What is the best way to display an animated icon in a QTableView?

For the record, I ended up using QAbstractItemView::setIndexWidget from inside the paint method of my delegate, to insert a QLabel displaying the QMovie inside the item (see code below).

This solution works quite nicely, and keep the display issues separated from the model. One drawback is that the display of a new frame in the label causes the entire item to be rendered again, resulting in almost continuous calls to the delegate's paint method...

To reduce the overhead inccured by these calls, I tried to minimize the work done for handling movies in the delegate by reusing the existing label if there is one. However, this results in weird behavior when resizing the windows: the animation gets shifted to the right, as if two labels were positioned side by side.

So well, here is a possible solution, feel free to comment on ways to improve it!

// Declaration

#ifndef MOVIEDELEGATE_HPP
#define MOVIEDELEGATE_HPP

#include <QtCore/QModelIndex>
#include <QtGui/QStyledItemDelegate>


class QAbstractItemView;
class QMovie;


class MovieDelegate : public QStyledItemDelegate
{
    Q_OBJECT

  public: // member functions

    MovieDelegate( QAbstractItemView & view, QObject * parent = NULL );

    void paint( QPainter * painter, 
                const QStyleOptionViewItem & option, 
                const QModelIndex & index ) const;


  private: // member functions

    QMovie * qVariantToPointerToQMovie( const QVariant & variant ) const;


  private: // member variables

    mutable QAbstractItemView & view_;
};

#endif // MOVIEDELEGATE_HPP


// Definition

#include "movieDelegate.hpp"

#include <QtCore/QVariant>
#include <QtGui/QAbstractItemView>
#include <QtGui/QLabel>
#include <QtGui/QMovie>


Q_DECLARE_METATYPE( QMovie * )


//---------------------------------------------------------
// Public member functions
//---------------------------------------------------------

MovieDelegate::MovieDelegate( QAbstractItemView & view, QObject * parent )
  : QStyledItemDelegate( parent ), view_( view )
{
}


void MovieDelegate::paint( QPainter * painter, 
                           const QStyleOptionViewItem & option, 
                           const QModelIndex & index ) const
{
    QStyledItemDelegate::paint( painter, option, index );

    const QVariant & data = index.data( Qt::DecorationRole );

    QMovie * movie = qVariantToPointerToQMovie( data );

    if ( ! movie )
    {
        view_.setIndexWidget( index, NULL );
    }
    else
    {
        QObject * indexWidget = view_.indexWidget( index );
        QLabel  * movieLabel  = qobject_cast< QLabel * >( indexWidget );

        if ( movieLabel )
        {
            // Reuse existing label

            if ( movieLabel->movie() != movie )
            {
                movieLabel->setMovie( movie );
            }
        }
        else
        {
            // Create new label;

            movieLabel = new QLabel;

            movieLabel->setMovie( movie );

            view_.setIndexWidget( index, movieLabel );
        }
    }
}


//---------------------------------------------------------
// Private member functions
//---------------------------------------------------------

QMovie * MovieDelegate::qVariantToPointerToQMovie( const QVariant & variant ) const
{
    if ( ! variant.canConvert< QMovie * >() ) return NULL;

    return variant.value< QMovie * >();
}

In my application, I have a typical spinning circle icon to indicate a waiting/processing state for some of the cells in a table. However I ended up using an approach, which is different from the one suggested in the currently accepted answer, mine is in my view simpler and somewhat more performant (UPDATE: I wrote this when a different answer was set as accepted - the one suggesting using QAbstractItemView::setIndexWidget). Using widgets seems as an overkill which will destroy the performance if there are too many of them. All the functionality in my solution is only implemented in my model layer (descendant of QAbstractItemModel) class. I do not need to make any changes in the view nor the delegate. I am however only animating one GIF and all animations are synchronized. This is the current limitation of my simple approach.

The model class which is used to implement this behavior needs to have the following:

• the vector of QImages - I use QImageReader, which allows me read all animation frames, I store them into a QVector<QImage>

• a QTimer ticking with the periodicity of the animated GIF - the time period is obtained using QImageReader::nextImageDelay().

• the index (int) of the current frame (I suppose the frame is the same for all animated cells - they are synchronized; if you want unsynchronized then you can use an integer offset for each of them)

• some knowledge of which cells should be animated and ability to translate the cell to QModelIndex (this is up to your custom code to implement this, depend on your specific needs)

• override QAbstractItemModel::data() part of your model to respond to Qt::DecorationRole for any animated cell (QModelIndex) and return the current frame as a QImage

• a slot which is triggered by the QTimer::timeout signal

The key part is the slot which reacts to the timer. It must do this:

1. Increase the current frame, e.g. m_currentFrame = (m_currentFrame + 1) % m_frameImages.size();

2. Get the list of indices (e.g. QModelIndexList getAnimatedIndices();) of the cells which have to be animated. This code of getAnimatedIndices() is up to you to develop - use brute force querying all cells in your model or some clever optimization...

3. emit dataChanged() signal for each animated cell, e.g. for (const QModelIndex &idx : getAnimatedIndices()) emit dataChanged(idx, idx, {Qt::DecorationRole});

Thats all. I estimate that depending on the complexity of your functions for determining which indices are animated, the whole implementation can have something like 15 to 25 lines, without need to alter the view nor delegate, just the model.

The best solution is to use QSvgRenderer within delegate.

It's very easy to implement and unlike gif, SVG is lightweight and supports transparency.

    TableViewDelegate::TableViewDelegate(TableView* view, QObject* parent)
    : QStyledItemDelegate(parent), m_view(view)
{
    svg_renderer = new QSvgRenderer(QString{ ":/res/img/spinning_icon.svg" }, m_view);

    connect(svg_renderer, &QSvgRenderer::repaintNeeded,
        [this] {
        m_view->viewport()->update();
    });
}


void TableViewDelegate::paint(QPainter* painter, const QStyleOptionViewItem& option,
    const QModelIndex& index) const
{
    QStyleOptionViewItem opt{ option };
    initStyleOption(&opt, index);

    if (index.column() == 0) {
        if (condition)
        {
            // transform bounds, otherwise fills the whole cell
            auto bounds = opt.rect;
            bounds.setWidth(28);
            bounds.moveTo(opt.rect.center().x() - bounds.width() / 2,
                opt.rect.center().y() - bounds.height() / 2);

            svg_renderer->render(painter, bounds);
        }
    }

    QStyledItemDelegate::paint(painter, opt, index);
}

Here's a nice website where you can generate your own spinning icon and export in SVG.