// Copyright (C) 2023 The Qt Company Ltd. // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only /*! \example imagescaling \meta tags {widgets, threads, network} \title Image Scaling \ingroup qtconcurrentexamples \examplecategory {Networking} \brief Demonstrates how to asynchronously download and scale images. This example shows how to use the QFuture, QPromise, and QFutureWatcher classes to download a collection of images from the network and scale them, without blocking the UI. \image imagescaling.webp The application consists of 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: \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 \l{QFuture::then}{.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 \l 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 \l{QFuture::onFailed}{.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. The \c download() method is called from the \c Images::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. More on that later: \snippet imagescaling/imagescaling.cpp 4 \dots After that we attach \l {QFuture::}{onCanceled()} and \l {QFuture::}{onFailed()} handlers: \snippet imagescaling/imagescaling.cpp 5 \dots The handler attached via the \l {QFuture::onCanceled}{.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 \l {QFuture::onFailed}{.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 \l QNetworkReply::NetworkError as argument. If the \c downloadFuture is not canceled, and didn't report any error, the scaling continuation is executed. Since the scaling may be computationally heavy, and we don't want to block the main thread, we use \l QtConcurrent::run(), to launch the scaling step in a new thread. \snippet imagescaling/imagescaling.cpp 16 Since the scaling is launched in a separate thread, the user can potentially decide to close the application while the scaling operation is in progress. To handle such situations gracefully, we pass the \l QFuture returned by \l QtConcurrent::run() to the \l QFutureWatcher instance. The watcher's \l QFutureWatcher::finished signal is connected to the \c Images::scaleFinished slot: \snippet imagescaling/imagescaling.cpp 6 This slot is responsible for showing the scaled images in the UI, and also for handling the errors that could potentially happen during scaling: \snippet imagescaling/imagescaling.cpp 15 The error reporting is implemented by returning an optional from the \c Images::scaled() method: \snippet imagescaling/imagescaling.cpp 14 The \c Images::OptionalImages type here is simply a typedef for \c std::optional: \snippet imagescaling/imagescaling.h 1 \note We cannot handle the errors from the async scaling operation using the \l {QFuture::onFailed}{.onFailed()} handler, because the handler needs to be executed in the context of \c Images object in the UI thread. If the user closes the application while the async computation is done, the \c Images object will be destroyed, and accessing its members from the continuation will lead to a crash. Using \l QFutureWatcher and its signals allows us to avoid the problem, because the signals are disconnected when the \l QFutureWatcher is destroyed, so the related slots will never be executed in a destroyed context. The rest of the code is straightforward, you can check the example project for more details. \include examples-run.qdocinc */