NOT INTENDED FOR LANDING: A promises demo

components/scheduler/promises/promise.h

Index: components/scheduler/promises/promise.h
diff --git a/components/scheduler/promises/promise.h b/components/scheduler/promises/promise.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_SCHEDULER_PROMISES_PROMISE_H_
+#define COMPONENTS_SCHEDULER_PROMISES_PROMISE_H_
+
+#include "components/scheduler/promises/promise_internal.h"
+
+namespace promise {
+
+template <typename R>
+class SCHEDULER_EXPORT Promise {
+ public:
+  using ReturnType = R;
+  using InternalPromise = internal::Promise<ReturnType>;
+
+  Promise() {}
+
+  explicit Promise(const scoped_refptr<InternalPromise>& promise)
+      : promise_(promise) {}
+
+  // Note this is non-explicit to support implicit conversion to allow
+  // assigning promise::Race(...).Then() to a Promise<R>.
+  Promise(scoped_refptr<InternalPromise>&& promise)
+      : promise_(std::move(promise)) {}
+
+  // A directive that instructs the executor to resolve this promise before
+  // others if possible.
+  void SetEager(bool eager) { promise_->SetThisAndPrerequisitesAsEager(eager); }
+
+  // Cancels execution, can be restarted by PromiseExecutor::StartResolve.
+  void Cancel() { promise_->Cancel(); }
+
+  // This will DCHECK if the promise isn't resolved.
+  template <typename RR = R>
+  internal::enable_if_not_void_t<RR, RR&> GetResolved() {
+    return promise_->GetResolved();
+  }
+
+  // Resolves the promise. May schedule excution of callbacks.
+  template <typename RR = R>
+  internal::enable_if_not_void_t<RR, void> Resolve(const RR& r) {
+    promise_->Resolve(r);
+  }
+
+  // Resolves the promise with a moveable type. May schedule excution of
+  // callbacks.
+  template <typename RR = R>
+  internal::enable_if_not_void_t<RR, void> Resolve(RR&& r) {
+    promise_->Resolve(std::move(r));
+  }
+
+  // Resolves the promise. May schedule excution of callbacks.
+  template <typename RR = R>
+  internal::enable_if_void_t<RR, void> Resolve() {
+    promise_->Resolve();
+  }
+
+  // Rejects the promise. May schedule excution of callbacks.
+  template <typename RR = R, typename T>
+  typename enable_if_rejectable<RR, void>::type Reject(const T& r) {
+    promise_->Reject(r);
+  }
+
+  // Rejects the promise with a moveable type. May schedule excution callbacks.
+  template <typename RR = R, typename T>
+  typename enable_if_rejectable<RR, void>::type Reject(T&& r) {
+    promise_->Reject(std::move(r));
+  }
+
+  // Rejects the promise. May schedule excution of callbacks.
+  template <typename RR = R>
+  typename enable_if_rejectable<RR, void>::type Reject() {
+    promise_->Reject();
+  }
+
+  // Resolves the promise with another promise which is sheduled for execution.
+  // May schedule excution of callbacks.
+  void Resolve(const Promise<R>& promise) {
+    promise_->Resolve(promise->promise_);
+  }
+
+  bool is_resolved() const { return promise_->is_resolved(); }
+
+  bool is_rejected() const { return promise_->is_rejected(); }
+
+  template <typename ThenR, typename... ThenArgs>
+  Promise<ThenR> Then(base::Callback<ThenR(ThenArgs...)>&& cb);
+
+  template <typename ThenR>
+  Promise<ThenR> Then(base::Callback<ThenR()>&& cb);
+
+  template <typename ThenR, typename... ThenArgs>
+  Promise<ThenR> Then(base::Callback<Promise<ThenR>(ThenArgs...)>&& cb);
+
+  template <typename ThenR>
+  Promise<ThenR> Then(base::Callback<Promise<ThenR>()>&& cb);
+
+ private:
+  friend class internal::PromiseExecutor;
+
+  template <typename RR>
+  friend class internal::Promise;
+
+  template <typename RR>
+  friend class internal::RaceSpec;
+
+  template <typename... Promises>
+  friend class internal::AllSpec;
+
+  InternalPromise* promise() const { return promise_.get(); }
+
+  scoped_refptr<InternalPromise> promise_;
+};
+
+// Create promise that will run a callback.
+template <typename R, typename... Args>
+Promise<R> Create(base::Callback<R(Args...)> c) {
+  return Promise<R>(
+      make_scoped_refptr(new internal::FunctionPromise<R, Args...>(c)));
+}
+
+// Create a promise that is already resolved.
+template <typename R>
+Promise<R> Create(const R& r) {
+  scoped_refptr<internal::Promise<R>> promise(new internal::Promise<R>(
+      internal::PromiseBase::PrerequisitePolicy::NEVER));
+  promise->Resolve(r);
+  return Promise<R>(promise);
+}
+
+// Create a promise that is already resolved.
+template <typename R>
+Promise<R> Create(R&& r) {
+  scoped_refptr<internal::Promise<R>> promise(new internal::Promise<R>(
+      internal::PromiseBase::PrerequisitePolicy::NEVER));
+  promise->Resolve(std::move(r));
+  return Promise<R>(promise);
+}
+
+// Create a promise that is manually resolved.
+template <typename R>
+Promise<R> Create() {
+  return Promise<R>(make_scoped_refptr(new internal::Promise<R>(
+      internal::PromiseBase::PrerequisitePolicy::NEVER)));
+}
+
+// ===
+
+template <typename R>
+template <typename ThenR, typename... ThenArgs>
+Promise<ThenR> Promise<R>::Then(base::Callback<ThenR(ThenArgs...)>&& cb) {
+  using PromiseType = internal::Promise<R>;
+  return Promise<ThenR>(
+      make_scoped_refptr(new internal::RacePromises<ThenR, ThenR, PromiseType>(
+          std::move(cb), {promise_})));
+}
+
+template <>
+template <typename ThenR>
+Promise<ThenR> Promise<void>::Then(base::Callback<ThenR()>&& cb) {
+  return Promise<ThenR>(make_scoped_refptr(
+      new internal::RaceVoidPromises<ThenR, ThenR>(std::move(cb), {promise_})));
+}
+
+template <typename R>
+template <typename ThenR, typename... ThenArgs>
+Promise<ThenR> Promise<R>::Then(
+    base::Callback<Promise<ThenR>(ThenArgs...)>&& cb) {
+  using PromiseType = internal::Promise<R>;
+  return Promise<ThenR>(make_scoped_refptr(
+      new internal::RacePromises<ThenR, Promise<ThenR>, PromiseType>(
+          std::move(cb), {promise_})));
+}
+
+template <>
+template <typename ThenR>
+Promise<ThenR> Promise<void>::Then(base::Callback<Promise<ThenR>()>&& cb) {
+  return Promise<ThenR>(make_scoped_refptr<internal::Promise<ThenR>>(
+      new internal::RaceVoidPromises<ThenR, Promise<ThenR>>(std::move(cb),
+                                                            {promise_})));
+}
+
+// ===
+
+/*
+Promises::All(promises...).Then(callback) allows you to schedule a callback to
+run once all of the specified promises have been resolved.
+
+The promises specified must either be all void or all non-void.  In the case of
+non-void promises, the callback specified by Then() is called with results of
+the promises.
+
+Race(...).Then() returns a promise which you need to schedule for execution.
+E.g.
+
+Promise<int> p1;
+Promise<bool> p2;
+Promise<float> p3;
+
+void MyFunc(int, bool, float);
+
+...
+Promise<void> p(
+    promise::All(p1, p2, p3).Then(base::Bind(&MyFunc));
+
+// Schedule p1, p2 & p3 for execution and once they're run call MyFunc with
+// their return values.
+my_executor_->StartResolve(p);
+*/
+template <typename... Promises>
+internal::AllSpec<Promises...> All(Promises... promises) {
+  static_assert(
+      internal::check_all_same_type<Promise<void>, Promises...>::value ||
+          !internal::check_any_have_type<Promise<void>, Promises...>::value,
+      "All(...) requires the promises to either be all void or all non-void.");
+  return internal::AllSpec<Promises...>(promises...);
+}
+
+/*
+Promises::Race(promises...).Then(callback) allows you to schedule a callback to
+run once any of the specified promises have been resolved.
+
+The promises must all have the same type.  If the promises are non-void, then
+the callback specified by Then() is called with results of the sellected
+resolved promise.  The selection algorithm works as follows, when attempting to
+run the callback, the promises specifed in the Race(...) clause are examined in
+order and the first resolved one is selected.
+
+All(...).Then() returns a promise which you need to schedule for execution. E.g.
+
+  Promise<int> p1;
+  Promise<int> p2;
+  Promise<int> p3;
+
+  void MyFunc(int);
+
+  ...
+  Promise<void> p(
+      promise::Race(p1, p2, p3).Then(base::Bind(&MyFunc));
+
+  // Schedule p1, p2 & p3 for execution and once any of them have been run. When
+  // deciding which promise to call MyFunc the promises from the Race() clause
+  // scanned in order and the the first resolved one is used.
+  my_executor_->StartResolve(p);
+*/
+template <typename R, typename... Promises>
+internal::RaceSpec<R> Race(Promise<R> r1, Promises... promises) {
+  static_assert(internal::check_all_same_type<Promise<R>, Promises...>::value,
+                "Race(...) requires all the promises to have the same type.");
+  return internal::RaceSpec<R>(r1, promises...);
+}
+
+}  // namespace promise
+
+#endif  // COMPONENTS_SCHEDULER_PROMISES_PROMISE_H_