How do I reuse Qt's QTextCursor in my C++ application? - c++

Question

I am implementing a text editor on Windows using Qt in C++ and am using a QWidget as the surface on which I am displaying text using OpenGL. So far, I have my own layout engine and document model and am able to get the text to display on the widget.
Now I am trying to implement a text cursor that will be used similar to the one Qt provides, but QTextCursor is closely tied to its QTextDocument model and I am not able to subclass it for reuse using my model. Is there any way to reuse just the cursor without the model?
If not, how do I go about implementing a text cursor using Qt?
Note: I did go through the Caret methods that Windows provides here, but am hoping to avoid using them directly.

Answer 1

TL;DR: You can't. It's not a caret.
QTextCursor is an iterator, it has nothing to do whatsoever with the on-screen cursor. You can certainly reuse it if it is useful as an iterator, and iff your own text representation is built on top of QTextDocument. But it's not a caret.
The visible cursor control is provided by the QTextEdit implementation. Recall that QTextEdit is a view for a QTextDocument - it is entirely devoted to the graphical rendering of the text, controlling the visible cursor, etc.
There is no public Qt API that you can use for the caret. The WINAPI caret methods are completely useless when you're using Qt for rendering. You need your own caret implementation. Given that you already have a text representation and a renderer, you presumably have an iterator that works on the text representation, so implementing the caret should be a trivial affair.
The WINAPI caret is very simple and trivial to reimplement with the power of Qt:
class Caret : public QWidget {
Q_OBJECT
Q_PROPERTY(int period READ period WRITE setPeriod)
QPicture m_shape;
BasicTimer m_timer;
int m_period;
void updateSize() {
auto size = m_shape.boundingRect().size();
setFixedSize(size);
resize(size);
}
void timerEvent(QTimerEvent * ev) {
if (ev->timerId() != m_timer.timerId()) return;
if (isVisible()) hide(); else show();
}
public:
QPicture defaultShape(int w, int h) {
QPicture pic;
QPainter p(&pic);
p.fillRect(0, 0, w, h, Qt::black);
return pic;
}
Caret(QWidget * parent = 0, const QPicture & pic = defaultShape()) :
QWidget(parent), m_shape(pic), m_period(250) {
setAttribute(Qt::WA_TransparentForMouseEvents);
setAttribute(Qt::WA_TranslucentBackground);
m_timer.start(m_period);
updateSize();
}
void setShape(const QPicture & pic) {
m_shape = pic;
updateSize();
update();
}
void setPeriod(int period) {
if (period < 1) {
m_timer.stop();
if (m_period > 0) show();
} else
m_timer.start(period);
m_period = period;
}
int period() const { return m_period; }
void paintEvent(QPaintEvent *) Q_DECL_OVERRIDE {
auto painter = QPainter(this);
painter.fillRect(rect(), Qt::transparent);
painter.drawPicture(QPoint(), m_shape);
}
};