Dialog Field Validation

1. Dialog Field Validation Using MFC Data Exchange and Validation Code David J. Kruglinski

2. Summary • What is field validation? • Existing MFC validation support • The “Validator Class” solution • The CValidDialog class • Using CValidDialog in a sample app • Cross-validation • Technical details • The CValidForm class for form-view apps

3. The Field Validation Problem • More people are using Windows for data entry applications. • Users expect immediate feedback after they make an error in a field. • Windows dialog box prodedure not optimized to support field validation. • Field validation not easy with SDK or MFC.

4. What is Field Validation? • When the user tries to leave a control (in a dialog box) that contains text (edit control, combobox).... • A function validates the data just entered. • Allows user to leave if data is OK. • Displays message box if data not OK. • Returns user to the same field.

5. What is Field Validation? • All fields are validated on OK button. • No validation on Escape button. • User is allowed to leave a control when moving to another application.

6. Existing MFC Validation Support • Validation of all fields on OK button • Derived dialog class has data members corresponding to controls • Generated DDX/DDV code called from OnOK virtual function • Validation is “too late” • No immediate feedback to user • Does allow cross-validation • Full support from ClassWizard

7. Existing MFC Validation Support • Control subclassing for keystroke validation • CWnd::SubclassDlgItem traps control’s messages before standard WndProc • A derived control class can map messages • Example: A derived CEdit class can accept only numeric keystrokes

8. The “Validator Class” Solution • Standard base class CValidDialog • CValidDialog base class is MFC’s CDialog. • Your dialog classes should be derived from CValidDialog instead of CDialog. • You override one virtual function, ValidateDlgItem. • #include “valid.h” • Add VALID.CPP to your project.

9. Sample Application A modal dialog displayed from the main menu

10. Writing Validation Code • Standard MFC DDX/DDV code • Generated by ClassWizard void CAboutDlg::DoDataExchange(CDataExchange* pDX) { //{{AFX_DATA_MAP(CAboutDlg) DDX_Text(pDX, IDC_EDIT1, m_nEdit1); DDV_MinMaxInt(pDX, m_nEdit1, 0, 10); DDX_Text(pDX, IDC_EDIT2, m_nEdit2); DDV_MinMaxInt(pDX, m_nEdit2, 20, 30); DDX_Text(pDX, IDC_EDIT3, m_strEdit3); DDV_MaxChars(pDX, m_strEdit3, 5); //}}AFX_DATA_MAP }

11. Writing Validation Code • The ValidateDlgItem function void CAboutDlg::ValidateDlgItem(CDataExchange* pDX, UINT uID) { switch (uID) { case IDC_EDIT1: DDX_Text(pDX, IDC_EDIT1, m_nEdit1); DDV_MinMaxInt(pDX, m_nEdit1, 0, 10); break; case IDC_EDIT2: DDX_Text(pDX, IDC_EDIT2, m_nEdit2); DDV_MinMaxInt(pDX, m_nEdit2, 20, 30); break; case IDC_EDIT3: DDX_Text(pDX, IDC_EDIT3, m_strEdit3); DDV_MaxChars(pDX, m_strEdit3, 5); default: break; } }

12. Writing Validation Code • Cross-validation • Independent of field validation • Executed on OK button • Code added to end of DoDataExchange • after //}}AFX_DATA_MAP if(pDX->m_bSaveAndValidate) { if(m_nEdit1 + m_nEdit2 < 30) { AfxMessageBox("Sum of Edit1 and Edit2 must be >= 30"); pDX->Fail(); } }

13. How CValidDialog Works • Traps KILLFOCUS message as user tries to leave control • Controls send “notification” WM_COMMAND messages to their parent dialog window • Edit control sends EN_KILLFOCUS • Calls virtual ValidDlgItem to validate data • MFC exception processing returns user to the field if there is an error

14. How CValidDialog Works -- Complications • Can’t call CWnd::SetFocus while a KILLFOCUS operation is in progress • Post user-defined message WM_VALIDATE • Handler sets focus back after KILLFOCUS completes • Must recognize the KILLFOCUS that results from the SetFocus call • Use a flag data member, m_bValidateOn

15. How CValidDialog Works -- Complications • Must regognize a KILLFOCUS when user switches to another app • Can call CWnd::GetParent to test if new parent is our same dialog window • Similar logic for modeless dialogs

16. MFC Command ProcessingControl Notifications • MFC first processes WM_COMMAND messages with a function CWnd::OnCommand( wParam, lParam) • Both parameters are 32 bits (Win32) • wParam LOWORD = control child window ID • wParam HIWORD = notification code • lParam = control window HWND • Base class handler dispatches command through message map

17. CValidDialog::OnCommand BOOL CValidDialog::OnCommand(WPARAM wParam, LPARAM lParam) { if(m_bValidationOn) { // might be a killfocus UINT notificationCode = (UINT) HIWORD( wParam ); if((notificationCode == EN_KILLFOCUS) || (notificationCode == LBN_KILLFOCUS) || (notificationCode == CBN_KILLFOCUS) ) { CWnd* pFocus = CWnd::GetFocus(); // if we're changing focus to another control in same dialog if( pFocus && (pFocus->GetParent() == this) ){ if(pFocus->GetDlgCtrlID() != IDCANCEL) { // and focus not in Cancel button // validate AFTER drawing finished PostMessage(WM_VALIDATE, wParam); } } } } return CDialog::OnCommand(wParam, lParam); // pass it on }

18. CValidDialog::OnValidate • Handler for WM_VALIDATE message ON_MESSAGE(WM_VALIDATE, OnValidate) LONG CValidDialog::OnValidate(UINT wParam, LONG lParam) { TRACE("Entering CValidDialog::OnValidate\n"); CDataExchange dx(this, TRUE); m_bValidationOn = FALSE; // temporarily off UINT controlID = (UINT) LOWORD( wParam ); TRY { ValidateDlgItem(&dx, controlID); } CATCH(CUserException, pUE) { // fall through -- user already alerted via message box } END_CATCH m_bValidationOn = TRUE; return 0; // goes no further }

19. CFormView Considerations • Same as dialog except use CValidForm derived from MFC CFormView • CValidForm is the new base class for your form views • ValidateDlgItem, OnCommand, OnValidate are the same • Could use multiple inheritance to save code duplication