Support external task cancellation mechanisms in base::Callback::IsCancelled

base/bind_helpers.h

Index: base/bind_helpers.h
diff --git a/base/bind_helpers.h b/base/bind_helpers.h
index c7c7be8ee840a7b52206787fa8f717dcc4de4e30..1902341324e1c0bf42429191b522ef366df8239d 100644
--- a/base/bind_helpers.h
+++ b/base/bind_helpers.h
@@ -179,6 +179,9 @@ struct BindUnwrapTraits;
 
 namespace internal {
 
+template <typename Functor, typename SFINAE = void>
+struct FunctorTraits;
+
 template <typename T>
 class UnretainedWrapper {
  public:
@@ -521,6 +524,47 @@ struct BindUnwrapTraits<internal::PassedWrapper<T>> {
   }
 };
 
+// An injection point to support Callback::IsCancelled in a specific form of
+// base::Bind.
+// Specialize BindCancellationChecker to support Callback::IsCancelled, set
+// is_cancellable to true in the specialization, and implement Run() method.
+// BindCancellationChecker<>::Run() should return true if the resulting callback
+// is cancelled.
+template <typename Functor, typename BoundArgsTuple, typename SFINAE = void>
+struct BindCancellationChecker {

[dcheng, 2016/11/10 06:29:23]

My main thoughts are around naming.

Most of our injection points for Bind / Callback are named
SomethingSomethingTraits. For consistency in naming, it might be nice to keep a
similar theme here. Maybe CallbackCancellationTraits? Since it's a Callback
(which is the result of a Bind) that is cancellable.

[tzik, 2016/11/10 07:21:14]

Done.

+  static constexpr bool is_cancellable = false;
+};
+
+// Specialization for method bound to weak pointer receiver.
+template <typename Functor, typename... BoundArgs>
+struct BindCancellationChecker<Functor,
+                               std::tuple<BoundArgs...>,
+                               typename std::enable_if<internal::IsWeakMethod<
+                                   internal::FunctorTraits<Functor>::is_method,
+                                   BoundArgs...>::value>::type> {
+  static constexpr bool is_cancellable = true;
+
+  template <typename Receiver, typename... Args>
+  static bool Run(const Functor&, const Receiver& receiver, const Args&...) {

[dcheng, 2016/11/10 06:29:23]

Similarly, I think calling this Run() is a bit confusing (since it overlaps with
Callback::Run). Would it be OK to just call this IsCancelled()?

[tzik, 2016/11/10 07:21:14]

Done.

+    return !receiver;
+  }
+};
+
+// Specialization for a nested bind.
+template <typename Signature,
+          typename... BoundArgs,
+          internal::CopyMode copy_mode,
+          internal::RepeatMode repeat_mode>
+struct BindCancellationChecker<Callback<Signature, copy_mode, repeat_mode>,
+                               std::tuple<BoundArgs...>> {
+  static constexpr bool is_cancellable = true;
+
+  template <typename Functor>
+  static bool Run(const Functor& functor, const BoundArgs&...) {
+    return functor.IsCancelled();
+  }
+};
+
 }  // namespace base
 
 #endif  // BASE_BIND_HELPERS_H_