Improve the QtConcurrent ImageScaling example

Provide execution context to QFuture continuations, instead of using
QMetaObject::invokeMethod calls for running in the main thread.

Change-Id: Ica7de19494065d677ffc94224781bfbe292b4f21
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

2 files changed, 18 insertions, 12 deletions

diff --git a/examples/qtconcurrent/imagescaling/doc/src/qtconcurrent-imagescaling.qdoc b/examples/qtconcurrent/imagescaling/doc/src/qtconcurrent-imagescaling.qdoc
index a919b7e2ed..8814c826ed 100644
--- a/examples/qtconcurrent/imagescaling/doc/src/qtconcurrent-imagescaling.qdoc
+++ b/examples/qtconcurrent/imagescaling/doc/src/qtconcurrent-imagescaling.qdoc
@@ -128,23 +128,29 @@
 
     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.
+    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.
 
-    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
 
-    Note that showImages() needs to be invoked from the main thread, so we call it through
-    QMetaObject::invokeMethod().
+    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:
 
diff --git a/examples/qtconcurrent/imagescaling/imagescaling.cpp b/examples/qtconcurrent/imagescaling/imagescaling.cpp
index 59664f8a58..66bb0686c3 100644
--- a/examples/qtconcurrent/imagescaling/imagescaling.cpp
+++ b/examples/qtconcurrent/imagescaling/imagescaling.cpp
@@ -118,24 +118,24 @@ void Images::process()
         downloadFuture.then([this](auto) { cancelButton->setEnabled(false); })
                 .then(QtFuture::Launch::Async,
                       [this] {
-                          updateStatus(tr("Scaling..."));
+                          QMetaObject::invokeMethod(this,
+                                                    [this] { updateStatus(tr("Scaling...")); });
                           return scaled();
                       })
 //! [4]
 //! [5]
-                .then([this](const QList<QImage> &scaled) {
-                    QMetaObject::invokeMethod(this, [this, scaled] { showImages(scaled); });
+                .then(this, [this](const QList<QImage> &scaled) {
+                    showImages(scaled);
                     updateStatus(tr("Finished"));
                 })
 //! [5]
 //! [6]
                 .onCanceled([this] { updateStatus(tr("Download has been canceled.")); })
                 .onFailed([this](QNetworkReply::NetworkError error) {
-                    const auto msg = QString("Download finished with error: %1").arg(error);
-                    updateStatus(tr(msg.toStdString().c_str()));
+                    updateStatus(tr("Download finished with error: %1").arg(error));
 
                     // Abort all pending requests
-                    QMetaObject::invokeMethod(this, &Images::abortDownload);
+                    abortDownload();
                 })
                 .onFailed([this](const std::exception& ex) {
                     updateStatus(tr(ex.what()));
@@ -256,7 +256,7 @@ void Images::initLayout(qsizetype count)
 
 void Images::updateStatus(const QString &msg)
 {
-    QMetaObject::invokeMethod(this, [this, msg] { statusBar->showMessage(msg); });
+    statusBar->showMessage(msg);
 }
 
 void Images::abortDownload()