QQuickImageProvider, async loading many images and threads.

[0hno]

Hello,
Here are problem: 150+ elements with image in GridLayout (or GridView, not so important) and I want to implement caching/handling images before they come to QML via QQuickImageProvider (or QQuickAsyncImageProvider).

Approach 1: Make provider class that inherits from QQuickImageProvider, implement requestImage (like in official example imageprovider-example ) only with loading images via QNetworkManager + handling (rounding image) + caching.
Problem: If Image.asynchronous = false then image loading pretty slow and there are some visual laggs in qml (scene not redrawing while image loading); If Image.asynchronous = true then another 2 variant: 1.1 If NOT try to make method requestImage "reentrant" like written in docs then app crush; 1.2 If try to make MutexLocker in requestImage then no one image will load because loading get stucked on trying load first image (but app will work) right on string with eventLoop.exec():

QImage MenuImageProvider::requestImage(const QString &id, QSize *size, const QSize &requestedSize) {
  QMutexLocker locker(&mutex);
  QUrl url("path_to_image_in_internet" + id);    
  QNetworkAccessManager *manager = new QNetworkAccessManager;
  QNetworkReply* reply = manager->get(QNetworkRequest(url));
  QEventLoop eventLoop;
  QObject::connect(reply, SIGNAL(finished()), &eventLoop, SLOT(quit()));
  eventLoop.exec(); // all get stucked here if using QMutexLocker locker(&mutex);
    if (reply->error() != QNetworkReply::NoError)
      return QImage();    
  image = QImage::fromData(reply->readAll());
  // .. + some handling and caching stuff
  size->setWidth(image.width());
  size->setHeight(image.height());
  return image;
}

Approach 2. Make provider class that inherits from QQuickAsyncImageProvider, implement requestImageResponse and so on like in official example imageresponseprovider-example ) also with loading images via QNetworkManager + handling + caching.
Problem: after creating 150+ elements with images via Repeater + model I see how about 100 images successfully loads but after i got error in console output (but app still working):

QThread::start: Failed to create thread (Недостаточно памяти для обработки команды.)

translate from russian: Not enough memory for handle command
It happend on Windows. If I launch this app on slow Android with 1 gb memory then app got crush with message about unable creating thread.

And if I not using any ImageProvider, and use in Image.source url to internet it works correctly (with Image.asynchronous = true or false, not important). So how QML Engine by default handling this, and why that ImageProviders (from official docs) doesn't works? Where my mistake?

There are implemetation of QQuickAsyncImageProvider:

1. Base class for loading and handling images that inherits from QObject and QRunnable (like in docs)

class BaseAsyncImageResponseRunnable : public QObject, public QRunnable
{
    Q_OBJECT

signals:
    void done(QImage image);

public:
    BaseAsyncImageResponseRunnable(const QString &id, const QSize &requestedSize, QString imagesPath)
        : m_id(id), m_requestedSize(requestedSize), m_imagesPath(imagesPath) {}

    void run() override
    {
        QImage image;
        QUrl url(m_imagesPath + m_id);

        QNetworkAccessManager *manager = new QNetworkAccessManager;
        QNetworkReply* reply = manager->get(QNetworkRequest(url));
        QEventLoop eventLoop;
        QObject::connect(reply, SIGNAL(finished()), &eventLoop, SLOT(quit()));
        eventLoop.exec();
        if (reply->error() != QNetworkReply::NoError) {
            emit done(image);
            return;
        }
        image = QImage::fromData(reply->readAll());
        image = handleImage(image);
        emit done(image);
    }

    virtual QImage handleImage(QImage image) { return image; }

protected:
    QString m_id;
    QSize m_requestedSize;
    QString m_imagesPath;
};

2. Implement QQuickImageResponse in base class

class BaseAsyncImageResponse : public QQuickImageResponse
{
    public:
        BaseAsyncImageResponse(const QString &id, const QSize &requestedSize, QThreadPool *pool, BaseAsyncImageResponseRunnable *runnable)
        {
            connect(runnable, &BaseAsyncImageResponseRunnable::done, this, &BaseAsyncImageResponse::handleDone);
            pool->start(runnable);
        }

        void handleDone(QImage image) {
            m_image = image;
            emit finished();
        }

        QQuickTextureFactory *textureFactory() const override
        {
            return QQuickTextureFactory::textureFactoryForImage(m_image);
        }

        QImage m_image;
};

3. Implements QQuickAsyncImageProvider

class MyAsyncImageProvider : public QQuickAsyncImageProvider
{
public:
    MyAsyncImageProvider() : QQuickAsyncImageProvider()
    { }

    QQuickImageResponse *requestImageResponse(const QString &id, const QSize &requestedSize) override
    {
        BaseAsyncImageResponse *response;
        QString imagesPath = "pat_to_images_in_internet";
        auto runnable = new BaseAsyncImageResponseRunnable(id, requestedSize, imagesPath);
        response = new BaseAsyncImageResponse(id, requestedSize, &pool, runnable);
        return response;
    }

private:
    QThreadPool pool;
};