arc: Provide a per-service getter for ArcServiceManager

components/arc/arc_service_manager.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_ARC_SERVICE_MANAGER_H_
 #define COMPONENTS_ARC_ARC_SERVICE_MANAGER_H_
 
 #include <memory>
+#include <string>
+#include <type_traits>
+#include <unordered_map>
+#include <utility>
 #include <vector>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/observer_list.h"
 #include "base/task_runner.h"
 #include "base/threading/thread_checker.h"
 #include "components/arc/intent_helper/activity_icon_loader.h"
 #include "components/arc/intent_helper/local_activity_resolver.h"
 
 namespace arc {
 
 class ArcBridgeService;
 class ArcIntentHelperObserver;
 class ArcService;
 
+namespace internal {
+
+// If an ArcService is declared with a name, e.g.:
+//
+// class MyArcService : public ArcService {
+//  public:
+//   static const char kArcServiceName[];
+//   ...
+// };
+//
+// it can then be retrieved from ArcServiceManager in a type-safe way using
+// GetService<T>(). This two functions allow AddService() to get the name only
+// if it was provided, or use an empty string otherwise.
+//
+// In order to avoid collisions, kArcServiceName should be the fully-qualified

[hidehiko, 2017/01/11 05:17:45]

So, as you explicitly specify the template typename in caller,
argument looks not necessary (but actually is needed).
Could you add implementation note about it?

Note: I was original thinking to use it for type inference,
but I'm ok for the current implementation, as well.

[Luis Héctor Chávez, 2017/01/11 16:01:36]

Done.

+// name of the class.
+template <typename T>
+decltype(T::kArcServiceName, std::string()) GetArcServiceName(T* unused) {
+  if (strlen(T::kArcServiceName) == 0)

[hidehiko, 2017/01/11 05:17:45]

Optional: or, static_assert for compile time check?
static_assert(T::kArcServiceName[0] != '\0', ...);

[Luis Héctor Chávez, 2017/01/11 16:01:36]

Tried that before, compiler didn't like it since T::kArcServiceName is not
constexpr.

+    LOG(ERROR) << "kArcServiceName[] should be a fully-qualified class name.";
+  return T::kArcServiceName;
+}
+
+template <typename T>
+std::string GetArcServiceName(...) {
+  return std::string();
+}
+
+}  // namespace internal
+
 // Manages creation and destruction of services that communicate with the ARC
 // instance via the ArcBridgeService.
 class ArcServiceManager {
  public:
   class Observer {
    public:
     // Called when ArcServiceManager is being shut down. Observer
     // implementation should clean up ARC related stuff here. One of the
     // typical use cases is calling ArcServiceManager::RemoveObserver().
     virtual void OnArcShutdown() = 0;
 
     // Called when intent filters are added or removed.
     virtual void OnIntentFiltersUpdated() = 0;
 
    protected:
     virtual ~Observer() = default;
   };
 
   explicit ArcServiceManager(
       scoped_refptr<base::TaskRunner> blocking_task_runner);
   ~ArcServiceManager();
 
   // |arc_bridge_service| can only be accessed on the thread that this
   // class was created on.
   ArcBridgeService* arc_bridge_service();
 
-  // Adds a service to the managed services list.
-  void AddService(std::unique_ptr<ArcService> service);
+  // Adds a service to the managed services list. Returns false if another
+  // named service with that name had already been added.
+  template <typename T>
+  bool AddService(std::unique_ptr<T> service) {
+    return AddServiceInternal(internal::GetArcServiceName<T>(nullptr),
+                              std::move(service));
+  }
+
+  // Gets the named service from the managed services list. This uses SFINAE, so
+  // you can only call this function if the service specified by T provides a
+  // static member variable called kArcServiceName[] (otherwise this will not
+  // compile).
+  template <typename T>
+  T* GetService() {
+    return static_cast<T*>(GetNamedServiceInternal(T::kArcServiceName));
+  }
 
   // Gets the global instance of the ARC Service Manager. This can only be
   // called on the thread that this class was created on.
   static ArcServiceManager* Get();
 
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
   // Called to shut down all ARC services.
   void Shutdown();
@@ skipping 11 matching lines @@
   }
 
   // Returns the IntentHelperObserver instance owned by ArcServiceManager.
   ArcIntentHelperObserver* intent_helper_observer() {
     return intent_helper_observer_.get();
   }
 
  private:
   class IntentHelperObserverImpl;  // implemented in arc_service_manager.cc.
 
+  // Helper methods for AddService and GetService.
+  bool AddServiceInternal(const std::string& name,
+                          std::unique_ptr<ArcService> service);
+  ArcService* GetNamedServiceInternal(const std::string& name);
+
   base::ThreadChecker thread_checker_;
   scoped_refptr<base::TaskRunner> blocking_task_runner_;
 
   // An object for observing the ArcIntentHelper instance in |services_|.
   std::unique_ptr<ArcIntentHelperObserver> intent_helper_observer_;
 
   std::unique_ptr<ArcBridgeService> arc_bridge_service_;
-  std::vector<std::unique_ptr<ArcService>> services_;
+  std::unordered_multimap<std::string, std::unique_ptr<ArcService>> services_;
   scoped_refptr<ActivityIconLoader> icon_loader_;
   scoped_refptr<LocalActivityResolver> activity_resolver_;
 
   base::ObserverList<Observer> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(ArcServiceManager);
 };
 
 }  // namespace arc
 
 #endif  // COMPONENTS_ARC_ARC_SERVICE_MANAGER_H_