setWindowOpacity not working for a child widget - qt

Question

I've used QPropertyAnimation to change the opacity of a QWidget within a specific duration, let's say a QLabel. However I found that once I give a QWidget a parent, setWindowOpacity doesn't work for it.
By the way, should I call QLabel::show first or QPropertyAnimation::start first?
MainWindow w;
// code worked as expected
QLabel label;
label.setFixedSize(100, 100);
label.setStyleSheet("background-color: red");
label.setWindowOpacity(0);
QPropertyAnimation animation(&label, "windowOpacity");
animation.setDuration(2000);
animation.setStartValue(0.1);
animation.setEndValue(1.0);
label.show();
animation.start();
// windowOpaicty of label is always 1.0
QLabel label(&w);
// same code with QPropertyAnimation
w.show();
animation.start();
// windowOpaicty of label is always 1.0
MainWindow w;
QLabel label(&w);
label.setFixedSize(100, 100);
label.setStyleSheet("background-color: red");
label.setWindowOpacity(0);
w.show();

Answer 1

Looks like windowOpacity property is available only for widgets without parent which means "window" widgets. For any widgets inside "window" widget windowOpacity could be unavailable (at least on Windows OS).
Try to avoid using both qss and any style property (like "opacity"). Sometimes style properties and qss conflict with each other.
Opacity of widget can be implemented with qss only. If you subclass your QLabel and add custom property for opacity (with qss implementation) you can have an opacity animation.
Here is the example. QLabel subclass:
#include <QObject>
#include <QLabel>
class OpacityLabel : public QLabel
{
Q_OBJECT
Q_PROPERTY(float opacity READ opacity WRITE setOpacity)
public:
explicit OpacityLabel(QWidget *parent = nullptr);
float opacity() const;
void setOpacity(const float opacity);
private:
float m_opacity = 1;
};
And cpp
#include "OpacityLabel.h"
OpacityLabel::OpacityLabel(QWidget *parent) : QLabel(parent)
{
}
float OpacityLabel::opacity() const
{
return m_opacity;
}
void OpacityLabel::setOpacity(const float opacity)
{
if (opacity > 1)
{
m_opacity = 1;
}
else if (opacity < 0)
{
m_opacity = 0;
}
else
{
m_opacity = opacity;
}
int opacityValue = m_opacity * 255; //css accepts values 0..255
setStyleSheet(QString("QLabel{background-color: rgba(255, 0, 0, %1); color: rgba(0, 0, 0, %1); }").arg(opacityValue));
}
And using like this (ui->label here is OpacityLabel not QLabel):
QPropertyAnimation *animation = new QPropertyAnimation(ui->label, "opacity");
animation->setDuration(2000);
animation->setStartValue(0.1);
animation->setEndValue(1.0);
animation->start(QAbstractAnimation::DeleteWhenStopped);
You can use QGraphicsOpacityEffect on your QLabel and animate effect instead of label.
For example, set effect:
QGraphicsOpacityEffect *effect = new QGraphicsOpacityEffect(ui->label_2);
ui->label_2->setGraphicsEffect(effect);
And then set animation
QPropertyAnimation *effectAnimation = new QPropertyAnimation(ui->label_2->graphicsEffect(), "opacity");
effectAnimation->setDuration(2000);
effectAnimation->setStartValue(0.1);
effectAnimation->setEndValue(1.0);
effectAnimation->start(QAbstractAnimation::DeleteWhenStopped);