QPlainTextEdit maxlength implementation

QPlainTextEdit maxlength implementation.cpp

// given:
QPlainTextEdit* widget;
int maxlength;

auto* doc = widget->document();
assert(doc);
QObject::connect(doc, &QTextDocument::contentsChange, this, [maxlength, widget](int pos, int removed, int added) {
   if (added - removed <= 0)
      return;
   if (maxlength < 0)
      return;
   auto text = widget->toPlainText();
   if (text.size() <= maxlength)
      return;
   //
   auto* doc = widget->document();
   auto  cur = QTextCursor(doc);
   const auto blocker = QSignalBlocker(doc);
   cur.setPosition(pos);
   cur.setPosition(pos + added, QTextCursor::MoveMode::KeepAnchor);
   if (cur.hasSelection()) {
      cur.deleteChar();
      text = widget->toPlainText();
   }
   if (text.size() > maxlength) { // failsafe
      text = text.left(maxlength);
      widget->setPlainText(text);
   }
   QApplication::beep();
});

// Other approaches (some seen on the web), and their flaws:

// EVENT FILTER: Hook the keyEvent and block input if already at the max length
//  - Fails if the user pastes a multi-character string in.
//  - seriously, this is so flimsy, why does anyone suggest it

// TRUNCATE: Hook QPlainTextEdit::textChanged and truncate the length
//  - Intended to create the appearance of preventing input by instantly reverting it
//  - Fails if the user types into the middle of the value: we truncate the end
//  - Fails if the user pastes in a long string: we don't know how many characters to delete
//  - Leveraging QPlainTextEdit::textCursor fixes the middle-typing issue but not the pasting issue

// UNDO: Use QPlainTextEdit::undo
//  - Properly handles multi-character operations including pastes...
//  - ...except for pressing and holding a key.
//  - If you hold "A" until the max length is reached, all "A"s are deleted, not just the last.

It's worth noting that I think this would double textChanged events. The QPlainTextEdit internals hook the same signals on QTextDocument, and signal/slot connections are handled in the order they're made, so by the time we have a chance to enforce the max length, I believe textChange will have already been fired.

My approach is to subclass QPlainTextEdit and use a variant on this function which emits a signal when input isn't rejected; essentially it's a duplicate of the textChange signal.