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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Qt Software Information (qt-info@nokia.com)
**
** This file is part of the $MODULE$ of the Qt Toolkit.
**
** $TROLLTECH_DUAL_LICENSE$
**
****************************************************************************/
/*!
\example tools/offsetvector
\title Offset Vector Example
The Offset Vector example shows how to use QOffsetVector 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 QOffsetVector 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 QOffsetVector 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 examples/tools/offsetvector/randomlistmodel.cpp 0
After getting the row the class determines if the row is in the bounds
of the offset vector's current range. It would have been equally valid to
simply have the following code instead.
\code
while (row > m_words.lastIndex())
m_words.append(fetchWord(m_words.lastIndex()+1);
while (row < m_words.firstIndex())
m_words.prepend(fetchWord(m_words.firstIndex()-1);
\endcode
However a list will often jump rows if the scroll bar is used directly, and
the above code would cause every row between where the cache was last centered
to where the cache is currently centered to be cached before the requested
row is reached.
Using QOffsetVector::lastIndex() and QOffsetVector::firstIndex() allows
the example to determine where the list the vector is currently over. These values
don't represent the indexes into the vector own memory, but rather a virtual
infinite array that the vector represents.
By using QOffsetVector::append() and QOffsetVector::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 vector range. QOffsetVector::insert() can
potentially remove more than one item from the cache as QOffsetVector 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 cache:
\snippet examples/tools/offsetvector/randomlistmodel.cpp 1
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.
It is also worth considering pre-fetching items into the cache outside of the
applications paint routine. This can be done either with a separate thread
or using a QTimer to incrementally expand the range of the thread prior to
rows being requested out of the current vector range.
*/
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