QTextLayout::drawCursor() does not work when subclassing QAbstractTextDocumentLayout - qt

Question

currently QTextEdit and QPlainTextEdit does not meet my requirements, so I need to subclass QAbstractTextDocumentLayout to provide custom layout of the document. I reference QPlainTextDocumentLayout and QTextDocumentLayout a lot, and finally got a simple layout to display the text. However, I couldn't see the cursor in QTextEdit, which should be blinking. I need help to figure this out.
I am using Qt 5.9.1. The simple project is here. The draw() function of VTextDocumentLayout looks like this:
void VTextDocumentLayout::draw(QPainter *p_painter, const PaintContext &p_context)
{
qDebug() << "VTextDocumentLayout draw()" << p_context.clip << p_context.cursorPosition << p_context.selections.size();
// Find out the blocks.
int first, last;
blockRangeFromRect(p_context.clip, first, last);
if (first == -1) {
return;
}
QTextDocument *doc = document();
QPointF offset(m_margin, m_blocks[first].m_offset);
QTextBlock block = doc->findBlockByNumber(first);
QTextBlock lastBlock = doc->findBlockByNumber(last);
qreal maximumWidth = m_width;
while (block.isValid()) {
const BlockInfo &info = m_blocks[block.blockNumber()];
const QRectF &rect = info.m_rect;
QTextLayout *layout = block.layout();
if (!block.isVisible()) {
offset.ry() += rect.height();
if (block == lastBlock) {
break;
}
block = block.next();
continue;
}
QTextBlockFormat blockFormat = block.blockFormat();
QBrush bg = blockFormat.background();
if (bg != Qt::NoBrush) {
fillBackground(p_painter, rect, bg);
}
auto selections = formatRangeFromSelection(block, p_context.selections);
QPen oldPen = p_painter->pen();
p_painter->setPen(p_context.palette.color(QPalette::Text));
layout->draw(p_painter,
offset,
selections,
p_context.clip.isValid() ? p_context.clip : QRectF());
// Draw the cursor.
int blpos = block.position();
int bllen = block.length();
bool drawCursor = p_context.cursorPosition >= blpos
&& p_context.cursorPosition < blpos + bllen;
Q_ASSERT(p_context.cursorPosition >= -1);
if (drawCursor) {
int cpos = p_context.cursorPosition;
cpos -= blpos;
qDebug() << "draw cursor" << block.blockNumber() << blpos << bllen << p_context.cursorPosition << cpos;
layout->drawCursor(p_painter, offset, cpos);
}
p_painter->setPen(oldPen);
offset.ry() += rect.height();
if (block == lastBlock) {
break;
}
block = block.next();
}
}
I called layout->drawCursor() to draw the cursor but this function seems to do nothing.
Any help is appreciated! Thanks!
Update:
Add the log as following:
VTextDocumentLayout draw() QRectF(67,0 9x13) 19 0
block range 0 1
draw cursor 1 5 15 19 14
blockBoundingRect() 1 13 QRectF(0,0 75x17)
VTextDocumentLayout draw() QRectF(67,0 9x13) -1 0
block range 0 1
blockBoundingRect() 1 13 QRectF(0,0 75x17)
VTextDocumentLayout draw() QRectF(67,0 9x13) 19 0
When running this project in Linux, I coundn't see the cursor. However, when running it in Windows, I could see the cursor but not blinking.
Update:
It seems that QTextEdit pass a wrong clip rect to the layout. If I just inserted one line of text (only one block within the document), I could see the blinking cursor. Quite strange!!!

Answer 1

The default value of PaintContext class cursorPosition is -1.
http://doc.qt.io/qt-4.8/qabstracttextdocumentlayout-paintcontext.html#cursorPosition-var
If any default value is not provided to the cursor position ever (it is a public variable), then my guess is value remains -1, and drawCursor is always false (as the block position index minimum value is zero in worst case).
Try setting some default value to PaintContext::cursorPosition, which may display the cursor.

Answer 2

When subclassing QAbstractTextDocumentLayout, blockBoundingRect() should return the geometry of the block, not just the rect as what QPlainTextDocumentLayout does.
In one word, QPlainTextDocumentLayout provides a BAD example for subsclassing QAbstractTextDocumentLayout.