qt 6.5.1 original

examples/corelib/tools/doc/src/contiguouscache.qdoc

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+// Copyright (C) 2016 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
+
+/*!
+    \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 that's 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.
+*/

examples/corelib/tools/doc/src/customtype.qdoc

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+// Copyright (C) 2016 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
+
+/*!
+    \example tools/customtype
+    \title Custom Type Example
+
+    \brief The Custom Type example shows how to integrate a custom type into Qt's
+    meta-object system.
+
+    Contents:
+
+    \tableofcontents
+
+    \section1 Overview
+
+    Qt provides a range of standard value types that are used to provide
+    rich and meaningful APIs. These types are integrated with the meta-object
+    system, enabling them to be stored in QVariant objects, written out in
+    debugging information and sent between components in signal-slot
+    communication.
+
+    Custom types can also be integrated with the meta-object system as long as
+    they are written to conform to some simple guidelines. In this example, we
+    introduce a simple \c Message class, we describe how we make it work with
+    QVariant, and we show how it can be extended to generate a printable
+    representation of itself for use in debugging output.
+
+    \section1 The Message Class Definition
+
+    The \c Message class is a simple value class that contains two pieces
+    of information (a QString and a QStringList), each of which can be read
+    using trivial getter functions:
+
+    \snippet tools/customtype/message.h custom type definition
+
+    The default constructor, copy constructor and destructor are
+    all required, and must be public, if the type is to be integrated into the
+    meta-object system. Other than this, we are free to implement whatever we
+    need to make the type do what we want, so we also include a constructor
+    that lets us set the type's data members.
+
+    To enable the type to be used with QVariant, we declare it using the
+    Q_DECLARE_METATYPE() macro:
+
+    \snippet tools/customtype/message.h custom type meta-type declaration
+
+    We do not need to write any additional code to accompany this macro.
+
+    To allow us to see a readable description of each \c Message object when it
+    is sent to the debug output stream, we define a streaming operator:
+
+    \snippet tools/customtype/message.h custom type streaming operator
+
+    This facility is useful if you need to insert tracing statements in your
+    code for debugging purposes.
+
+    \section1 The Message Class Implementation
+
+    The streaming operator is implemented in the following way:
+
+    \snippet tools/customtype/message.cpp custom type streaming operator
+
+    Here, we want to represent each value depending on how many lines are stored
+    in the message body. We stream text to the QDebug object passed to the
+    operator and return the QDebug object obtained from its maybeSpace() member
+    function; this is described in more detail in the
+    \l{Creating Custom Qt Types#Making the Type Printable}{Creating Custom Qt Types}
+    document.
+
+    We include the code for the getter functions for completeness:
+
+    \snippet tools/customtype/message.cpp getter functions
+
+    With the type fully defined, implemented, and integrated with the
+    meta-object system, we can now use it.
+
+    \section1 Using the Message
+
+    In the example's \c{main()} function, we show how a \c Message object can
+    be printed to the console by sending it to the debug stream:
+
+    \snippet tools/customtype/main.cpp printing a custom type
+
+    You can use the type with QVariant in exactly the same way as you would
+    use standard Qt value types. Here's how to store a value using the
+    QVariant::setValue() function:
+
+    \snippet tools/customtype/main.cpp storing a custom value
+
+    Alternatively, the QVariant::fromValue() function can be used if
+    you are using a compiler without support for member template
+    functions.
+
+    The value can be retrieved using the QVariant::value() member template
+    function:
+
+    \snippet tools/customtype/main.cpp retrieving a custom value
+
+    \section1 Further Reading
+
+    The custom \c Message type can also be used with direct signal-slot
+    connections.
+
+    To register a custom type for use with queued signals and slots, such as
+    those used in cross-thread communication, see the
+    \l{Queued Custom Type Example}.
+
+    More information on using custom types with Qt can be found in the
+    \l{Creating Custom Qt Types} document.
+*/