Making QLabel fade out the current text and fade in the new one when setText is called

Violet Giraffe

I want any text change via setText to be done via transparency animation instead of immediate change. Is that doable with a standard QLabel? QPropertyAnimation, perhaps?

kshegunov

@Violet-Giraffe
QPropertyAnimation over a QGraphicsOpacityEffect would be the first thing I'd try. It should work with a generic label, or with any widget.

@Violet-Giraffe Hi! @kshegunov's post hints into the right direction. You can't use QPropertyAnimation to interpolate between two strings but you can create a custom widget with two overlapping labels and use QProperyAnimation and QGraphicsOpacityEffect to animate the fading between the two labels. Something like:

fadinglabel.h

#ifndef FADINGLABEL_H
#define FADINGLABEL_H

#include <QWidget>
#include <QString>

class QLabel;
class QGraphicsOpacityEffect;
class QPropertyAnimation;

class FadingLabel : public QWidget
{
    Q_OBJECT
public:
    explicit FadingLabel(QWidget *parent = 0);

    void setText(const QString &text);

private:
    QLabel *m_label_back;
    QLabel *m_label_front;
    QGraphicsOpacityEffect *m_effect_back;
    QGraphicsOpacityEffect *m_effect_front;
    QPropertyAnimation *m_animation_back;
    QPropertyAnimation *m_animation_front;
};

#endif // FADINGLABEL_H

fadinglabel.cpp

#include "fadinglabel.h"

#include <QHBoxLayout>
#include <QLabel>
#include <QGraphicsOpacityEffect>
#include <QPropertyAnimation>

FadingLabel::FadingLabel(QWidget *parent)
    : QWidget(parent)
{
    QHBoxLayout *layout = new QHBoxLayout;
    this->setLayout(layout);
    m_label_back = new QLabel(this);
    m_label_front = new QLabel(this);

    m_effect_back = new QGraphicsOpacityEffect();
    m_effect_back->setOpacity(0.0f);
    m_label_back->setGraphicsEffect(m_effect_back);

    m_effect_front = new QGraphicsOpacityEffect();
    m_effect_front->setOpacity(1.0f);
    m_label_front->setGraphicsEffect(m_effect_front);

    m_animation_back = new QPropertyAnimation(m_effect_back, "opacity");
    m_animation_back->setDuration(1000);
    m_animation_back->setStartValue(1.0f);
    m_animation_back->setEndValue(0.0f);

    m_animation_front = new QPropertyAnimation(m_effect_front, "opacity");
    m_animation_front->setDuration(1000);
    m_animation_front->setStartValue(0.0f);
    m_animation_front->setEndValue(1.0f);
}

void FadingLabel::setText(const QString &text)
{
    m_label_back->setText( m_label_front->text() );
    m_label_front->setText( text );
    m_animation_back->start();
    m_animation_front->start();
}

kshegunov

@Wieland
Nice example. I don't know why you use new so much, though ... ;)

@kshegunov said:

I don't know why you use new so much

To avoid too many #includes in the header. An actual implementation for production would use pimpl idiom.

Violet Giraffe

Thanks for the suggestions, colleagues! Let me try and see if I can make it work in my project.
Not sure if optimal, but certainly a clever idea with two labels.

Just my two cents: while I do try to minimize the number of #include in my headers as well, I wouldn't pay it too much attention. And PIMPL definitely makes for bloated code that's a bit harder to debug and maintain. Not much, but it is inconvenient. So I don't ever use it anymore without a really good reason.

kshegunov

@Wieland said:

To avoid too many #includes in the header. An actual implementation for production would use pimpl idiom.

I was just teasing. ;)
Still that's not a good argument, as you include the headers in the source already, and forward declare the classes ... :)

@Violet-Giraffe

So I don't ever use it anymore without a really good reason.

If you're writing an application it's of no consequence. If you're writing a library on the other hand it's a good idea to keep binary compatibility. You can either use PIMPL for that, or abstract classes and factories, which are much worse in my opinion.

Violet Giraffe

Binary compatibility is not necessarily a concern. Another reason for using PIMPL would be preventing your internals (private declarations) from bleeding into the public API headers.

kshegunov

@Violet-Giraffe

Binary compatibility is not necessarily a concern.

Depends on the viewpoint I suppose. When I have to use and build a library that's not binary compatible I often swear to high heavens. Not a fun thing rebuilding dependencies over dependencies just because that wasn't a consideration for the developer who wrote it ...

Another reason for using PIMPL would be preventing your internals (private declarations) from bleeding into the public API headers.

I agree. It does provide nice clean class interface.

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