doc/src/examples/fetchmore.qdoc
author eckhart.koppen@nokia.com
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/****************************************************************************
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**
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** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
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** All rights reserved.
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** Contact: Nokia Corporation (qt-info@nokia.com)
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**
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** This file is part of the documentation of the Qt Toolkit.
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**
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** $QT_BEGIN_LICENSE:LGPL$
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** No Commercial Usage
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** This file contains pre-release code and may not be distributed.
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** You may use this file in accordance with the terms and conditions
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** contained in the Technology Preview License Agreement accompanying
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** this package.
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**
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** GNU Lesser General Public License Usage
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** Alternatively, this file may be used under the terms of the GNU Lesser
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** General Public License version 2.1 as published by the Free Software
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** Foundation and appearing in the file LICENSE.LGPL included in the
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** packaging of this file.  Please review the following information to
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** ensure the GNU Lesser General Public License version 2.1 requirements
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** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
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**
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** In addition, as a special exception, Nokia gives you certain additional
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** rights.  These rights are described in the Nokia Qt LGPL Exception
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** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
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**
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** If you have questions regarding the use of this file, please contact
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** Nokia at qt-info@nokia.com.
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**
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**
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**
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**
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**
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**
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**
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**
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** $QT_END_LICENSE$
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**
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****************************************************************************/
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/*!
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    \example itemviews/fetchmore
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    \title Fetch More Example
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    The Fetch More example shows how two add items to an item view
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    model on demand.
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    \image fetchmore-example.png
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    The user of the example can enter a directory in the \gui
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    Directory line edit. The contents of the directory will
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    be listed in the list view below.
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    When you have large - or perhaps even infinite - data sets, you
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    will need to add items to the model in batches, and preferably only
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    when the items are needed by the view (i.e., when they are visible
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    in the view).
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    In this example, we implement \c FileListModel - an item view
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    model containing the entries of a directory. We also have \c
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    Window, which sets up the GUI and feeds the model with
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    directories.
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    Let's take a tour of \c {FileListModel}'s code.
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    \section1 FileListModel Class Definition
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    The \c FileListModel inherits QAbstractListModel and contains the
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    contents of a directory. It will add items to itself only when
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    requested to do so by the view.
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    \snippet examples/itemviews/fetchmore/filelistmodel.h 0
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    The secret lies in the reimplementation of
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    \l{QAbstractItemModel::}{fetchMore()} and
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    \l{QAbstractItemModel::}{canFetchMore()} from QAbstractItemModel.
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    These functions are called by the item view when it needs more
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    items. 
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    The \c setDirPath() function sets the directory the model will
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    work on. We emit \c numberPopulated() each time we add a batch of
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    items to the model.
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    We keep all directory entries in \c fileList. \c fileCount is the
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    number of items that have been added to the model.
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    \section1 FileListModel Class Implementation
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    We start by checking out the \c setDirPath().
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    \snippet examples/itemviews/fetchmore/filelistmodel.cpp 0
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    We use a QDir to get the contents of the directory. We need to
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    inform QAbstractItemModel that we want to remove all items - if
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    any - from the model.
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    \snippet examples/itemviews/fetchmore/filelistmodel.cpp 1
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    The \c canFetchMore() function is called by the view when it needs
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    more items. We return true if there still are entries that we have
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    not added to the model; otherwise, we return false.
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    And now, the \c fetchMore() function itself:
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    \snippet examples/itemviews/fetchmore/filelistmodel.cpp 2
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    We first calculate the number of items to fetch.
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    \l{QAbstractItemModel::}{beginInsertRows()} and
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    \l{QAbstractItemModel::}{endInsertRows()} are mandatory for
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    QAbstractItemModel to keep up with the row insertions. Finally, we
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    emit \c numberPopulated(), which is picked up by \c Window.
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    To complete the tour, we also look at \c rowCount() and \c data().
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    \snippet examples/itemviews/fetchmore/filelistmodel.cpp 4
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    Notice that the row count is only the items we have added so far,
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    i.e., not the number of entries in the directory.
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    In \c data(), we return the appropriate entry from the \c
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    fileList. We also separate the batches with a different background
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    color.
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*/
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