[mDns] Make DnsSdRegistry a leaky singleton

chrome/browser/media/router/discovery/mdns/dns_sd_registry.h

 // Copyright 2013 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_MEDIA_ROUTER_DISCOVERY_MDNS_DNS_SD_REGISTRY_H_
 #define CHROME_BROWSER_MEDIA_ROUTER_DISCOVERY_MDNS_DNS_SD_REGISTRY_H_
 
 #include <map>
 #include <memory>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/macros.h"
+#include "base/memory/singleton.h"
 #include "base/observer_list.h"
+#include "base/threading/thread_checker.h"
 #include "chrome/browser/media/router/discovery/mdns/dns_sd_delegate.h"
 
 namespace local_discovery {
 class ServiceDiscoverySharedClient;
 }
 
+namespace extensions {
+namespace {
+class MockDnsSdRegistry;

[mark a. foltz, 2017/05/11 23:22:24]

This is a sign this mock should live in c/b/media/router/discovery.  Is that
possible to fix in this patch?

[zhaobin, 2017/05/12 17:35:00]

Done.

+}
+}  // namespace extensions
+
 namespace media_router {
 
 class DnsSdDeviceLister;
 class ServiceTypeData;
 
 // Registry class for keeping track of discovered network services over DNS-SD.
 class DnsSdRegistry : public DnsSdDelegate {
  public:
   typedef std::vector<DnsSdService> DnsSdServiceList;
 
   class DnsSdObserver {
    public:
     virtual void OnDnsSdEvent(const std::string& service_type,
                               const DnsSdServiceList& services) = 0;
 
    protected:
     virtual ~DnsSdObserver() {}
   };
 
-  DnsSdRegistry();
-  explicit DnsSdRegistry(local_discovery::ServiceDiscoverySharedClient* client);
-  virtual ~DnsSdRegistry();
+  static DnsSdRegistry* GetInstance();
 
   // Publishes the current device list for |service_type| to event listeners
   // whose event filter matches the service type.
   virtual void Publish(const std::string& service_type);
 
   // Immediately issues a multicast DNS query for all service types of the
   // calling extension.
   virtual void ForceDiscovery();
 
   // Observer registration for parties interested in discovery events.
@@ skipping 47 matching lines @@
   void ServiceChanged(const std::string& service_type,
                       bool added,
                       const DnsSdService& service) override;
   void ServiceRemoved(const std::string& service_type,
                       const std::string& service_name) override;
   void ServicesFlushed(const std::string& service_type) override;
 
   std::map<std::string, std::unique_ptr<ServiceTypeData>> service_data_map_;
 
  private:
+  friend struct base::DefaultSingletonTraits<DnsSdRegistry>;
+  friend class extensions::MockDnsSdRegistry;
+  friend class TestDnsSdRegistry;
+
+  DnsSdRegistry();
+  explicit DnsSdRegistry(local_discovery::ServiceDiscoverySharedClient* client);
+  virtual ~DnsSdRegistry();
+
   void DispatchApiEvent(const std::string& service_type);
   bool IsRegistered(const std::string& service_type);
 
   scoped_refptr<local_discovery::ServiceDiscoverySharedClient>
       service_discovery_client_;
   base::ObserverList<DnsSdObserver> observers_;
+  base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(DnsSdRegistry);
 };
 
 }  // namespace media_router
 
 #endif  // CHROME_BROWSER_MEDIA_ROUTER_DISCOVERY_MDNS_DNS_SD_REGISTRY_H_