Text Object Example¶
The Text Object example shows how to insert an SVG file into a
QTextDocument
.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’slayout
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
set
on aQTextCharFormat
. We also register the text object with the layout of the document, enabling it to drawQTextCharFormat
s governed by our text object. We can summarize the procedure with the following steps:
Implement the text object.
Register the text object with the layout of the text document.
Set the text object on a
QTextCharFormat
.Insert a
ObjectReplacementCharacter
with that text char format into the document.The example consists of the following classes:
SvgTextObject
implements the text object.
Window
shows aQTextEdit
into which SVG images can be inserted.
SvgTextObject Class Definition¶
Let’s take a look at the header file of
SvgTextObject
:class SvgTextObject(QObject, QTextObjectInterface): def __init__(self,...): super(SvgTextObject, self).__init__(...) ... public: QSizeF intrinsicSize(QTextDocument *doc, int posInDocument, const QTextFormat &format); void drawObject(QPainter *painter, const QRectF &rect, QTextDocument *doc, int posInDocument, const QTextFormat &format); };A text object is a
QObject
that implementsQTextObjectInterface
. Note that the first class inherited must beQObject
, and that you must useQ_INTERFACES
to let Qt know that your class implementsQTextObjectInterface
.The document layout keeps a collection of text objects stored as
QObject
s, each of which has an associated object type. The layout casts theQObject
for the associated object type into theQTextObjectInterface
.The
intrinsicSize()
anddrawObject()
functions are then used to calculate the size of the text object and draw it.
SvgTextObject Class Implementation¶
We start of by taking a look at the
intrinsicSize()
function:QSizeF SvgTextObject::intrinsicSize(QTextDocument * /*doc*/, int /*posInDocument*/, const QTextFormat &format) { QImage bufferedImage = qvariant_cast<QImage>(format.property(Window::SvgData)); QSize size = bufferedImage.size(); if (size.height() > 25) size *= 25.0 / (double) size.height(); return QSizeF(size); }
intrinsicSize()
is called by the layout to calculate the size of the text object. Notice that we have drawn the SVG image on aQImage
. This is because SVG rendering is quite expensive. The example would lag seriously for large images if we drew them with aQSvgRenderer
each time.void SvgTextObject::drawObject(QPainter *painter, const QRectF &rect, QTextDocument * /*doc*/, int /*posInDocument*/, const QTextFormat &format) { QImage bufferedImage = qvariant_cast<QImage>(format.property(Window::SvgData)); painter->drawImage(rect, bufferedImage); }In
drawObject()
, we paint the SVG image using theQPainter
provided by the layout.
Window Class Definition¶
The
Window
class is a self-contained window that has aQTextEdit
in which SVG images can be inserted.class Window : public QWidget { Q_OBJECT public: enum { SvgTextFormat = QTextFormat::UserObject + 1 }; enum SvgProperties { SvgData = 1 }; Window(); private slots: void insertTextObject(); private: void setupTextObject(); void setupGui(); private: QTextEdit *textEdit; QLabel *fileNameLabel; QLineEdit *fileNameLineEdit; QPushButton *insertTextObjectButton; };The
insertTextObject()
slot inserts an SVG image at the current cursor position, whilesetupTextObject()
creates and registers the SvgTextObject with the layout of the text edit’s document.The constructor simply calls
setupTextObject()
andsetupGui()
, which creates and lays out the widgets of theWindow
.
Window Class Implementation¶
We will now take a closer look at the functions that are relevant to our text object, starting with the
setupTextObject()
function.void Window::setupTextObject() { QObject *svgInterface = new SvgTextObject; svgInterface->setParent(this); textEdit->document()->documentLayout()->registerHandler(SvgTextFormat, svgInterface); }
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 theSvgTextFormat
object type.Let’s move on to the
insertTextObject()
function:void Window::insertTextObject() { QString fileName = fileNameLineEdit->text(); QFile file(fileName); if (!file.open(QIODevice::ReadOnly)) { QMessageBox::warning(this, tr("Error Opening File"), tr("Could not open '%1'").arg(fileName)); } QByteArray svgData = file.readAll();First, the
.svg
file is opened and its contents are read into thesvgData
array.QTextCharFormat svgCharFormat; svgCharFormat.setObjectType(SvgTextFormat); QSvgRenderer renderer(svgData); QImage svgBufferImage(renderer.defaultSize(), QImage::Format_ARGB32); QPainter painter(&svgBufferImage); renderer.render(&painter, svgBufferImage.rect()); svgCharFormat.setProperty(SvgData, svgBufferImage); QTextCursor cursor = textEdit->textCursor(); cursor.insertText(QString(QChar::ObjectReplacementCharacter), svgCharFormat); textEdit->setTextCursor(cursor); }To speed things up, we buffer the SVG image in a
QImage
. We usesetProperty()
to store theQImage
in the in theQTextCharFormat
. We can retrieve it later withproperty()
.We insert the char format in the standard way - using a
QTextCursor
. Notice that we use the specialQChar
ObjectReplacementCharacter
.
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