source: trunk/doc/src/examples/textobject.qdoc

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/*!
    \example richtext/textobject
    \title Text Object Example
    
    The Text Object example shows how to insert an SVG file into a
    QTextDocument.

    \image textobject-example.png

    A QTextDocument consists of a hierarchy of elements, such as text blocks and
    frames. A text object describes the structure or format of one or more of these
    elements. For instance, images imported from HTML are implemented using text
    objects. Text objects are used by the document's
    \l{QAbstractTextDocumentLayout}{layout} to lay out and render (paint) the
    document. Each object knows how to paint the elements they govern, and
    calculates their size. 

    To be able to insert an SVG image into a text document, we create
    a text object, and implement painting for that object. This object
    can then be \l{QTextCharFormat::setObjectType()}{set} on a
    QTextCharFormat. We also register the text object with the layout
    of the document, enabling it to draw \l{QTextCharFormat}s governed
    by our text object. We can summarize the procedure with the
    following steps:

    \list
        \o Implement the text object.
        \o Register the text object with the layout of the text
           document.
        \o Set the text object on a QTextCharFormat.
        \o Insert a QChar::ObjectReplacementCharacter with that
           text char format into the document.
    \endlist

    The example consists of the following classes:

    \list
        \o \c{SvgTextObject} implements the text object.
        \o \c{Window} shows a QTextEdit into which SVG images can be
           inserted.
    \endlist

    \section1 SvgTextObject Class Definition

    Let's take a look at the header file of \c {SvgTextObject}:

    \snippet examples/richtext/textobject/svgtextobject.h 0

    A text object is a QObject that implements QTextObjectInterface.
    Note that the first class inherited must be QObject, and that
    you must use Q_INTERFACES to let Qt know that your class
    implements QTextObjectInterface.

    The document layout keeps a collection of text objects stored as
    \l{QObject}s, each of which has an associated object type. The
    layout casts the QObject for the associated object type into the
    QTextObjectInterface.

    The \l{QTextObjectInterface::}{intrinsicSize()} and
    \l{QTextObjectInterface::}{drawObject()} functions are then used
    to calculate the size of the text object and draw it.

    \section1 SvgTextObject Class Implementation

    We start of by taking a look at the
    \l{QTextObjectInterface::}{intrinsicSize()} function:

    \snippet examples/richtext/textobject/svgtextobject.cpp 0

    \c intrinsicSize() is called by the layout to calculate the size
    of the text object. Notice that we have drawn the SVG image on a
    QImage. This is because SVG rendering is quite expensive. The
    example would lag seriously for large images if we drew them
    with a QSvgRenderer each time.

    \snippet examples/richtext/textobject/svgtextobject.cpp 1

    In \c drawObject(), we paint the SVG image using the QPainter
    provided by the layout.

    \section1 Window Class Definition

    The \c Window class is a self-contained window that has a
    QTextEdit in which SVG images can be inserted.

    \snippet examples/richtext/textobject/window.h 0

    The \c insertTextObject() slot inserts an SVG image at the current
    cursor position, while \c setupTextObject() creates and registers
    the SvgTextObject with the layout of the text edit's document.

    The constructor simply calls \c setupTextObject() and \c
    setupGui(), which creates and lays out the widgets of the \c
    Window.

    \section1 Window Class Implementation

    We will now take a closer look at the functions that are relevant
    to our text object, starting with the \c setupTextObject()
    function.
    
    \snippet examples/richtext/textobject/window.cpp 3

    \c {SvgTextFormat}'s value is the number of our object type. It is
    used to identify object types by the document layout. 

    Note that we only create one SvgTextObject instance; it will be
    used for all QTextCharFormat's with the \c SvgTextFormat object
    type.

    Let's move on to the \c insertTextObject() function:

    \snippet examples/richtext/textobject/window.cpp 1

    First, the \c .svg file is opened and its contents are read
    into the \c svgData array. 

    \snippet examples/richtext/textobject/window.cpp 2

    To speed things up, we buffer the SVG image in a QImage.  We use
    \l{QTextFormat::}{setProperty()} to store the QImage in the in the
    QTextCharFormat. We can retrieve it later with
    \l{QTextCharFormat::}{property()}.

    We insert the char format in the standard way - using a
    QTextCursor. Notice that we use the special QChar
    \l{QChar::}{ObjectReplacementCharacter}.
*/