How to show QImage from http url?

jkosonen · wrote on 18 Jul 2011, 19:44

Hey, I would like to cache images from an url and show them from local file system, but if file doesn't exists then I would need to download it from url. Any advices how I should proceed?

I found this class:
QDeclarativeImageProvider, where I can return Image from Qt side and there I can do something like this:

@QImage requestImage(const QString &id;, QSize *size, const QSize &requestedSize;)
{

     QDir dir;
     QFile file&#40; dir.absolutePath(&#41; + "images/" + id + ".jpg" );

     if (file.exists())
     {
        QImage img;
        img.load(dir.absolutePath() + "images/" + id + ".jpg");
        return img; 
     }
     else {

        Load from url

     }

}@

How to do that load from url part.

I found this example: http://www.qtcentre.org/threads/1483-Qt4-How-to-load-Url-image-into-QImage
But how can I return the image, when it's created in slot?

Thanks for any tips.

DenisKormalev · wrote on 18 Jul 2011, 21:03

You can also use custom "QDeclarativeNetworkAccessManagerFactory":http://doc.qt.nokia.com/4.7/qdeclarativenetworkaccessmanagerfactory.html to cache images with cache-enabled QNAM.

jkosonen · wrote on 19 Jul 2011, 06:35

[quote author="Denis Kormalev" date="1311023033"]You can also use custom "QDeclarativeNetworkAccessManagerFactory":http://doc.qt.nokia.com/4.7/qdeclarativenetworkaccessmanagerfactory.html to cache images with cache-enabled QNAM.[/quote]

That sounds even trickier task

Chuck Gao · wrote on 19 Jul 2011, 06:55

Hi jkosonen, you want to show your images with QML?

jkosonen · wrote on 19 Jul 2011, 08:04

[quote author="Chuck Gao" date="1311058507"]Hi jkosonen, you want to show your images with QML?[/quote]

Yes :)

Chuck Gao · wrote on 19 Jul 2011, 08:12

If so, i think the key point is the file path(local or internet). QML Image element's source can be a url, so you just need to get the file path. The element itself will take care of the http part.

jkosonen · wrote on 19 Jul 2011, 08:22

[quote author="Chuck Gao" date="1311063146"]If so, i think the key point is the file path(local or internet). QML Image element's source can be a url, so you just need to get the file path. The element itself will take care of the http part.[/quote]

Yes, I know. but I don't want to download it from the internet everytime, but make a local copy and then take it from there if exists.

Chuck Gao · wrote on 19 Jul 2011, 08:32

You want cache the images yourself ? I mean, store them into the cache and get them from the cache. I'm not sure if the QML Image element use caching when load url image. But you can try QNetworkDiskCache. I found this in the document: "Images are cached and shared internally, so if several Image elements have the same source, only one copy of the image will be loaded". Let's look into the source code. :-)

jkosonen · wrote on 19 Jul 2011, 08:38

Yes, I would like to cache by myself :) I don't think that the QML caches those automatically since it always takes so long to get the images to the screen.

Maybe need to investigate that QNetworkDiscCache, but I wouldn't mind if someone could give me a solution to problem I had in the main question :)

Chuck Gao · wrote on 19 Jul 2011, 08:56

I have a look at the source code, and QML will cache those images for you, with
@
QDeclarativeImageBase::load()
{
...
QDeclarativePixmap::load(QDeclarativeEngine *engine, const QUrl &url;, const QSize &requestSize;, bool async)
{
if (d) { d->release(); d = 0; }

QDeclarativePixmapKey key = { &url;, &requestSize; };
QDeclarativePixmapStore *store = pixmapStore();

QHash<QDeclarativePixmapKey, QDeclarativePixmapData *>::Iterator iter = store->m_cache.find(key);

if (iter == store->m_cache.end()) {
    if (async) {
        // pixmaps can only be loaded synchronously
        if (url.scheme() == QLatin1String("image") 
                && QDeclarativeEnginePrivate::get(engine)->getImageProviderType(url) == QDeclarativeImageProvider::Pixmap) {
            async = false;
        }
    }

    if (!async) {
        bool ok = false;
        d = createPixmapDataSync(engine, url, requestSize, &ok;);
        if (ok) {
            d->addToCache();
            return;
        }
        if (d)  // loadable, but encountered error while loading
            return;
    } 

    if (!engine)
        return;

    QDeclarativePixmapReader *reader = QDeclarativePixmapReader::instance(engine);

    d = new QDeclarativePixmapData(url, requestSize);
    d->addToCache();

    d->reply = reader->getImage(d);
} else {
    d = *iter;
    d->addref();
}

}

.....
}
@

jkosonen · wrote on 19 Jul 2011, 08:58

Then it takes awful lot of time to show the image if it already comes from cache...

DenisKormalev · wrote on 19 Jul 2011, 08:59

[quote author="jkosonen" date="1311064731"]Yes, I would like to cache by myself :) I don't think that the QML caches those automatically since it always takes so long to get the images to the screen.

Maybe need to investigate that QNetworkDiscCache, but I wouldn't mind if someone could give me a solution to problem I had in the main question :)[/quote]

QNetworkDiskCache is the same way I've suggested. It is really not hard (maybe a 5 or 10 lines of code)

jkosonen · wrote on 19 Jul 2011, 09:03

[quote author="Denis Kormalev" date="1311065955"][quote author="jkosonen" date="1311064731"]Yes, I would like to cache by myself :) I don't think that the QML caches those automatically since it always takes so long to get the images to the screen.

Maybe need to investigate that QNetworkDiscCache, but I wouldn't mind if someone could give me a solution to problem I had in the main question :)[/quote]

QNetworkDiskCache is the same way I've suggested. It is really not hard (maybe a 5 or 10 lines of code)[/quote]

Can you point me to any site with example, with QML?

DenisKormalev · wrote on 19 Jul 2011, 09:10

"example about QDeclarativeNetworkAccessManagerFactory":http://doc.qt.nokia.com/4.7/declarative-cppextensions-networkaccessmanagerfactory.html
"QNetworkDiskCache":http://doc.qt.nokia.com/4.7/qnetworkdiskcache.html (there is an example of how to use it with QNAM)

jkosonen · wrote on 19 Jul 2011, 09:17

[quote author="Denis Kormalev" date="1311066630"]"example about QDeclarativeNetworkAccessManagerFactory":http://doc.qt.nokia.com/4.7/declarative-cppextensions-networkaccessmanagerfactory.html
"QNetworkDiskCache":http://doc.qt.nokia.com/4.7/qnetworkdiskcache.html (there is an example of how to use it with QNAM)[/quote]

Thanks, will try that!

but there is this:

When sending requests, to control the preference of when to use the cache and when to use the network, consider the following:

@ // do a normal request (preferred from network, as this is the default)
QNetworkRequest request(QUrl(QString("http://qt.nokia.com")));
manager->get(request);

// do a request preferred from cache
QNetworkRequest request2(QUrl(QString("http://qt.nokia.com")));
request2.setAttribute(QNetworkRequest::CacheLoadControlAttribute, QNetworkRequest::PreferCache);
manager->get(request2);@

How do I say in QML to prfer cache?

DenisKormalev · wrote on 19 Jul 2011, 09:19

jkosonen, it will prefer a cache in most part of cases (if we are talking about images)

jkosonen · wrote on 19 Jul 2011, 09:22

[quote author="Denis Kormalev" date="1311067186"]jkosonen, it will prefer a cache in most part of cases (if we are talking about images)[/quote]

OK, but how to say then not to use cache, or clear cache?
Cache isn't always preferred, would be nice to know when it is used and when it's not.

DenisKormalev · wrote on 19 Jul 2011, 09:24

jkosonen, clearing cache is possbile with simple removing all contents of cache dir. I'm not aware of any other ways.

Chuck Gao · wrote on 20 Jul 2011, 02:10

You can try Qt 4.8, it has a better image caching for QML. Hope it will work for you :D

jkosonen · wrote on 7 Aug 2011, 07:08

Can anyone help with the question in the first post, how that could be done?