I am painting a block of text on a widget with QTextDocument::drawContents. My current goal is to intercept mouse events and emulate text selection. It's pretty clear how to handle the mouse, but displaying the result puzzles me a lot.

Just before we start: I can not use QLabel and let it handle selection on it's own (it has no idea how to draw unusual characters and messes up line height (https://git.macaw.me/blue/squawk/issues/59)), nor I can not use QTextBrowser there - it's just a message bubble, I'm not ready to sacrifice performance there.

There is a very rich framework around QTextDocument, but I can not find any way to make it color the background of some fragment of text that I would consider selected. Found a way to make a frame around a text, found a way to draw under-over-lined text, but it looks like there is simply just no way this framework can draw a background behind text.

I have tried doing this, to see if I can take selected fragment under some QTextFrame and set it's style:

QTextDocument* bodyRenderer = new QTextDocument();
bodyRenderer->setHtml("some text");
bodyRenderer->setTextWidth(50);
painter->setBackgroundMode(Qt::BGMode::OpaqueMode); //this at least makes it color background under all text
QTextFrameFormat format = bodyRenderer->rootFrame()->frameFormat();
format.setBackground(option.palette.brush(QPalette::Active, QPalette::Highlight));
bodyRenderer->rootFrame()->setFrameFormat(format);
bodyRenderer->drawContents(painter);

Nothing of this works too:

QTextBlock b = bodyRenderer->begin();
QTextBlockFormat format = b.blockFormat();
format.setBackground(option.palette.brush(QPalette::Active, QPalette::Highlight));
format.setProperty(QTextFormat::BackgroundBrush, option.palette.brush(QPalette::Active, QPalette::Highlight));
QTextCursor cursor(bodyRenderer);
cursor.setBlockFormat(format);
b = bodyRenderer->begin();
while (b.isValid() > 0) {
    QTextLayout* lay = b.layout();
    QTextLayout::FormatRange range;
    range.format = b.charFormat();
    range.start = 0;
    range.length = 2;
    lay->draw(painter, option.rect.topLeft(), {range});
    b = b.next();
}

Is there any way I can make this framework do a simple thing - draw a selection background behind some text? If not - is there a way I can unproject cursor position into coordinate translation, like can I do reverse operation from QAbstractTextDocumentLayout::hitTest just to understand where to draw that selection rectangle myself?

CodePudding user response：

You can use QTextCursor to change the background of the selected text. You only need to select one character at a time to keep the formatting. Here is an example of highlighting in blue (the color of the text is highlighted in white for contrast):

void MainWindow::paintEvent(QPaintEvent *event) {
  QPainter painter(this);
  painter.fillRect(contentsRect(), QBrush(QColor("white")));
  QTextDocument document;
  document.setHtml(QString("Hello <font size='20'>world</font> with Qt!"));

  int selectionStart = 3;
  int selectionEnd = selectionStart   10;
  QTextCursor cursor(&document);
  cursor.setPosition(selectionStart);
  while (cursor.position() < selectionEnd) {
    cursor.movePosition(QTextCursor::Right, QTextCursor::KeepAnchor); // select one symbol
    QTextCharFormat selectFormat = cursor.charFormat();
    selectFormat.setBackground(Qt::blue);
    selectFormat.setForeground(Qt::white);
    cursor.setCharFormat(selectFormat);                                // set format for selection
    cursor.movePosition(QTextCursor::Right, QTextCursor::MoveAnchor, 1);
  }

  document.drawContents(&painter, contentsRect());
  QMainWindow::paintEvent(event);
}