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/*!
    \example imagescaling
    \title Image Scaling Example
    \brief Demonstrates how to asynchronously download and scale images.
    \ingroup qtconcurrentexamples
    \image imagescaling_example.png

    This example shows how to use the QFuture and QPromise classes to download a
    collection of images from the network and scale them, without blocking the UI.

    The application consists of the the following steps:

    \list 1
    \li Download images form the list of URLs specified by the user.
    \li Scale the images.
    \li Show the scaled images in a grid layout.
    \endlist

    Let's start with the download:

    \snippet imagescaling/imagescaling.cpp 8

    The \c download() method takes a list of URLs and returns a QFuture. The QFuture
    stores the byte array data received for each downloaded image. To store the data
    inside the QFuture, we create a QPromise object and report that it has started to
    indicate the start of the download:

    \snippet imagescaling/imagescaling.cpp 9
    \dots
    \snippet imagescaling/imagescaling.cpp 13

    The future associated with the promise is returned to the caller.

    Without going into details yet, let's note that the promise object is wrapped
    inside a QSharedPointer. This will be explained later.

    We use QNetworkAccessManager to send network requests and download data for each
    url:

    \snippet imagescaling/imagescaling.cpp 10

    And here starts the interesting part:

    \dots
    \snippet imagescaling/imagescaling.cpp 11
    \dots

    Instead of connecting to QNetworkReply's signals using the QObject::connect()
    method, we use QtFuture::connect(). It works similar to QObject::connect(), but
    returns a QFuture object, that becomes available as soon as the
    QNetworkReply::finished() signal is emitted. This allows us to attach continuations
    and failure handlers, as it is done in the example.

    In the continuation attached via \b{.then()}, we check if the user has requested to
    cancel the download. If that's the case, we stop processing the request. By calling
    the \c QPromise::finish() method, we notify the user that processing has been finished.
    In case the network request has ended with an error, we throw an exception. The
    exception will be handled in the failure handler attached using the \b{.onFailed()}
    method. Note that we have two failure handlers: the first one captures the network
    errors, the second one all other exceptions thrown during the execution. Both handlers
    save the exception inside the promise object (to be handled by the caller of the
    \c download() method) and report that the computation has finished. Also note that,
    for simplicity, in case of an error we interrupt all pending downloads.

    If the request has not been canceled and no error occurred, we read the data from
    the network reply and add it to the list of results of the promise object:

    \dots
    \snippet imagescaling/imagescaling.cpp 12
    \dots

    If the number of results stored inside the promise object is equal to the number
    of the \c {url}s to be downloaded, there are no more requests to process, so we also
    report that the promise has finished.

    As mentioned earlier, we've wrapped the promise inside a QSharedPointer.
    Since the promise object is shared between handlers connected to each network reply,
    we need to copy and use the promise object in multiple places simultaneously. Hence,
    a QSharedPointer is used.

    \c download() method is called from the \c QImage::process method. It is invoked
    when the user presses the \e {"Add URLs"} button:

    \dots
    \snippet imagescaling/imagescaling.cpp 1
    \dots

    After clearing the possible leftovers from previous download, we create a dialog
    so that the user can specify the URLs for the images to download. Based on the
    specified URL count, we initialize the layout where the images will be shown and
    start the download. The future returned by the \c download() method is saved, so that
    the user can cancel the download if needed:

    \snippet imagescaling/imagescaling.cpp 3
    \dots

    Next, we attach a continuation to handle the scaling step:

    \snippet imagescaling/imagescaling.cpp 4
    \dots

    Since the scaling may be computationally heavy, and we don't want to block the main
    thread, we pass the \c QtFuture::Launch::Async option, to launch the scaling step in
    a new thread. The \c scaled() method returns a list of the scaled images to the next
    step, which takes care of showing images in the layout.

    Note that \c updateStatus() is called through QMetaObject::invokeMethod(),
    because it updates the UI and needs to be invoked from the main thread.

    \dots
    \snippet imagescaling/imagescaling.cpp 5
    \dots

    For the same reason \c showImages() also needs to be invoked from the main thread, so
    we pass \c this as a context to \c .then(). By default, \c .then() is launched in the
    parent's thread, but if a context object is specified, it is launched in the context
    object's thread.

    Then we add cancellation and failure handlers:

    \dots
    \snippet imagescaling/imagescaling.cpp 6

    We don't need to specify the context anymore, because \c .onCanceled() and the next
    handlers will be launched in their parent's context.

    The handler attached via the \c .onCanceled() method will be called if the user has
    pressed the \e "Cancel" button:

    \dots
    \snippet imagescaling/imagescaling.cpp 2
    \dots

    The \c cancel() method simply aborts all the pending requests:

    \snippet imagescaling/imagescaling.cpp 7

    The handlers attached via \c .onFailed() method will be called in case an
    error occurred during one of the previous steps. For example, if a network error
    has been saved inside the promise during the download step, it will be propagated to
    the handler that takes \c QNetworkReply::NetworkError as argument. A failure can
    happen also during the scaling step:

    \snippet imagescaling/imagescaling.cpp 14

    The rest of the code is straightforward, you can check the example project for
    more details.
*/
