48e0c4df23
Change-Id: Ic804938fc352291d011800d21e549c10acac66fb Reviewed-by: Lars Knoll <lars.knoll@digia.com>
84 lines
4.0 KiB
Plaintext
84 lines
4.0 KiB
Plaintext
/****************************************************************************
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**
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** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
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** Contact: http://www.qt-project.org/legal
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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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** $QT_BEGIN_LICENSE:FDL$
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** Commercial License Usage
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** Licensees holding valid commercial Qt licenses may use this file in
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** accordance with the commercial license agreement provided with the
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** Software or, alternatively, in accordance with the terms contained in
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** a written agreement between you and Digia. For licensing terms and
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** conditions see http://qt.digia.com/licensing. For further information
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** use the contact form at http://qt.digia.com/contact-us.
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**
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** GNU Free Documentation License Usage
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** Alternatively, this file may be used under the terms of the GNU Free
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** Documentation License version 1.3 as published by the Free Software
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** Foundation and appearing in the file included in the packaging of
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** this file. Please review the following information to ensure
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** the GNU Free Documentation License version 1.3 requirements
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** will be met: http://www.gnu.org/copyleft/fdl.html.
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** $QT_END_LICENSE$
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**
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****************************************************************************/
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/*!
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\example contiguouscache
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\title Contiguous Cache Example
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\brief The Contiguous Cache example shows how to use QContiguousCache to manage memory usage for
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very large models. In some environments memory is limited and, even when it
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isn't, users still dislike an application using excessive memory.
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Using QContiguousCache to manage a list, rather than loading
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the entire list into memory, allows the application to limit the amount
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of memory it uses, regardless of the size of the data set it accesses
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The simplest way to use QContiguousCache is to cache as items are requested. When
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a view requests an item at row N it is also likely to ask for items at rows near
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to N.
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\snippet contiguouscache/randomlistmodel.cpp 0
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After getting the row, the class determines if the row is in the bounds
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of the contiguous cache's current range. It would have been equally valid to
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simply have the following code instead.
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\code
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while (row > m_rows.lastIndex())
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m_rows.append(fetchWord(m_rows.lastIndex()+1);
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while (row < m_rows.firstIndex())
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m_rows.prepend(fetchWord(m_rows.firstIndex()-1);
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\endcode
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However a list will often jump rows if the scroll bar is used directly, resulting in
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the code above causing every row between the old and new rows to be fetched.
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Using QContiguousCache::lastIndex() and QContiguousCache::firstIndex() allows
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the example to determine what part of the list the cache is currently caching.
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These values don't represent the indexes into the cache's own memory, but rather
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a virtual infinite array that the cache represents.
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By using QContiguousCache::append() and QContiguousCache::prepend() the code ensures
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that items that may be still on the screen are not lost when the requested row
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has not moved far from the current cache range. QContiguousCache::insert() can
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potentially remove more than one item from the cache as QContiguousCache does not
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allow for gaps. If your cache needs to quickly jump back and forth between
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rows with significant gaps between them consider using QCache instead.
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And thats it. A perfectly reasonable cache, using minimal memory for a very large
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list. In this case the accessor for getting the words into the cache
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generates random information rather than fixed information. This allows you
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to see how the cache range is kept for a local number of rows when running the
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example.
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\snippet contiguouscache/randomlistmodel.cpp 1
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It is also worth considering pre-fetching items into the cache outside of the
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application's paint routine. This can be done either with a separate thread
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or using a QTimer to incrementally expand the range of the cache prior to
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rows being requested out of the current cache range.
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*/
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