How to create custom icons for QFileSystemModel in a background thread

Question

I am making a file browser in qt for some custom design-files. I want to load their preview as their thumbnail and for that reason I am using QIconProvider to return the Icon to my QFileSystemModel.

The problem is that the algorithm that creates the QIcon needs some resources and as a result my application is not responsive until it finishes loading all the thumbnails.

I am wondering if there is any way to put my QIconProvider in a background thread, so that I have my application responsive.

Answer 1

Unfortunately, there's an impedance mismatch between the QFileIconProvider API and the model api: the QFileSystemModel provides asynchronous notifications to the view when things change, but the icon provider can't asynchronously notify the model when icons change or become known.

You can install an identity proxy between the file system model and the view(s). That proxy's data method would then query the icons asynchronously. The model's synchronous icon provider is then unused and unnecessary.

// https://github.com/KubaO/stackoverflown/tree/master/questions/icon-proxy-39144638
#include <QtWidgets>
#include <QtConcurrent>

/// A thread-safe function that returns an icon for an item with a given path.
/// If the icon is not known, a null icon is returned.
QIcon getIcon(const QString & path);

class IconProxy : public QIdentityProxyModel {
    Q_OBJECT
    QMap<QString, QIcon> m_icons;
    Q_SIGNAL void hasIcon(const QString&, const QIcon&, const QPersistentModelIndex& index) const;
    void onIcon(const QString& path, const QIcon& icon, const QPersistentModelIndex& index) {
        m_icons.insert(path, icon);
        emit dataChanged(index, index, QVector<int>{QFileSystemModel::FileIconRole});
    }
public:
    QVariant data(const QModelIndex & index, int role = Qt::DisplayRole) const override {
        if (role == QFileSystemModel::FileIconRole) {
            auto path = index.data(QFileSystemModel::FilePathRole).toString();
            auto it = m_icons.find(path);
            if (it != m_icons.end()) {
                if (! it->isNull()) return *it;
                return QIdentityProxyModel::data(index, role);
            }
            QPersistentModelIndex pIndex{index};
            QtConcurrent::run([this,path,pIndex]{
                emit hasIcon(path, getIcon(path), pIndex);
            });
            return QVariant{};
        }
        return QIdentityProxyModel::data(index, role);
    }
    IconProxy(QObject * parent = nullptr) : QIdentityProxyModel{parent} {
        connect(this, &IconProxy::hasIcon, this, &IconProxy::onIcon);
    }
};