Programming with Microsoft Visual C++.NET 6ed [Electronic resources] نسخه متنی

The Ex15a Example: A Simple Document-View Interaction

The first of this chapter's two examples shows a very simple document-view interaction. The CEx15aDoc document class, which is derived from CDocument, allows for a single embedded CStudent object. The CStudent class represents a student record composed of a CString name and an integer grade. The CEx15aView view class is derived from CFormView. It is a visual representation of a student record that has edit controls for the name and grade. The default Enter pushbutton updates the document with data from the edit controls. Figure 15-1 shows the Ex15a program window.

Figure 15-1: The Ex15a program in action.

The code for the CStudent class is shown below. Most of the class's features serve Ex15a, but a few items carry forward to Ex15b and the programs discussed in Chapter 16. For now, take note of the two data members, the default constructor, the operators, and the Dump function declaration. The DECLARE_DYNAMIC and IMPLEMENT_DYNAMIC macros ensure that the class name is available for the diagnostic dump.

Student.h

// student.h
#ifndef _INSIDE_VISUAL_CPP_STUDENT
#define _INSIDE_VISUAL_CPP_STUDENT
class CStudent : public CObject
{
DECLARE_DYNAMIC(CStudent)
public:
CString m_strName;
int m_nGrade;
CStudent()
{
m_nGrade = 0;
}
CStudent(const char* szName, int nGrade) : m_strName(szName)
{
m_nGrade = nGrade;
}
CStudent(const CStudent& s) : m_strName(s.m_strName)
{
// copy constructor
m_nGrade = s.m_nGrade;
}
const CStudent& operator =(const CStudent& s)
{
m_strName = s.m_strName;
m_nGrade = s.m_nGrade;
return *this;
}
BOOL operator ==(const CStudent& s) const
{
if ((m_strName == s.m_strName) && (m_nGrade == s.m_nGrade)) {
return TRUE;
}
else {
return FALSE;
}
}
BOOL operator !=(const CStudent& s) const
{
// Let's make use of the operator we just defined!
return !(*this == s);
}
#ifdef _DEBUG
void Dump(CDumpContext& dc) const;
#endif // _DEBUG
};
#endif // _INSIDE_VISUAL_CPP_STUDENT

Student.cpp

#include "stdafx.h"
#include "student.h"
IMPLEMENT_DYNAMIC(CStudent, CObject)
#ifdef _DEBUG
void CStudent::Dump(CDumpContext& dc) const
{
CObject::Dump(dc);
dc << "m_strName = " << m_strName << "\nm_nGrade = " << m_nGrade;
}
#endif // _DEBUG

Follow these steps to build the Ex15a example:

Run the MFC Application Wizard to generate the Ex15a project. Make it an SDI application. On the Generated Classes page, change the view's base class to CFormView, as shown here.








Use the menu editor to replace the Edit menu commands. Delete the current Edit menu commands and replace them with a Clear All command. Use the default constant ID_EDIT_CLEARALL, which is assigned by the application framework.





Use the dialog editor to modify the IDD_EX15A_FORM dialog box.Open the MFC Application Wizard–generated dialog box IDD_EX15A_FORM, and then add controls as shown here.




Be sure that you set the following properties in the dialog editor's Properties window: Style = Child, Border = None, Visible = False. Use the following IDs for the controls.




















Control	ID
Name edit control	IDC_NAME
Grade edit control	IDC_GRADE
Enter button	IDC_ENTER








Use Class View's Properties window to add message handlers for CEx15aView. Select the CEx15aView class, and then add handlers for the following messages. Accept the default function names.




















Object ID	Message	Member Function
IDC_ENTER	BN_CLICKED	OnBnClickedEnter
ID_EDIT_CLEARALL	COMMAND	OnEditClearall
ID_EDIT_CLEARALL	UPDATE_COMMAND_UI	OnUpdateEditClearall








Use the Add Member Variable Wizard to add variables for CEx15aView. In Class View, right-click on CEx15aView and choose Add Variable. Add the following variables:

















Control ID	Member Variable	Category	Variable Type
IDC_GRADE	m_nGrade	Value	int
IDC_NAME	m_strName	Value	CString




For m_nGrade, enter a minimum value of 0 and a maximum value of 100. Notice that the Add Member Variable Wizard generates the code necessary to validate data entered by the user.





Add a prototype for the helper function UpdateControlsFromDoc. In Class View, right-click on CEx15aView and choose Add Function. Fill out the dialog box to add the following function:

private:
void UpdateControlsFromDoc(void);






Edit the file

Ex15aView.cpp . The MFC Application Wizard generated the skeleton OnInitialUpdate function, and the Add Member Function Wizard available from Class View generated the skeleton UpdateControlsFromDoc function. UpdateControlsFromDoc is a private helper member function that transfers data from the document to the CEx15aView data members and then to the dialog edit controls. Edit the code as shown here:

void CEx15aView::OnInitialUpdate()
{   // called on startup
    CFormView::OnInitialUpdate();
    UpdateControlsFromDoc();
}
void CEx15aView::UpdateControlsFromDoc(void)
{   // called from OnInitialUpdate and OnEditClearall
    CEx15aDoc* pDoc = GetDocument();    
    m_nGrade = pDoc->m_student.m_nGrade;
    m_strName = pDoc->m_student.m_strName;
    UpdateData(FALSE); // calls DDX
}




The OnBnClickedEnter function replaces the OnOK function you'd expect to see in a dialog class. The function transfers data from the edit controls to the view's data members and then to the document. Add the boldface code shown here:


void CEx15aView::OnBnClickedEnter()
{
    CEx15aDoc* pDoc = GetDocument();
    UpdateData(TRUE);
    pDoc->m_student.m_nGrade = m_nGrade;
    pDoc->m_student.m_strName = m_strName;
}




In a complex multi-view application, the Edit Clear All command would be routed directly to the document. In this simple example, it's routed to the view. The update command user interface handler disables the menu command if the document's student object is already blank. Add the following boldface code:

void CEx15aView::OnEditClearall()
{
    GetDocument()->m_student = CStudent(); // "blank" student object
    UpdateControlsFromDoc();
}
void CEx15aView::OnUpdateEditClearall(CCmdUI* pCmdUI)
{
    pCmdUI->Enable(GetDocument()->m_student != CStudent()); // blank?
}








Edit the Ex15a project to add the files for CStudent. Be sure that

Student.h and

Student.cpp are in your project directory. Choose Add Existing Item from the Project menu and select the

Student.h header and the

Student.cpp source code files. Visual C++ .NET will add the files' names to the project's project file so that they will be compiled when you build the project.





Add a CStudent data member to the CEx15aDoc class. Edit the code in

Ex15aDoc.h and remember to include

Student.h in the C

Ex15aDoc.h file.

public:
CStudent m_student;




The CStudent constructor is called when the document object is constructed, and the CStudent destructor is called when the document object is destroyed.





Edit the

Ex15aDoc.cpp file. Use the CEx15aDoc constructor to initialize the student object, as shown here:

CEx15aDoc::CEx15aDoc() : m_student("default value", 0)
{
    TRACE("Document object constructed\n");
}




We can't tell whether the Ex15a program works properly unless we dump the document when the program exits. We'll use the destructor to call the document's Dump function, which calls the CStudent::Dump function shown here:

CEx15aDoc::~CEx15aDoc()
{
#ifdef _DEBUG
    Dump(afxDump);
#endif // _DEBUG
}
void CEx15aDoc::Dump(CDumpContext& dc) const
{
CDocument::Dump(dc);
dc << "\n" << m_student << "\n";
}






Build and test the Ex15a application. Type a name and a grade, and then click Enter. Now exit the application. Does the Debug window show messages similar to those shown here?

a CEx15aDoc at $411580 
m_strTitle = Untitled
m_strPathName = 
m_bModified = 0
m_pDocTemplate = $4113A0
a CStudent at $4115D4 
m_strName = Sullivan, Walter
m_nGrade = 78