Added QFuture::then() methods to allow chaining multiple asynchronous computations. Continuations can use the following execution policies: * QtFuture::Launch::Sync - the continuation will be launched in the same thread in which the parent has been executing. * QtFuture::Launch::Async - the continuation will be launched in a new thread. * QtFuture::Launch::Inherit - the continuation will inherit the launch policy of the parent, or its thread pool (if it was using a custom one). * Additionally then() also accepts a custom QThreadPool* instance. Note, that if the parent future gets canceled, its continuation(s) will be also canceled. If the parent throws an exception, it will be propagated to the continuation's future, unless it is caught inside the continuation (if it has a QFuture arg). Some example usages: QFuture<int> future = ...; future.then([](int res1){ ... }).then([](int res2){ ... })... QFuture<int> future = ...; future.then([](QFuture<int> fut1){ /* do something with fut1 */ })... In the examples above all continuations will run in the same thread as future. QFuture<int> future = ...; future.then(QtFuture::Launch::Async, [](int res1){ ... }) .then([](int res2){ ... }).. In this example the continuations will run in a new thread (but on the same one). QThreadPool pool; QFuture<int> future = ...; future.then(&pool, [](int res1){ ... }) .then([](int res2){ ... }).. In this example the continuations will run in the given thread pool. [ChangeLog][QtCore] Added support for attaching continuations to QFuture. Task-number: QTBUG-81587 Change-Id: I5b2e176694f7ae8ce00404aca725e9a170818955 Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io> Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io> Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>