How to implement undo in a QLineEdit heir? - c++

Question

How to use QLineEdit's built-in undo/redo functionality while implementing custom input logic?
I am implementing a custom editor for amounts of money for a cashier's efficient work. Basic features are:
The format is always ^\d+,\d{2}$;
When cursor is left to comma, it edits the integer part (e.g., when 0,00 is edited, the first digit input replaces the zero; i.e., 0,00 becomes 1,00 after '1' is entered);
Comma or period entered moves cursor to the fractional part;
Deletes and backspaces respect the format.
I can't use masks since they can't manage arbitrary length without unnecessary spaces. I can't use validators because of they are not enough flexible to handle logic of editing leading zero in place. That's why I sublassed QLineEdit and wrote the necessary logic myself.
The question is: is there a way to support undo functionality on QLineEdit's own logic? I use setText(), that resets undo/redo history. I could make two stacks of states and override undo() and redo() correspondingly, or make sequences of selections and insertions/deletions (what can cause unnecessary blinks), but I feel there may be even simpler way.
class MoneyLineEdit : public QLineEdit {
Q_OBJECT
public:
MoneyLineEdit(QWidget *parent = 0);
protected:
void keyPressEvent(QKeyEvent * event);
};
void MoneyLineEdit::keyPressEvent(QKeyEvent *event) {
if (event->key() == Qt::Key_Comma || event->key() == Qt::Key_Period) {
setCursorPosition(text().length() - 2);
} else {
QString text = this->text();
int pos = cursorPosition();
if (event->key() == Qt::Key_Backspace) {
if (pos == text.length()) {
text.replace(text.length() - 1, 1, QChar('0'));
--pos;
} else if (pos == text.length() - 1) {
text.replace(text.length() - 2, 1, text.at(text.length() - 1));
text.replace(text.length() - 1, 1, QChar('0'));
--pos;
} else if (pos == text.length() - 2) {
--pos;
} else if (text.length() == 4 && pos == 1) {
text.replace(0, 1, QChar('0'));
}
setText(text);
setCursorPosition(pos);
} else if ((pos == text.length() - 3 || pos == text.length() - 2) && event->key() == Qt::Key_Delete && selectionStart() == -1){
text.replace(text.length() - 2, 1, text.at(text.length() - 1));
text.replace(text.length() - 1, 1, QChar('0'));
setText(text);
setCursorPosition(pos);
} else if (pos >= text.length() - 2 && pos <= text.length() - 1 && !event->text().isEmpty()){
text.replace(cursorPosition(), 1, event->text());
++pos;
setText(text);
setCursorPosition(pos);
} else if ((pos == 0 || pos == 1) && !text.isEmpty() && text.length() == 4 && text.at(0) == QChar('0') && !event->text().isEmpty()) {
text.replace(0, 1, event->text());
setText(text);
setCursorPosition(1);
} else {
QLineEdit::keyPressEvent(event);
}
}
}

Answer 1

Obviously, two stacks of Pair(text,cursorPosition) for undo()/redo() overloaded are sufficient.