Improve QtConcurrent ImageScaling example to demo new features

In order to demonstrate the new functionality, changed the example to
download the images from the network, scale and show them by attaching
different continuations to QFuture. Because QtConcurrent::map is not
used anymore, removed the suspension functionality (supporting
suspension of download would complicate the logic).

Task-number: QTBUG-87205
Change-Id: I5a48b63195d28025ae8c5de28bc6d6178dad03db
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

2 files changed, 136 insertions, 4 deletions

diff --git a/examples/qtconcurrent/imagescaling/doc/images/imagescaling_example.png b/examples/qtconcurrent/imagescaling/doc/images/imagescaling_example.png
index 7c6794132a..a3860e1974 100644
--- a/examples/qtconcurrent/imagescaling/doc/images/imagescaling_example.png
+++ b/examples/qtconcurrent/imagescaling/doc/images/imagescaling_example.png
diff --git a/examples/qtconcurrent/imagescaling/doc/src/qtconcurrent-imagescaling.qdoc b/examples/qtconcurrent/imagescaling/doc/src/qtconcurrent-imagescaling.qdoc
index 0134f0f8e8..a919b7e2ed 100644
--- a/examples/qtconcurrent/imagescaling/doc/src/qtconcurrent-imagescaling.qdoc
+++ b/examples/qtconcurrent/imagescaling/doc/src/qtconcurrent-imagescaling.qdoc
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the documentation of the Qt Toolkit.
@@ -28,10 +28,142 @@
 /*!
     \example imagescaling
     \title Image Scaling Example
-    \brief Demonstrates how to asynchronously scale images.
+    \brief Demonstrates how to asynchronously download and scale images.
     \ingroup qtconcurrentexamples
     \image imagescaling_example.png
 
-    The QtConcurrent Map example shows how to use the asynchronous
-    QtConcurrent API to load and scale a collection of images.
+    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:
+
+    \dots
+    \snippet imagescaling/imagescaling.cpp 5
+    \dots
+
+    Note that showImages() needs to be invoked from the main thread, so we call it through
+    QMetaObject::invokeMethod().
+
+    Then we add cancellation and failure handlers:
+
+    \dots
+    \snippet imagescaling/imagescaling.cpp 6
+
+    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.
 */