bkietz commented on a change in pull request #8680:
URL: https://github.com/apache/arrow/pull/8680#discussion_r534351861
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File path: cpp/src/arrow/util/future.h
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@@ -264,11 +335,125 @@ class Future {
}
/// \brief Make a finished Future<> with the provided Status.
- template <typename E = ValueType, typename = detail::Empty::EnableIfSame<E>>
+ template <typename E = ValueType, typename = typename std::enable_if<
+ std::is_same<E,
detail::Empty>::value>::type>
static Future<> MakeFinished(Status s = Status::OK()) {
return MakeFinished(E::ToResult(std::move(s)));
}
+ /// \brief Consumer API: Register a callback to run when this future
completes
+ template <typename OnComplete>
+ void AddCallback(OnComplete&& on_complete) const {
+ struct Callback {
+ void operator()() && {
+ auto self = weak_self.get();
+ std::move(on_complete)(*self.GetResult());
+ }
+
+ WeakFuture<T> weak_self;
+ OnComplete on_complete;
+ };
+
+ // We know impl_ will not be dangling when invoking callbacks because at
least one
+ // thread will be waiting for MarkFinished to return. Thus it's safe to
keep a
+ // weak reference to impl_ here
+ impl_->AddCallback(
+ Callback{WeakFuture<T>(*this), std::forward<OnComplete>(on_complete)});
+ }
+
+ /// \brief Consumer API: Register a continuation to run when this future
completes
+ ///
+ /// The continuation will run in the same thread that called MarkFinished
(whatever
+ /// callback is registered with this function will run before MarkFinished
returns).
+ /// Avoid long-running callbacks in favor of submitting a task to an
Executor and
+ /// returning the future.
+ ///
+ /// Two callbacks are supported:
+ /// - OnSuccess, called against the result (const ValueType&) on successul
completion.
+ /// - OnFailure, called against the error (const Status&) on failed
completion.
+ ///
+ /// Then() returns a Future whose ValueType is derived from the return type
of the
+ /// callbacks. If a callback returns:
+ /// - void, a Future<> will be produced which will completes successully as
soon
+ /// as the callback runs.
+ /// - Status, a Future<> will be produced which will complete with the
returned Status
+ /// as soon as the callback runs.
+ /// - V or Result<V>, a Future<V> will be produced which will complete with
the result
+ /// of invoking the callback as soon as the callback runs.
+ /// - Future<V>, a Future<V> will be produced which will be marked complete
when the
+ /// future returned by the callback completes (and will complete with the
same
+ /// result).
+ ///
+ /// The continued Future type must be the same for both callbacks.
+ ///
+ /// Note that OnFailure can swallow errors, allowing continued Futures to
successully
+ /// complete even if this Future fails.
+ ///
+ /// If this future is already completed then the callback will be run
immediately
+ /// (before this method returns) and the returned future may already be
marked complete
+ /// (it will definitely be marked complete if the callback returns a
non-future or a
+ /// completed future).
+ template <typename OnSuccess, typename OnFailure,
+ typename ContinuedFuture =
+ detail::ContinueFuture::ForSignature<OnSuccess && (const T&)>>
+ ContinuedFuture Then(OnSuccess&& on_success, OnFailure&& on_failure) const {
+ static_assert(
+ std::is_same<detail::ContinueFuture::ForSignature<OnFailure && (const
Status&)>,
+ ContinuedFuture>::value,
+ "OnSuccess and OnFailure must continue with the same future type");
+
+ auto next = ContinuedFuture::Make();
+
+ struct Callback {
+ void operator()(const Result<T>& result) && {
+ if (ARROW_PREDICT_TRUE(result.ok())) {
+ detail::Continue(std::move(next), std::move(on_success),
result.ValueOrDie());
+ } else {
+ detail::Continue(std::move(next), std::move(on_failure),
result.status());
Review comment:
not necessary; since this is in a FnOnce all members will be dropped as
soon as it's invoked
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