complete serialization and versionable application there are few significant steps to follow.
Next, I will present to you a Dialog base sample application that supports serialization and is versionable.

    Sample application – SerAddressBook

Next, I will present to you how you can create an address book application (based on a MFC Dialog application architecture).

Suppose that initially our client requested us an address book that contaions: name, prename, address and phone number. But, once with the mobile phone and Internet area extensions our client needs two new fields for mobile phone number and for email address.
File versions structure

Our application with a file version 2 looks like this:
Application window

Because this is a demo application I kept on my window the possibility to save both version, using two radio buttons.
A good application design helps us if we have new requirements and we have to change the application structure. The code changes have to be done without too many code interactions. Ideally, with add code only.
That’s why, my application classes design looks like this:
Classes Hierarchy

Although Contact class and CAddressBook class are serializable, the objects serialization is implemented into Contact class.

    Contact class

From Contact’s interface class you can observe:
• I derived this class from the abstract CObject;
DECLARE_SERIAL, macro calling;
Serialize()‘s method declaration in order to overwrite the parent class;
• Our class attributes.

The last line represents an “alias” definition for a MFC list definition, used in order to store displayed data. This list is using for Contact object administration.
In the implementation file we declare the IMPLEMENT_SERIAL macro and we are initializing the static variable with our current application version.

Into this declaration you can observe the VERSIOABLE_SCHEMA constant combined on OR logic with 2 (my demo application last version). This, the third macro argument is essential for objects versioning, combined with CArchive::GetObjectSchema() and CArchive::SetObjectSchema().
More details about this constant and it using process or about these methods you can find here.
The implementation of Contacte::Serialize() looks like this:

If CArchive constructor sets store-load flag on CArchive::store (save to file) then the code flow will follow true if’s block and objects data is sending to archive and stored in the file (including file version, too).
When we want to open an existing file, our CArchive’s constructor receives CArcuhive::load flag and enter to else if’s block of Serialize(). Is extracting file version, and after that is loading all Contact objects.

    CAddressBook class

CAddressBook class make the link between interface dialog class (CSerAddressBookDlg) and the serialized class Contact. This class contains a Contact objects list. CAddressBook class is administrating this contact list and is realizing load/store object.

The interface of this class is looking like this:

Into this declaration you can see the existence of a contact list instance (m_cContactsList). This class contains add, update or remove contact methods.

Because our class has to be a serialized method we have to overwrite Serialize() method, the method has to me used by the client class ( in our case the interface class – CSerAddressBookDlg).

Because CArchive class doesn’t provides any method or attribute in order to obtain the objects (I’m counting when I’m loading or storing), I decided to save the objects count into my files. That’s why, if I’m storing, I call next line:

Same story, for file version, before starting Contact objects serialization:

Then, into a while loop I’m iterating over the contact list. I am serializing the data and I’m storing to my new file.
If I load a file from disk (else branch) then I follow next steps:
• I’m cleaning contact list;
• Get the objects count;
• Get file version and serialize all objects for load;
• Add all data to my Contact object list;

    CSerAddressBookDlg – The application interface class

Once we have implemented this serialization mechanism the using of this one into our application became very easy.

For instance, when the user wants to save into a file all he’s new data, he will call next method:

As you can see, I have a CFile object that I’m using it, combined with a CArchive instance, for data storing to a file. Although my local CArchive instance receive as a first parameter the address of the file handler and the store flag CArchive::store.
Next I call CAddressBook::Serialize() method and I’m closing the store operation.

Loading file method, based on my Contact serialization mechanism looks like this:

As you can see, I am creating a local CFile object, needed for reading operation. Although, I’m creating a local CArchive instance that received as constructor parameter the file handler address with CArchive::load flag.
Then, I’m calling CAddressBook::Serialize() method. Is entering on else branch and finally we are disconnected the object from file.
The last line contains PopulateList() call and is my populate list method. It populates my list control (a CListCtrl instance) with the file loaded data in order to display it into our dialog.

Conclusions:
The MFC’s Document View architecture offers complete serialization support. Each MDI/SDI application contains default serialization support. My demo solution presented is an adapted serialization version for dialog base applications.

Download demo application: SerAddressBook (Visual C++ 2005 project)

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