[Device Discovery] Make DialRegistry a Singleton

chrome/browser/extensions/api/dial/dial_api.h

 // Copyright (c) 2012 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 CHROME_BROWSER_EXTENSIONS_API_DIAL_DIAL_API_H_
 #define CHROME_BROWSER_EXTENSIONS_API_DIAL_DIAL_API_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 14 matching lines @@
 
 // Dial API which is a ref-counted KeyedService that manages
 // the DIAL registry. It takes care of creating the registry on the IO thread
 // and is an observer of the registry. It makes sure devices events are sent out
 // to extension listeners on the right thread.
 //
 // TODO(mfoltz): This should probably inherit from BrowserContextKeyedAPI
 // instead; ShutdownOnUIThread below is a no-op, which is the whole point of
 // RefcountedKeyedService.
 //
 // TODO(mfoltz): The threading model for this API needs to be rethought.  At a

[mark a. foltz, 2017/03/18 18:04:58]

Since the DialRegistry is going to now be shared among multiple clients running
on the IO and UI threads, this paragraph no longer applies and can be removed.

It would be nice to simplify this API to post fewer tasks to/from the IO thread
to use the registry, but if the plan is to deprecate and remove chrome.dial then
maybe it's no longer worth it.

 // minimum, DialRegistry should move to the UI thread to avoid extra thread hops
 // here.  This would also allow a straightforward GetDeviceList implementation
 // (crbug.com/576817), cleanup some long-tail crashes (crbug.com/640011) that
 // are likely due to lifetime issues, and simplify unit tests
 // (crbug.com/661457).
 //
 // Also, DialRegistry should be an interface that can be mocked and injected for
 // tests; this would allow us to remove code that injects test data into the
 // real DialRegsitry.
 class DialAPI : public RefcountedKeyedService,
@@ skipping 39 matching lines @@
   // listeners.
   void NotifyListenerAddedOnIOThread();
   void NotifyListenerRemovedOnIOThread();
 
   // Fills the |device| API struct from |device_data|.
   void FillDialDevice(const media_router::DialDeviceData& device_data,
                       api::dial::DialDevice* device) const;
 
   Profile* profile_;
 
-  // Created lazily on first access on the IO thread.
-  std::unique_ptr<media_router::DialRegistry> dial_registry_;
+  // Created lazily on first access on the IO thread. Does not take ownership of
+  // |dial_registry_|.
+  scoped_refptr<media_router::DialRegistry> dial_registry_;
 
   // Device data for testing.
   std::unique_ptr<media_router::DialDeviceData> test_device_data_;
   std::unique_ptr<media_router::DialDeviceDescriptionData>
       test_device_description_;
 
   DISALLOW_COPY_AND_ASSIGN(DialAPI);
 };
 
 // DiscoverNow function. This function needs a round-trip from the IO thread
@@ skipping 50 matching lines @@
       device_description_fetcher_;
 
   DialAPI* dial_;
 
   DISALLOW_COPY_AND_ASSIGN(DialFetchDeviceDescriptionFunction);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_DIAL_DIAL_API_H_