doc/src/examples/textobject.qdoc
author Eckhart Koeppen <eckhart.koppen@nokia.com>
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/****************************************************************************
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**
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** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
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** All rights reserved.
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** Contact: Nokia Corporation (qt-info@nokia.com)
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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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** $QT_BEGIN_LICENSE:LGPL$
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** No Commercial Usage
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** This file contains pre-release code and may not be distributed.
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** You may use this file in accordance with the terms and conditions
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** contained in the Technology Preview License Agreement accompanying
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** this package.
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**
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** GNU Lesser General Public License Usage
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** Alternatively, this file may be used under the terms of the GNU Lesser
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** General Public License version 2.1 as published by the Free Software
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** Foundation and appearing in the file LICENSE.LGPL included in the
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** packaging of this file.  Please review the following information to
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** ensure the GNU Lesser General Public License version 2.1 requirements
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** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
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**
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** In addition, as a special exception, Nokia gives you certain additional
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** rights.  These rights are described in the Nokia Qt LGPL Exception
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** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
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**
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** If you have questions regarding the use of this file, please contact
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** Nokia at qt-info@nokia.com.
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**
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**
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**
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**
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**
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**
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**
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**
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** $QT_END_LICENSE$
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**
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****************************************************************************/
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/*!
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    \example richtext/textobject
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    \title Text Object Example
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    The Text Object example shows how to insert an SVG file into a
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    QTextDocument.
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    \image textobject-example.png
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    A QTextDocument consists of a hierarchy of elements, such as text blocks and
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    frames. A text object describes the structure or format of one or more of these
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    elements. For instance, images imported from HTML are implemented using text
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    objects. Text objects are used by the document's
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    \l{QAbstractTextDocumentLayout}{layout} to lay out and render (paint) the
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    document. Each object knows how to paint the elements they govern, and
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    calculates their size. 
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    To be able to insert an SVG image into a text document, we create
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    a text object, and implement painting for that object. This object
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    can then be \l{QTextCharFormat::setObjectType()}{set} on a
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    QTextCharFormat. We also register the text object with the layout
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    of the document, enabling it to draw \l{QTextCharFormat}s governed
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    by our text object. We can summarize the procedure with the
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    following steps:
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    \list
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        \o Implement the text object.
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        \o Register the text object with the layout of the text
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           document.
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        \o Set the text object on a QTextCharFormat.
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        \o Insert a QChar::ObjectReplacementCharacter with that
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           text char format into the document.
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    \endlist
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    The example consists of the following classes:
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    \list
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        \o \c{SvgTextObject} implements the text object.
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        \o \c{Window} shows a QTextEdit into which SVG images can be
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           inserted.
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    \endlist
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    \section1 SvgTextObject Class Definition
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    Let's take a look at the header file of \c {SvgTextObject}:
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    \snippet examples/richtext/textobject/svgtextobject.h 0
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    A text object is a QObject that implements QTextObjectInterface.
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    Note that the first class inherited must be QObject, and that
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    you must use Q_INTERFACES to let Qt know that your class
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    implements QTextObjectInterface.
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    The document layout keeps a collection of text objects stored as
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    \l{QObject}s, each of which has an associated object type. The
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    layout casts the QObject for the associated object type into the
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    QTextObjectInterface.
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    The \l{QTextObjectInterface::}{intrinsicSize()} and
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    \l{QTextObjectInterface::}{drawObject()} functions are then used
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    to calculate the size of the text object and draw it.
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    \section1 SvgTextObject Class Implementation
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    We start of by taking a look at the
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    \l{QTextObjectInterface::}{intrinsicSize()} function:
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    \snippet examples/richtext/textobject/svgtextobject.cpp 0
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    \c intrinsicSize() is called by the layout to calculate the size
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    of the text object. Notice that we have drawn the SVG image on a
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    QImage. This is because SVG rendering is quite expensive. The
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    example would lag seriously for large images if we drew them
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    with a QSvgRenderer each time.
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    \snippet examples/richtext/textobject/svgtextobject.cpp 1
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    In \c drawObject(), we paint the SVG image using the QPainter
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    provided by the layout.
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    \section1 Window Class Definition
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    The \c Window class is a self-contained window that has a
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    QTextEdit in which SVG images can be inserted.
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    \snippet examples/richtext/textobject/window.h 0
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    The \c insertTextObject() slot inserts an SVG image at the current
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    cursor position, while \c setupTextObject() creates and registers
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    the SvgTextObject with the layout of the text edit's document.
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    The constructor simply calls \c setupTextObject() and \c
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    setupGui(), which creates and lays out the widgets of the \c
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    Window.
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    \section1 Window Class Implementation
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    We will now take a closer look at the functions that are relevant
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    to our text object, starting with the \c setupTextObject()
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    function.
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    \snippet examples/richtext/textobject/window.cpp 3
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    \c {SvgTextFormat}'s value is the number of our object type. It is
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    used to identify object types by the document layout. 
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    Note that we only create one SvgTextObject instance; it will be
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    used for all QTextCharFormat's with the \c SvgTextFormat object
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    type.
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    Let's move on to the \c insertTextObject() function:
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    \snippet examples/richtext/textobject/window.cpp 1
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    First, the \c .svg file is opened and its contents are read
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    into the \c svgData array. 
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    \snippet examples/richtext/textobject/window.cpp 2
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    To speed things up, we buffer the SVG image in a QImage.  We use
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    \l{QTextFormat::}{setProperty()} to store the QImage in the in the
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    QTextCharFormat. We can retrieve it later with
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    \l{QTextCharFormat::}{property()}.
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    We insert the char format in the standard way - using a
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    QTextCursor. Notice that we use the special QChar
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    \l{QChar::}{ObjectReplacementCharacter}.
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*/
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