source: trunk/doc/src/porting/qt4-interview.qdoc@ 846

/****************************************************************************
**
** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation ([email protected])
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial Usage
** Licensees holding valid Qt Commercial licenses may use this file in
** accordance with the Qt Commercial License Agreement provided with the
** Software or, alternatively, in accordance with the terms contained in a
** written agreement between you and Nokia.
**
** 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.
**
** If you have questions regarding the use of this file, please contact
** Nokia at [email protected].
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \page qt4-interview.html
    \title The Interview Framework

    \contentspage {What's New in Qt 4}{Home}
    \previouspage The Tulip Container Classes
    \nextpage The Arthur Paint System

    The Interview classes provide a model/view framework for Qt
    applications based on the well known Model-View-Controller design
    pattern. In this document, we will describe Qt's model/view
    architecture, provide some examples, and show the improvements
    offered over Qt 3's item view classes.

    \tableofcontents

    \section1 Overview of The Model/View Architecture

    The model/view architecture is a variation of the Model-View-Controller
    (MVC) design pattern, originating from Smalltalk, that is often used when
    building user interfaces.

    In the model/view architecture, the view and the controller objects are
    combined. This still separates the way that data is stored from the way
    that it is presented to the user, but provides a simpler framework based
    on the same principles. This separation makes it possible to display the
    same data in several different views, and to implement new types of views,
    without changing the underlying data structures.

    User input is handled by \e delegates. The advantage of this approach is
    that it allows rendering and editing of individual items of data to be
    customized to suit each data type in use.

    \table
    \row \i \inlineimage modelview-overview.png
    \i \bold{The model/view architecture}

    The model communicates with a source of data, providing an \e interface
    for the other components in the architecture. The nature of the
    communication depends on the type of data source, and the way the model
    is implemented.

    The view obtains \e{model indexes} from the model; these are references
    to items of data. By supplying model indexes to the model, the view can
    retrieve items of data from the data source.

    In standard views, a \e delegate renders the items of data. When an item
    is edited, the delegate communicates with the model directly using
    model indexes.
    \endtable

    \section1 Model/View Classes

    On a fundamental level, the Interview classes define the interfaces and
    common functionality for models, views, and delegates. All implemented
    components subclass QAbstractItemModel, QAbstractItemView, or
    QAbstractItemDelegate. The use of a common API ensures a level of
    interoperability between the components.

    \image standard-views.png

    Interview provides ready-to-use implementations of views for table,
    tree, and list widgets: QTableView, QTreeView, and QListView.
    These standard views are suitable for displaying the most common
    types of data structures used in applications, and can be used with
    the ready-made models supplied with Qt:

    \list
    \o QStandardItemModel is a minimal convenience model that developers
       can use to manage items of data.
    \o QFileSystemModel provides directory information for use with QListView
       and QTreeView.
    \o QStringListModel is a convenience model that can be used to hold
       strings for views such as QListView and QComboBox.
    \endlist

    Two specialized abstract models are provided that can be subclassed
    and extended (see the
    \l{model-view-programming.html#related-examples}{Model/View Programming}
    examples):

    \list
    \o QAbstractTableModel is a useful starting point for providing a custom
       model that can be used with QTableView.
    \o QAbstractListModel can be subclassed to produce a list-based model
       for use with QListView.
    \endlist

    Operations on items, such as filtering and sorting, are handled by \e{proxy
    models} that allow views to display processed data without having to
    copy or modify data obtained from a source model. Interview provides
    the QSortFilterProxyModel class to allow items of data from a source model
    to be sorted and filtered before they are supplied to views.

    Developers who are familiar with the conventional list, tree, and table
    widgets may find QListWidget, QTreeWidget, and QTableWidget useful.
    These present a simplified interface to the views that does not require a
    knowledge of the underlying model/view architecture.

    For details about how to use the model/view classes, see the
    \l{Model/View Programming} document.

    See also the \l{The Qt 4 Database GUI Layer}{Database GUI Layer} document
    for information about Qt 4's database models.

    \section1 Example Code

    To illustrate how the Interview classes are used, we present two
    examples that show different aspects of the model/view architecture.

    \section2 Sharing a Model Between Views

    In this example, we display the contents of a model using two
    different views, and share the user's selection between
    them. We will use the QFileSystemModel supplied with Qt because it
    requires very little configuration, and provides existing data to
    the views.

    The main() function for this example demonstrates all the
    principles involved in setting up a model and two views. We also
    share the selection between the two views:

    \snippet doc/src/snippets/shareddirmodel/main.cpp 1

    In the above function, we construct a directory model to display
    the contents of a default directory. The two views are constructed
    and given the same model to work with. By default, each view will
    maintain and display its own selection of items from the model,
    so we explicitly create a new selection that is shared between the
    tree view and the list view. As a result, changes to the selection
    in either of these views will automatically cause the selection in
    the other to change.

    \image interview-shareddirmodel.png

    The model/view architecture allows us to replace the QFileSystemModel in
    this example with a completely different model, one that will perhaps
    obtain data from a remote server, or from a database.

    \section2 Creating a Custom Model

    In this example, we display items of data obtained from a custom list
    model using a standard view. The custom model is a subclass of
    QAbstractListModel and provides implementations of a core set of
    functions.

    The complete declaration of our model is as follows:

    \snippet doc/src/snippets/stringlistmodel/model.h 0
    \snippet doc/src/snippets/stringlistmodel/model.h 1
    \codeline
    \snippet doc/src/snippets/stringlistmodel/model.h 5

    The model takes a list of strings when constructed, and supplies these
    to views as required. Since this is only a simple read-only model, we
    only need to implement a few functions.

    The underlying data structure used to hold the strings is a QStringList.
    Since the model maps each item in the list to a row in the model, the
    rowCount() function is quite simple:

    \snippet doc/src/snippets/stringlistmodel/model.cpp 0

    The data() function returns an item of data for each model index
    supplied by a view:

    \snippet doc/src/snippets/stringlistmodel/model.cpp 1

    The data() function returns a QVariant containing the information
    referred to by the model index. Items of data are returned to the view,
    but only if a number of checks are satisfied; for example, if the view
    specifies an invalid model index, the model indicates this by returning
    an invalid QVariant.

    Vertical and horizontal headers are supplied by the headerData()
    function. In this model, the value returned for these items is the row
    or column number, depending on the header:

    \snippet doc/src/snippets/stringlistmodel/model.cpp 2

    We only include an excerpt from the main() function for this short
    example:

    \snippet doc/src/snippets/stringlistmodel/main.cpp 1
    \dots
    \snippet doc/src/snippets/stringlistmodel/main.cpp 3

    We create a string list to use with the model, and we supply it to the
    model when it is constructed. The information in the string list is
    made available to the view via the model.

    \image stringlistmodel.png

    This example shows that it can be easy to populate views with data
    from a simple model. The standard models and views planned for
    Qt 4 will make the process even easier, and the convenience widgets
    supplied provide support for the classic item-based approach.

    \section1 What's Changed Since Qt 3?

    The table and item view classes in Qt 3 implemented widgets that
    both stored data and presented it to the user. These classes were
    designed to be easy-to-use and consistent, but were sometimes
    difficult to customize and extend.

    The equivalent classes in Qt 4 are designed to be extensible while
    remaining easy-to-use; the introduction of the model/view
    architecture ensures that they will be more consistent than their
    predecessors. The view classes provided can be summarized in the
    following way:

    \list
    \i QListView class provides a view widget that looks similar to
       Qt 3's QListBox widget, but displays data provided by a model.
       It can also be used to display icons in a similar way to Qt 3's
       QIconView.
    \i The QTableView class is a view widget that displays tabular data
       like Qt 3's QTable widget, but uses data provided by a model.
    \i The QTreeView class provides a view widget that behaves like
       Qt 3's QListView widget, except that it displays data provided
       by a model.
    \endlist

    Since the model takes responsibility for supplying items of data,
    and the view takes care of their presentation to the user, we do
    not require item classes to represent individual items.
    Delegates handle the painting and editing of data obtained from
    the model.

    Qt continues to provide a number of classic item view widgets with
    familiar item-based interfaces that are not based on compatibility
    classes:

    \list
    \i The QListWidget class provides a widget to display a
       list of items, as found in Qt 3's QListBox class.
    \i The QTreeWidget class implements the equivalent of Qt 3's
       QListView class.
    \i The QTableWidget class provides comparable functionality to
       Qt 3's QTable class.
    \endlist

    Each of the convenience classes have a corresponding item class:
    QListWidgetItem, QTreeWidgetItem, and QTableWidgetItem are the Qt 4
    equivalents of Qt 3's QListBoxItem, QListViewItem, and QTableItem
    respectively.

    The move towards a model/view architecture presents both challenges
    and opportunities for developers. Although the approach may appear to
    be rather powerful for simple applications, it encourages greater
    reuse of components within applications.
*/