fixing return values and handling of invalid files

[wxWidgets.git] / docs / doxygen / overviews / validator.h

/////////////////////////////////////////////////////////////////////////////
// Name:        validator.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
/////////////////////////////////////////////////////////////////////////////

/**

@page overview_validator wxValidator Overview

Classes: wxValidator, wxTextValidator, wxGenericValidator

@li @ref overview_validator_intro
@li @ref overview_validator_anatomy
@li @ref overview_validator_dialogs


<hr>


@section overview_validator_intro Validator basic concepts

The aim of the validator concept is to make dialogs very much easier to write.
A validator is an object that can be plugged into a control (such as a
wxTextCtrl), and mediates between C++ data and the control, transferring the
data in either direction and validating it. It also is able to intercept events
generated by the control, providing filtering behaviour without the need to
derive a new control class.

You can use a stock validator, such as wxTextValidator (which does text control
data transfer, validation and filtering) and wxGenericValidator (which does
data transfer for a range of controls); or you can write your own.

Here is an example of wxTextValidator usage.

@code
wxTextCtrl *txt1 = new wxTextCtrl(
    this, -1, wxT(""), wxDefaultPosition, wxDefaultSize, 0,
    wxTextValidator(wxFILTER_ALPHA, &g_data.m_string));
@endcode

In this example, the text validator object provides the following
functionality:

@li It transfers the value of g_data.m_string (a wxString variable) to the
    wxTextCtrl when the dialog is initialised.
@li It transfers the wxTextCtrl data back to this variable when the dialog is
    dismissed.
@li It filters input characters so that only alphabetic characters are allowed.

The validation and filtering of input is accomplished in two ways. When a
character is input, wxTextValidator checks the character against the allowed
filter flag (@c wxFILTER_ALPHA in this case). If the character is inappropriate,
it is vetoed (does not appear) and a warning beep sounds (unless
wxValidator::SetBellOnError(false) has been called).
The second type of validation is performed when the dialog is about to be dismissed,
so if the default string contained invalid characters already, a dialog box is shown
giving the error, and the dialog is not dismissed.

Note that any wxWindow may have a validator; using the @c wxWS_EX_VALIDATE_RECURSIVELY
style (see wxWindow extended styles) you can also implement recursive validation.


@section overview_validator_anatomy Anatomy of a Validator

A programmer creating a new validator class should provide the following
functionality.

A validator constructor is responsible for allowing the programmer to specify
the kind of validation required, and perhaps a pointer to a C++ variable that
is used for storing the data for the control. If such a variable address is not
supplied by the user, then the validator should store the data internally.

The wxValidator::Validate member function should return @true if the data in
the control (not the C++ variable) is valid. It should also show an appropriate
message if data was not valid.

The wxValidator::TransferToWindow member function should transfer the data from
the validator or associated C++ variable to the control.

The wxValidator::TransferFromWindow member function should transfer the data
from the control to the validator or associated C++ variable.

There should be a copy constructor, and a wxValidator::Clone function which
returns a copy of the validator object. This is important because validators
are passed by reference to window constructors, and must therefore be cloned
internally.

You can optionally define event handlers for the validator, to implement
filtering. These handlers will capture events before the control itself does
(see @ref overview_events_processing).
For an example implementation, see the @c valtext.h and @c valtext.cpp files in the
wxWidgets library.


@section overview_validator_dialogs How Validators Interact with Dialogs

For validators to work correctly, validator functions must be called at the
right times during dialog initialisation and dismissal.

When a wxDialog::Show is called (for a modeless dialog) or wxDialog::ShowModal
is called (for a modal dialog), the function wxWindow::InitDialog is
automatically called. This in turn sends an initialisation event to the dialog.
The default handler for the @c wxEVT_INIT_DIALOG event is defined in the wxWindow
class to simply call the function wxWindow::TransferDataToWindow.
This function finds all the validators in the window's children and calls the
wxValidator::TransferToWindow function for each. Thus, data is transferred from C++
variables to the dialog just as the dialog is being shown.

@note If you are using a window or panel instead of a dialog, you will need to
call wxWindow::InitDialog explicitly before showing the window.

When the user clicks on a button, for example the OK button, the application
should first call wxWindow::Validate, which returns @false if any of the child
window validators failed to validate the window data. The button handler should
return immediately if validation failed. Secondly, the application should call
wxWindow::TransferDataFromWindow and return if this failed. It is then safe to
end the dialog by calling wxDialog::EndModal (if modal) or wxDialog::Show (if modeless).

In fact, wxDialog contains a default command event handler for the @c wxID_OK
button. It goes like this:

@code
void wxDialog::OnOK(wxCommandEvent& event)
{
    if ( Validate() && TransferDataFromWindow() )
    {
        if ( IsModal() )
            EndModal(wxID_OK);
        else
        {
            SetReturnCode(wxID_OK);
            this->Show(false);
        }
    }
}
@endcode

So if using validators and a normal OK button, you may not even need to write
any code for handling dialog dismissal.

If you load your dialog from a resource file, you will need to iterate through
the controls setting validators, since validators can't be specified in a
dialog resource.

*/