[mojo-bindings] Support reuse of native enum classes

mojo/public/cpp/bindings/enum_traits.h

+// 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 MOJO_PUBLIC_CPP_BINDINGS_ENUM_TRAITS_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_ENUM_TRAITS_H_
+
+namespace mojo {
+
+// Specialize this over any enum class |T| in order to use the enum with
+// Mojo bindings in place of a mojom enum alias.
+//
+// For example, if you have an enum class:
+//
+//   enum Feels : int32_t {
+//     HAPPY,
+//     SAD,
+//     OK_I_GUESS
+//   };
+//
+// you can leverage this in Mojo interfaces by declaring in mojom
+//
+//   [Native=True]
+//   enum MojoFeels;
+//
+// and generating those bindings with a typemap applied, mapping MojoFeels to
+// the native Feels type.
+//
+// NOTE: If this seems a bit convoluted, that's because it is. It's only meant
+// to avoid maintaining duplicate enum definitions in cases where you for some
+// reason cannot move an enum definition into mojom.
+//
+// Once you have this setup in place, you may (but are not required to) define
+// a mojo::EnumTraits<Feels> specialization with a single static method:
+//
+//   namespace mojo {
+//
+//   template <>
+//   class EnumTraits<Feels> {
+//     static bool IsKnownValue(int32_t value) { /* ... */ }
+//   };
+//
+//   }  // namespace mojo
+//
+// This is used to check deserialized values on incoming messages. If it returns
+// |false| and the mojom enum declaration does not have the attribute
+// IsExtensible=True set, the message will be rejected without dispatch.
+
+template <typename T>
+struct EnumTraits {
+  static bool IsKnownValue(int32_t value) { return true; }
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_ENUM_TRAITS_H_