Add support for unwrapping QFuture<QFuture<T>>

Added QFuture::unwrap() for unwrapping the future nested inside
QFuture<QFuture<T>>. QTBUG-86725 suggests doing the unwrapping
automatically inside .then(), but this will change the return type
of .then() that used to return QFuture<QFuture<T>> and might cause
SC breaks. Apart from that, QFuture::unwrap() might be helpful in
general, for asynchronous computations that return a nested QFuture.

[ChangeLog][QtCore][QFuture] Added QFuture::unwrap() for unwrapping the
future nested inside QFuture<QFuture<T>>.

Task-number: QTBUG-86725
Change-Id: I8886743aca261dca46f62d9dfcaead4a141d3dc4
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 34 insertions, 0 deletions

diff --git a/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp b/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
index ffb78dc4c8..9ece90553a 100644
--- a/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
@@ -398,3 +398,37 @@ QtFuture::whenAny(intFuture, stringFuture, voidFuture).then([](const FuturesVari
     ...
 });
 //! [27]
+
+//! [28]
+
+QFuture<QFuture<int>> outerFuture = ...;
+QFuture<int> unwrappedFuture = outerFuture.unwrap();
+
+//! [28]
+
+//! [29]
+
+auto downloadImages = [] (const QUrl &url) {
+    QList<QImage> images;
+    ...
+    return images;
+};
+
+auto processImages = [](const QList<QImage> &images) {
+   return QtConcurrent::mappedReduced(images, scale, reduceImages);
+}
+
+auto show = [](const QImage &image) { ... };
+
+auto future = QtConcurrent::run(downloadImages, url)
+               .then(processImages)
+               .unwrap()
+               .then(show);
+//! [29]
+
+//! [30]
+
+QFuture<QFuture<QFuture<int>>>> outerFuture;
+QFuture<int> unwrappedFuture = outerFuture.unwrap();
+
+//! [30]