How to add a hover transition to QPushButton?

UPDATE

For some reason the proposed solution does not work as expected on Windows 10. I have updated the answer using painter.setOpacity(0.25); and painter.fillRect(rect(), m_currentColor); as a workaround. The code in the GitHub repository is updated as well.

Cause

QSS is not CSS. There is no transition property. Here is a list of all available properties.

Solution

Instead of using stylesheets, I would suggest you to take another path, which is longer, but gives you more flexibility. Here is the solution:

1. Create a subclass of QPushButton, e.g. AnimatedHoverButton

2. Get notified about QEvent::HoverEnter and QEvent::HoverLeave events by reimplementing QPushButton::event

    bool AnimatedHoverButton::event(QEvent *event)
    {
        switch (event->type()) {
            case QEvent::HoverEnter:
                animateHover(true);
                break;
            case QEvent::HoverLeave:
                animateHover(false);
                break;
            default:
                break;
        }
   
        return QPushButton::event(event);
    }
   

3. Create the in and out transition by using QVariantAnimation

    void AnimatedHoverButton::animateHover(bool in)
    {
        if (m_transition)
            m_transition->stop();
   
        m_transition = new QVariantAnimation(this);
        m_transition->setDuration(m_duration);
        m_transition->setStartValue(m_currentColor);
        m_transition->setEndValue(in ? palette().highlight().color()
                                     : Qt::transparent);
   
        connect(m_transition, &QVariantAnimation::valueChanged,
                this, [this](const QVariant &value){
            m_currentColor = value.value<QColor>();
            repaint();
        });
   
        connect(m_transition, &QVariantAnimation::destroyed,
                this, [this](){
            m_transition = nullptr;
            repaint();
        });
   
        m_transition->start(QAbstractAnimation::DeleteWhenStopped);
    }
   

4. Paint the button by reimplementing the QPushButton::paintEvent event handler and taking into account the current value of the animation

    void AnimatedHoverButton::paintEvent(QPaintEvent */*event*/)
    {
        QStylePainter painter(this);
        QStyleOptionButton option;
   
        initStyleOption(&option);
   
        option.state &= ~QStyle::State_MouseOver;
   
        painter.drawControl(QStyle::CE_PushButton, option);
        painter.setOpacity(0.25);
        painter.fillRect(rect(), m_currentColor);
    }
   

Note: This solution uses the widget's palette to set the start and end values of the animation.

Example

The solution might seem complicated, but fortunatelly I have prepared a working example for you of how to implement and use the AnimatedHoverButton class.

The following code fragment uses the AnimatedHoverButton class to produce a result, similar to the CSS example you have provided:

#include <QApplication>
#include "AnimatedHoverButton.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    AnimatedHoverButton button(QObject::tr("Hover Over Me"));

    button.setTransitionDuration(300);
    button.resize(300, 150);
    button.show();

    return a.exec();
}

The full code of the example is available on GitHub.

Result

The given example produces the following result:

You can use Animation.

MyButton.h

#include <QPushButton>
#include <QColor>
#include <QPropertyAnimation>

class MyButton : public QPushButton
{
    Q_OBJECT
    Q_PROPERTY(QColor color READ GetColor WRITE SetColor)

public:
    explicit MyButton(QWidget *parent = 0);

    void SetColor(const QColor& color);
    const QColor& GetColor() const;

protected:
    bool eventFilter(QObject *obj, QEvent *e);

private:
    QColor m_currentColor;

    QPropertyAnimation m_colorAnimation;

    void StartHoverEnterAnimation();
    void StartHoverLeaveAnimation();
};

MyButton.cpp

#include "MyButton.h"

#include <QEvent>
#include <QDebug>

MyButton::MyButton(QWidget *parent) :
    QPushButton(parent),
    m_colorAnimation(this, "color")
{
    this->installEventFilter(this);
}

void MyButton::SetColor(const QColor& color)
{
    m_currentColor = color;
    QString css = "QPushButton { border-radius: 5px; ";
    css.append("border: 1.5px solid rgb(91,231,255); ");
    QString strColor = QString("rgb(%1, %2, %3)").arg(color.red()).arg(color.green()).arg(color.blue());
    css.append("background-color: " + strColor + "; }");
    setStyleSheet(css);
}

const QColor& MyButton::GetColor() const
{
    return m_currentColor;
}

bool MyButton::eventFilter(QObject *obj, QEvent *e)
{
    if (e->type() == QEvent::HoverEnter) {
        StartHoverEnterAnimation();
    }

    if (e->type() == QEvent::HoverLeave) {
        StartHoverLeaveAnimation();
    }

    return false;
}

void MyButton::StartHoverEnterAnimation()
{
    m_colorAnimation.stop();

    m_colorAnimation.setDuration(900); //set your transition
    m_colorAnimation.setStartValue(GetColor()); //starts from current color
    m_colorAnimation.setEndValue(QColor(100, 100, 100));//set your hover color

    m_colorAnimation.setEasingCurve(QEasingCurve::Linear);//animation style

    m_colorAnimation.start();
}

void MyButton::StartHoverLeaveAnimation()
{
    m_colorAnimation.stop();

    m_colorAnimation.setDuration(900); //set your transition
    m_colorAnimation.setStartValue(GetColor()); //starts from current color
    m_colorAnimation.setEndValue(QColor(255, 0, 0));//set your regular color

    m_colorAnimation.setEasingCurve(QEasingCurve::Linear);//animation style

    m_colorAnimation.start();
}

It will conflict with external qss setting. So set all qss in SetColor.