qt-everywhere-src-5.14.1/qtbase/examples/corelib/tools/doc/src/contiguouscache.qdoc

/****************************************************************************
**
** Copyright (C) 2016 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** 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. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \example tools/contiguouscache
    \title Contiguous Cache Example

    \brief The Contiguous Cache example shows how to use QContiguousCache to manage memory usage for
    very large models.  In some environments memory is limited and, even when it
    isn't, users still dislike an application using excessive memory.
    Using QContiguousCache to manage a list, rather than loading
    the entire list into memory, allows the application to limit the amount
    of memory it uses, regardless of the size of the data set it accesses.

    The simplest way to use QContiguousCache is to cache as items are requested. When
    a view requests an item at row N it is also likely to ask for items at rows near
    to N.

    \snippet tools/contiguouscache/randomlistmodel.cpp 0

    After getting the row, the class determines if the row is in the bounds
    of the contiguous cache's current range.  It would have been equally valid to
    simply have the following code instead.

    \code
    while (row > m_rows.lastIndex())
        m_rows.append(fetchWord(m_rows.lastIndex()+1);
    while (row < m_rows.firstIndex())
        m_rows.prepend(fetchWord(m_rows.firstIndex()-1);
    \endcode

    However a list will often jump rows if the scroll bar is used directly, resulting in
    the code above causing every row between the old and new rows to be fetched.

    Using QContiguousCache::lastIndex() and QContiguousCache::firstIndex() allows
    the example to determine what part of the list the cache is currently caching.
    These values don't represent the indexes into the cache's own memory, but rather
    a virtual infinite array that the cache represents.

    By using QContiguousCache::append() and QContiguousCache::prepend() the code ensures
    that items that may be still on the screen are not lost when the requested row
    has not moved far from the current cache range.  QContiguousCache::insert() can
    potentially remove more than one item from the cache as QContiguousCache does not
    allow for gaps.  If your cache needs to quickly jump back and forth between
    rows with significant gaps between them consider using QCache instead.

    And thats it.  A perfectly reasonable cache, using minimal memory for a very large
    list.  In this case the accessor for getting the words into the cache
    generates random information rather than fixed information.  This allows you
    to see how the cache range is kept for a local number of rows when running the
    example.

    \snippet tools/contiguouscache/randomlistmodel.cpp 1

    It is also worth considering pre-fetching items into the cache outside of the
    application's paint routine.  This can be done either with a separate thread
    or using a QTimer to incrementally expand the range of the cache prior to
    rows being requested out of the current cache range.
*/