blob: 3a207c24cdb6057cbc32db768ecd8eff5d7f1433 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
|
/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** 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.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\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 examples/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 examples/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.
*/
|
