How to reset QLineEdit text by pressing Escape key? - c++

Question

I'm working on a Qt4 project. I have a QLineEdit and I want to re-use behavior that I see when I click the Escape key inside the QLineEdit, but I'm not sure how.
When I press the escape key, I get the field to reset to the value that it was before I started editing. This is useful to me and I want this behavior on-hand.
Ideally, I would like a signal I can fire off that triggers the QLineEdit to reset to the value it was before. I would prefer not to try and fake an escape key event. I can cache the old value of the line edit, but this seems more work if the behavior already exists when I click escape. Thanks.

Answer 1

How can I make QLineEdit widget to respond on Escape key by setting
the text programmatically?
Either by overriding QWidget::event virtual function with the child of QLineEdit or a bit more "local", like installing the event filter:
class MyLineEditEventFilter : public QObject
{
public:
explicit MyLineEditEventFilter(QLineEdit *parent) : QObject(parent)
{}
bool eventFilter(QObject *obj, QEvent *e)
{
switch (e->type())
{
case QEvent::KeyPress:
{
QKeyEvent *keyEvent = static_cast<QKeyEvent *>(e);
if (keyEvent->key() == Qt::Key_Escape)
{
// or set the other text from the variable
reinterpret_cast<QLineEdit *>(parent())->setText("Escape!");
}
break;
}
}
// standard event processing
return QObject::eventFilter(obj, e);
}
};
And the caller is like that:
m_pLineEditSearch = new QLineEdit;
auto* pLineEditEvtFilter = new MyLineEditEventFilter(m_pLineEditSearch);
m_pLineEditSearch->installEventFilter(pLineEditEvtFilter);
For getting the previous text to reset with Escape pressed you may use different methods but you need to obtain the pointer to the object holding the string somehow. That is hard to answer without seeing your code.

Answer 2

You can also use QDataWidgetMapper and map your QLineEdit to its model. The Esc key behavior you're mentioning is already built-in there. You can also take advantage of the other functionalities that QDataWidgetMapper provides.
Here's how you would use it:
//Your model can have multiple rows and columns, let's assume it's just one
//QLineEdit, that will mean one row and one column
QStandardItemModel *model = new QStandardItemModel(1,1,this);
QStandardItem* item = new QStandardItem("QLineEdit initial value");
QLineEdit* myLineEdit = new QLineEdit(this);
QDataWidgetMapper* dataWidgetMapper = new QDataWidgetMapper(this);
dataWidgetMapper->setModel(model);
dataWidgetMapper->addMapping(myLineEdit, 0);
dataWidgetMapper->toFirst();
Hope this helps.
UPDATE:
There have been some valid concerns raised in the comments under my section about some of the pitfalls of QDataWidgetMapper, namely the fact that it not only responds to Escape key but also other keys like Enter/Return. So what happens is that after calling QDataWidgetMapper::addMapping(QWidget*, int) an event filter will be installed on your widget, which will consume Escape and Enter/Return keys. If you are still interested in finding out whether Enter or Escape have been pressed, you can add one more event filter right after that where you can check for other keys pressed. I have tried it and it works.