[mojo] Expose interface method min_versions

mojo/public/cpp/bindings/interface_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_INTERFACE_TRAITS_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_INTERFACE_TRAITS_H_
+
+#include <stdint.h>
+
+#include <string>
+
+namespace mojo {
+
+// Metadata information for each method in an interface.
+struct MethodMetadata {
+  uint32_t min_version;

[yzshen1, 2016/12/22 18:53:28]

At the moment, it seems that min_version is what is really needed.

Besides, there is an internal conversation (background: crbug.com/673417, we
could chat in person if necessary). So storing mapping to ordinal/name may not
be a good idea.

Without |name|, we won't be able to do something like
GetInstanceForMethod<mojom::Foo::kBarMethodIndex>() and let it logs a string
name. Instead we would have to do either:

  GetInstanceForMethod<mojom::Foo::kBarMethodIndex>("Bar")

or if we really want to avoid duplication of "Bar" in the call above, use macro:
  GET_INSTANCE_FOR_METHOD(mojom::Foo, Bar);

Getting rid of |name| probably also helps to avoid binary size increase.
WDYT?

+  uint32_t ordinal;
+  const char* name;
+};
+
+// Exposes metadata about a method. MethodIndex should be one of the
+// kMethodNameMethodIndex constants in Interface::kMethodIndices.
+template <typename Interface, uint32_t MethodIndex>
+struct MethodTraits {
+  static uint32_t MinVersion() {
+    return Interface::kMethodMetadata[MethodIndex].min_version;
+  }
+
+  static const char* Name() {
+    return Interface::kMethodMetadata[MethodIndex].name;
+  }
+
+  static uint32_t Ordinal() {
+    return Interface::kMethodMetadata[MethodIndex].ordinal;
+  }
+
+  static_assert(MethodIndex < Interface::kMethodMetadataSize,
+                "Invalid method index");
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_INTERFACE_TRAITS_H_