qt5base-lts/examples/widgets/doc/combowidgetmapper.qdoc

/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \example widgets/itemviews/combowidgetmapper
    \title Combo Widget Mapper Example

    The Combo Widget Mapper example shows how to use a custom delegate to
    map information from a model to specific widgets on a form.

    \image combowidgetmapper-example.png

    In the \l{Simple Widget Mapper Example}, we showed the basic use of a
    widget mapper to relate data exposed by a model to simple input widgets
    in a user interface. However, sometimes we want to use input widgets that
    expose data as choices to the user, such as QComboBox, and we need a way
    to relate their input to the values stored in the model.

    This example is very similar to the \l{Simple Widget Mapper Example}.
    Again, we create a \c Window class with an almost identical user interface,
    except that, instead of providing a spin box so that each person's age
    can be entered, we provide a combo box to allow their addresses to be
    classified as "Home", "Work" or "Other".

    \section1 Window Class Definition

    The class provides a constructor, a slot to keep the buttons up to date,
    and a private function to set up the model:

    \snippet widgets/itemviews/combowidgetmapper/window.h Window definition

    In addition to the QDataWidgetMapper object and the controls used to make
    up the user interface, we use a QStandardItemModel to hold our data and
    a QStringListModel to hold information about the types of address that
    can be applied to each person's data.

    \section1 Window Class Implementation

    The constructor of the \c Window class can be explained in three parts.
    In the first part, we set up the widgets used for the user interface:

    \snippet widgets/itemviews/combowidgetmapper/window.cpp Set up widgets

    Note that we set up the mapping the combo box in the same way as for other
    widgets, but that we apply its own model to it so that it will display
    data from its own model, the \c typeModel, rather than from the model
    containing data about each person.

    Next, we set up the widget mapper, relating each input widget to a column
    in the model specified by the call to \l{QDataWidgetMapper::}{setModel()}:

    \snippet widgets/itemviews/combowidgetmapper/window.cpp Set up the mapper

    For the combo box, we pass an extra argument to tell the widget mapper
    which property to relate to values from the model. As a result, the user
    is able to select an item from the combo box, and the corresponding
    value stored in the widget's \c currentIndex property will be stored in
    the model.

    \omit
    However, we also set a delegate on the mapper. As with \l{Delegate Classes},
    this changes the way that data is presented to the user. In this case, the
    delegate acts as a proxy between the mapper and the input widgets,
    translating the data into a suitable form for the combo box but not
    interfering with the other input widgets. The implementation is shown later.
    \endomit

    The rest of the constructor is very similar to that of the
    \l{Simple Widget Mapper Example}:

    \snippet widgets/itemviews/combowidgetmapper/window.cpp Set up connections and layouts

    The model is initialized in the window's \c{setupModel()} function. Here,
    we create a standard model with 5 rows and 3 columns. In each row, we
    insert a name, address, and a value that indicates the type of address.
    The address types are stored in a string list model.

    \snippet widgets/itemviews/combowidgetmapper/window.cpp Set up the model

    As we insert each row into the model, like a record in a database, we
    store values that correspond to items in \c typeModel for each person's
    address type. When the widget mapper reads these values from the final
    column of each row, it will need to use them as references to values in
    \c typeModel, as shown in the following diagram. This is where the
    delegate is used.

    \image widgetmapper-combo-mapping.png

    We show the implementation of the \c{updateButtons()} slot for
    completeness:

    \snippet widgets/itemviews/combowidgetmapper/window.cpp Slot for updating the buttons

    \omit
    \section1 Delegate Class Definition and Implementation

    The delegate we use to mediate interaction between the widget mapper and
    the input widgets is a small QItemDelegate subclass:

    \snippet widgets/itemviews/combowidgetmapper/delegate.h Delegate class definition

    This provides implementations of the two standard functions used to pass
    data between editor widgets and the model (see the \l{Delegate Classes}
    documentation for a more general description of these functions).

    Since we only provide an empty implementation of the constructor, we
    concentrate on the other two functions.

    The \l{QItemDelegate::}{setEditorData()} implementation takes the data
    referred to by the model index supplied and processes it according to
    the presence of a \c currentIndex property in the editor widget:

    \snippet widgets/itemviews/combowidgetmapper/delegate.cpp setEditorData implementation

    If, like QComboBox, the editor widget has this property, it is set using
    the value from the model. Since we are passing around QVariant values,
    the strings stored in the model are automatically converted to the integer
    values needed for the \c currentIndex property.

    As a result, instead of showing "0", "1" or "2" in the combo box, one of
    its predefined set of items is shown. We call QItemDelegate::setEditorData()
    for widgets without the \c currentIndex property.

    The \l{QItemDelegate::}{setModelData()} implementation performs the reverse
    process, taking the value stored in the widget's \c currentIndex property
    and storing it back in the model:

    \snippet widgets/itemviews/combowidgetmapper/delegate.cpp setModelData implementation
    \endomit

    \section1 Summary and Further Reading

    The use of a separate model for the combo box provides a menu of choices
    that are separate from the data stored in the main model. Using a named
    mapping that relates the combo box's \c currentIndex property to a column
    in the model effectively allows us to store a look-up value in the model.

    However, when reading the model outside the context of the widget mapper,
    we need to know about the \c typeModel to make sense of these look-up
    values. It would be useful to be able to store both the data and the
    choices held by the \c typeModel in one place.
    This is covered by the \l{SQL Widget Mapper Example}.
*/