Recognize if a QLineEdit loses a focus

Question

In one of my projects I have a series of QLineEdit Widgets, that should accept double numbers, that are lying in a certain range. For certain reasons I can not use QDoubleSpinBox.

Now I'm using QDoubleValidator to check, if my number lies in the given range. Unfortunately, the signal editingFinished is only emitted if QValidator gives QValidator::Acceptable.

Now assume, that I might have a series of such QLineEdit widgets. In one I enter a bad number and then I switch the focus to another widget. The user is left with a bad value inside the QLineEdit.

My desired behavior instead would be to set the focus to the widget containing the bad input and giving a warning.

For some reasons, I'm failing to implement this feature. Even after I consulted the Qt docs.

Here is my full code.

ValidatedDoubleEditorWidget.h

#pragma once

#include <QWidget>

namespace Ui {
    class validatedDoubleEditorWidget;
}

class QDoubleValidator;

    class ValidatedDoubleEditorWidget : public QWidget {
        Q_OBJECT
    public:
        ValidatedDoubleEditorWidget(double min, double max, double value);

        double getValue() const;
        void setValue(const double value);

    private slots:
        void on_lineEdit_editingFinished();

    protected:
        virtual void focusOutEvent(QFocusEvent *event) override;


        virtual void focusInEvent(QFocusEvent *event) override;

    private:
        Ui::validatedDoubleEditorWidget* mWidget = nullptr;
        double mValue = 0.;
        double mMin = 0.;
        double mMax = 0.;
        QDoubleValidator* mValidator = nullptr;

    };

ValidatedDoubleEditorWidget.cpp

#include "ValidatedDoubleEditorWidget.h"
#include <QDoubleValidator>
#include <QMessageBox>
#include <QDebug>
#include "ui_ValidatedDoubleEditorWidget.h"

ValidatedDoubleEditorWidget::ValidatedDoubleEditorWidget(double min, double max, double value)
{
    mWidget = new Ui::validatedDoubleEditorWidget;
    mWidget->setupUi(this);
    mValue = value;
    mWidget->lineEdit->setText(QString("%1").arg(value));
    mValidator = new QDoubleValidator(min, max, 20, this);
    mWidget->lineEdit->setValidator(mValidator);
    setFocusProxy(mWidget->lineEdit);
    setFocusPolicy(Qt::StrongFocus);
}

double ValidatedDoubleEditorWidget::getValue() const
{
    return mValue;
}

void ValidatedDoubleEditorWidget::setValue(const double value)
{
    mValue = value;
    mWidget->lineEdit->setText(QString("%1").arg(value));
}


void ValidatedDoubleEditorWidget::on_lineEdit_editingFinished()
{
    QString text = mWidget->lineEdit->text();
    qDebug() << "Editing finished";
    bool ok;
    double value = text.toDouble(&ok);
    if (!ok) {
        //
    }
    else {
        mValue = value;
    }
}

void ValidatedDoubleEditorWidget::focusOutEvent(QFocusEvent *event)
{
    qDebug() << "OutFocus";
    QString text = mWidget->lineEdit->text();
    int i;
    auto state=mValidator->validate(text, i);
    if (state != QValidator::Acceptable) {
        QMessageBox::warning(this, tr("Invalid Input!"), tr("Please check your input."), QMessageBox::Ok); 
        mWidget->lineEdit->setText(QString("%1").arg(mValue));
        mWidget->lineEdit->setFocus();
    }

}

void ValidatedDoubleEditorWidget::focusInEvent(QFocusEvent *event)
{
    qDebug() << "InFocus";
}

TestRunner.cpp

#include <QApplication>
#include <QMap>
#include <QFrame>
#include <QHBoxLayout>
#include "ValidatedDoubleEditorWidget.h"

int main(int argc, char** args) {
    QApplication app(argc, args);
    QFrame frame;
    frame.setLayout(new QHBoxLayout);
    frame.layout()->addWidget(new ValidatedDoubleEditorWidget(-1., 4., 1.));
    frame.layout()->addWidget(new ValidatedDoubleEditorWidget(-2., 4., 5.));

    frame.show();
    app.exec();
    return 0;
}

Answer 1

You can subclass QLineEdit and reimplement focusOutEvent method. Call QLineEdit::hasAcceptableInput to check if the input is valid. If it is not, you can call setFocus to regain input focus. You can also show a warning dialog or emit some signal. Here is an example:

#include <QtWidgets>

class CustomLineEdit : public QLineEdit
{
    Q_OBJECT
public:
    CustomLineEdit(QWidget *parent = nullptr) : QLineEdit(parent){}
protected:
    void focusOutEvent(QFocusEvent *event)
    {
        QLineEdit::focusOutEvent(event);
        if(!hasAcceptableInput())
        {
            setFocus();
            emit validationError();
        }
    }
signals:
    void validationError();
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow m;
    m.setCentralWidget(new QWidget);
    m.centralWidget()->setLayout(new QVBoxLayout);

    QDoubleValidator d_validator(0, 10, 2);
    CustomLineEdit l1;
    CustomLineEdit l2;
    l1.setValidator(&d_validator);
    l2.setValidator(&d_validator);

    QObject::connect(&l1, &CustomLineEdit::validationError, [=]{qDebug() << "Validation error!";});
    QObject::connect(&l2, &CustomLineEdit::validationError, [=]{qDebug() << "Validation error!";});

    m.centralWidget()->layout()->addWidget(&l1);
    m.centralWidget()->layout()->addWidget(&l2);

    m.show();
    return a.exec();
}

#include "main.moc"