[mojo] Expose interface method min_versions

components/arc/instance_holder.h

Index: components/arc/instance_holder.h
diff --git a/components/arc/instance_holder.h b/components/arc/instance_holder.h
index 8455814581c61997f4d4ec602c6373cdbd9bb41b..4387da5c6ae1f82a1f7529ad75ade7f82df76f53 100644
--- a/components/arc/instance_holder.h
+++ b/components/arc/instance_holder.h
@@ -12,6 +12,7 @@
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "base/threading/thread_checker.h"
+#include "mojo/public/cpp/bindings/interface_traits.h"
 
 namespace arc {
 
@@ -63,6 +64,23 @@ class InstanceHolder {
     return instance_;
   }
 
+  template <uint32_t MethodIndex>
+  T* GetInstanceForMethod() {
+    using Method = mojo::MethodTraits<T, MethodIndex>;

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

I feel that mojo::MethodTraits is not needed, we could simply do:
const auto& kMethodMetaData = T::kMethodMetadata[MethodIndex];

And then use kMethodMetaData.{name, min_version} below. WDYT?

[yzshen1, 2016/12/22 19:01:09]

But please see my comments in the other file.
If we only have min_version as meta data, we could direct do:

  x->GetInstanceForVersion(kFooMinVersion, "Foo" /* for logging */);

or macro GET_INSTANCE_FOR_VERSION(x, Foo);

+    if (!instance_) {
+      VLOG(1) << "Instance for " << T::Name_ << "::" << Method::Name()
+              << " not available.";
+      return nullptr;
+    }
+    if (version_ < Method::MinVersion()) {
+      LOG(ERROR) << "Instance for " << T::Name_ << "::" << Method::Name()
+                 << " version mismatch. Expected " << Method::MinVersion()
+                 << " got " << version_;
+      return nullptr;
+    }
+    return instance_;
+  }
+
   // Same as the above, but for the version zero.
   T* GetInstanceForMethod(const std::string& method_name_for_logging) {
     return GetInstanceForMethod(method_name_for_logging, 0u);