arc: Add InstanceHelper::GetInstanceForMethod()

components/arc/instance_holder.h

 // Copyright 2015 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_ARC_INSTANCE_HOLDER_H_
 #define COMPONENTS_ARC_INSTANCE_HOLDER_H_
 
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 24 matching lines @@
 
   InstanceHolder() = default;
 
   // Gets the Mojo interface for all the instance services. This will return
   // nullptr if that particular service is not ready yet. Use an Observer if you
   // want to be notified when this is ready. This can only be called on the
   // thread that this class was created on.
   T* instance() const { return instance_; }
   uint32_t version() const { return version_; }
 
+  // Gets the Mojo interface that's intended to call for
+  // |method_name_for_logging|, but only if its reported version is at least
+  // |min_version|. Returns nullptr if the instance is either not ready or does
+  // not have the requested version, and logs appropriately.
+  T* GetInstanceForMethod(const char* method_name_for_logging,
+                          uint32_t min_version) {
+    DCHECK_LT(0u, min_version) << T::Name_ << "::" << method_name_for_logging;

[Yusuke Sato, 2016/09/17 04:28:05]

nit: you can't have this check anymore since line 68 passes 0u in.

[Luis Héctor Chávez, 2016/09/19 15:33:11]

Done.

+    if (!instance_) {
+      VLOG(1) << "Instance for " << T::Name_ << "::" << method_name_for_logging
+              << " not available.";
+      return nullptr;
+    }
+    if (version_ < min_version) {
+      VLOG(1) << "Instance for " << T::Name_ << "::" << method_name_for_logging
+              << " version mismatch. Expected " << min_version << " got "
+              << version_;
+      return nullptr;
+    }
+    return instance_;
+  }
+
+  // Same as the above, but for the version zero.
+  T* GetInstanceForMethod(const char* method_name_for_logging) {
+    return GetInstanceForMethod(method_name_for_logging, 0u);
+  }
+
   // Adds or removes observers. This can only be called on the thread that this
   // class was created on. RemoveObserver does nothing if |observer| is not in
   // the list.
   void AddObserver(Observer* observer) {
     DCHECK(thread_checker_.CalledOnValidThread());
     observer_list_.AddObserver(observer);
 
     if (instance())
       observer->OnInstanceReady();
   }
@@ skipping 32 matching lines @@
 
   base::ThreadChecker thread_checker_;
   base::ObserverList<Observer> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(InstanceHolder<T>);
 };
 
 }  // namespace arc
 
 #endif  // COMPONENTS_ARC_INSTANCE_HOLDER_H_