diff --git a/src/DefaultPieceStorage.cc b/src/DefaultPieceStorage.cc
index 954765ef..baba8be8 100644
--- a/src/DefaultPieceStorage.cc
+++ b/src/DefaultPieceStorage.cc
@@ -494,7 +494,7 @@ void DefaultPieceStorage::completePiece(const SharedHandle<Piece>& piece)
         util::executeHookByOptName(downloadContext_->getOwnerRequestGroup(),
                                    option_, PREF_ON_BT_DOWNLOAD_COMPLETE);
         SingletonHolder<Notifier>::instance()->
-          notifyDownloadEvent(Notifier::ON_BT_DOWNLOAD_COMPLETE,
+          notifyDownloadEvent(EVENT_ON_BT_DOWNLOAD_COMPLETE,
                               downloadContext_->getOwnerRequestGroup());
       }
     }
diff --git a/src/DownloadEngine.cc b/src/DownloadEngine.cc
index c014eb80..71607ca0 100644
--- a/src/DownloadEngine.cc
+++ b/src/DownloadEngine.cc
@@ -69,6 +69,9 @@
 #ifdef ENABLE_BITTORRENT
 # include "BtRegistry.h"
 #endif // ENABLE_BITTORRENT
+#ifdef ENABLE_WEBSOCKET
+# include "WebSocketSessionMan.h"
+#endif // ENABLE_WEBSOCKET
 
 namespace aria2 {
 
@@ -596,4 +599,12 @@ void DownloadEngine::setAsyncDNSServers(ares_addr_node* asyncDNSServers)
 }
 #endif // HAVE_ARES_ADDR_NODE
 
+#ifdef ENABLE_WEBSOCKET
+void DownloadEngine::setWebSocketSessionMan
+(const SharedHandle<rpc::WebSocketSessionMan>& wsman)
+{
+  webSocketSessionMan_ = wsman;
+}
+#endif // ENABLE_WEBSOCKET
+
 } // namespace aria2
diff --git a/src/DownloadEngine.h b/src/DownloadEngine.h
index ff14a93e..afe51307 100644
--- a/src/DownloadEngine.h
+++ b/src/DownloadEngine.h
@@ -68,6 +68,11 @@ class Command;
 #ifdef ENABLE_BITTORRENT
 class BtRegistry;
 #endif // ENABLE_BITTORRENT
+#ifdef ENABLE_WEBSOCKET
+namespace rpc {
+class WebSocketSessionMan;
+} // namespace rpc
+#endif // ENABLE_WEBSOCKET
 
 class DownloadEngine {
 private:
@@ -144,6 +149,10 @@ private:
 
   SharedHandle<AuthConfigFactory> authConfigFactory_;
 
+#ifdef ENABLE_WEBSOCKET
+  SharedHandle<rpc::WebSocketSessionMan> webSocketSessionMan_;
+#endif // ENABLE_WEBSOCKET
+
   /**
    * Delegates to StatCalc
    */
@@ -349,6 +358,15 @@ public:
     return asyncDNSServers_;
   }
 #endif // HAVE_ARES_ADDR_NODE
+
+#ifdef ENABLE_WEBSOCKET
+  void setWebSocketSessionMan
+  (const SharedHandle<rpc::WebSocketSessionMan>& wsman);
+  const SharedHandle<rpc::WebSocketSessionMan>& getWebSocketSessionMan() const
+  {
+    return webSocketSessionMan_;
+  }
+#endif // ENABLE_WEBSOCKET
 };
 
 } // namespace aria2
diff --git a/src/MultiUrlRequestInfo.cc b/src/MultiUrlRequestInfo.cc
index eaf2608c..d7723a86 100644
--- a/src/MultiUrlRequestInfo.cc
+++ b/src/MultiUrlRequestInfo.cc
@@ -133,11 +133,7 @@ int MultiUrlRequestInfo::prepare()
 {
   global::globalHaltRequested = 0;
   try {
-    SharedHandle<rpc::WebSocketSessionMan> wsSessionMan;
-    if(option_->getAsBool(PREF_ENABLE_RPC)) {
-      wsSessionMan.reset(new rpc::WebSocketSessionMan());
-    }
-    SharedHandle<Notifier> notifier(new Notifier(wsSessionMan));
+    SharedHandle<Notifier> notifier(new Notifier());
     SingletonHolder<Notifier>::instance(notifier);
 
 #ifdef ENABLE_SSL
@@ -166,6 +162,14 @@ int MultiUrlRequestInfo::prepare()
     // Avoid keeping RequestGroups alive longer than necessary
     requestGroups_.clear();
 
+    if(option_->getAsBool(PREF_ENABLE_RPC)) {
+      SharedHandle<rpc::WebSocketSessionMan> wsSessionMan
+        (new rpc::WebSocketSessionMan());
+      e_->setWebSocketSessionMan(wsSessionMan);
+      SingletonHolder<Notifier>::instance()->addDownloadEventListener
+        (wsSessionMan);
+    }
+
     if(!option_->blank(PREF_LOAD_COOKIES)) {
       File cookieFile(option_->get(PREF_LOAD_COOKIES));
       if(cookieFile.isFile() &&
diff --git a/src/Notifier.cc b/src/Notifier.cc
index 6d843340..5228e76f 100644
--- a/src/Notifier.cc
+++ b/src/Notifier.cc
@@ -35,47 +35,25 @@
 #include "Notifier.h"
 #include "RequestGroup.h"
 #include "LogFactory.h"
-#ifdef ENABLE_WEBSOCKET
-#  include "WebSocketSessionMan.h"
-#else // !ENABLE_WEBSOCKET
-#  include "NullWebSocketSessionMan.h"
-#endif // !ENABLE_WEBSOCKET
 
 namespace aria2 {
 
-Notifier::Notifier(const SharedHandle<rpc::WebSocketSessionMan>& wsSessionMan)
-  : wsSessionMan_(wsSessionMan)
-{}
+Notifier::Notifier() {}
 
 Notifier::~Notifier() {}
 
-void Notifier::addWebSocketSession
-(const SharedHandle<rpc::WebSocketSession>& wsSession)
+void Notifier::addDownloadEventListener
+(const SharedHandle<DownloadEventListener>& listener)
 {
-  A2_LOG_DEBUG("WebSocket session added.");
-  wsSessionMan_->addSession(wsSession);
+  listeners_.push_back(listener);
 }
 
-void Notifier::removeWebSocketSession
-(const SharedHandle<rpc::WebSocketSession>& wsSession)
-{
-  A2_LOG_DEBUG("WebSocket session removed.");
-  wsSessionMan_->removeSession(wsSession);
-}
-
-const std::string Notifier::ON_DOWNLOAD_START = "aria2.onDownloadStart";
-const std::string Notifier::ON_DOWNLOAD_PAUSE = "aria2.onDownloadPause";
-const std::string Notifier::ON_DOWNLOAD_STOP = "aria2.onDownloadStop";
-const std::string Notifier::ON_DOWNLOAD_COMPLETE = "aria2.onDownloadComplete";
-const std::string Notifier::ON_DOWNLOAD_ERROR = "aria2.onDownloadError";
-const std::string Notifier::ON_BT_DOWNLOAD_COMPLETE =
-  "aria2.onBtDownloadComplete";
-
 void Notifier::notifyDownloadEvent
-(const std::string& event, const RequestGroup* group)
+(DownloadEvent event, const RequestGroup* group)
 {
-  if(wsSessionMan_) {
-    wsSessionMan_->addNotification(event, group);
+  for(std::vector<SharedHandle<DownloadEventListener> >::const_iterator i =
+        listeners_.begin(), eoi = listeners_.end(); i != eoi; ++i) {
+    (*i)->onEvent(event, group);
   }
 }
 
diff --git a/src/Notifier.h b/src/Notifier.h
index efc9c225..39787326 100644
--- a/src/Notifier.h
+++ b/src/Notifier.h
@@ -36,45 +36,37 @@
 #define D_NOTIFIER_H
 
 #include "common.h"
+
+#include <vector>
+
+#include <aria2/aria2.h>
 #include "SharedHandle.h"
 
 namespace aria2 {
 
 class RequestGroup;
-class Option;
-struct Pref;
 
-namespace rpc {
-
-class WebSocketSessionMan;
-class WebSocketSession;
-
-} // namespace rpc
+struct DownloadEventListener {
+  virtual ~DownloadEventListener() {}
+  virtual void onEvent(DownloadEvent event, const RequestGroup* group) = 0;
+};
 
 class Notifier {
 public:
-  // The string constants for download events.
-  static const std::string ON_DOWNLOAD_START;
-  static const std::string ON_DOWNLOAD_PAUSE;
-  static const std::string ON_DOWNLOAD_STOP;
-  static const std::string ON_DOWNLOAD_COMPLETE;
-  static const std::string ON_DOWNLOAD_ERROR;
-  static const std::string ON_BT_DOWNLOAD_COMPLETE;
-
-  Notifier(const SharedHandle<rpc::WebSocketSessionMan>& wsSessionMan);
+  Notifier();
   ~Notifier();
-  void addWebSocketSession(const SharedHandle<rpc::WebSocketSession>& wsSes);
-  void removeWebSocketSession(const SharedHandle<rpc::WebSocketSession>& wsSes);
+  void addDownloadEventListener
+  (const SharedHandle<DownloadEventListener>& listener);
   // Notifies the download event to all listeners.
-  void notifyDownloadEvent(const std::string& event, const RequestGroup* group);
+  void notifyDownloadEvent(DownloadEvent event, const RequestGroup* group);
 
-  void notifyDownloadEvent(const std::string& event,
+  void notifyDownloadEvent(DownloadEvent event,
                            const SharedHandle<RequestGroup>& group)
   {
     notifyDownloadEvent(event, group.get());
   }
 private:
-  SharedHandle<rpc::WebSocketSessionMan> wsSessionMan_;
+  std::vector<SharedHandle<DownloadEventListener> > listeners_;
 };
 
 } // namespace aria2
diff --git a/src/RequestGroupMan.cc b/src/RequestGroupMan.cc
index 54cfe5be..3fdf064f 100644
--- a/src/RequestGroupMan.cc
+++ b/src/RequestGroupMan.cc
@@ -211,7 +211,7 @@ bool RequestGroupMan::removeReservedGroup(a2_gid_t gid)
 namespace {
 
 void notifyDownloadEvent
-(const std::string& event, const SharedHandle<RequestGroup>& group)
+(DownloadEvent event, const SharedHandle<RequestGroup>& group)
 {
   // Check NULL to make unit test easier.
   if(SingletonHolder<Notifier>::instance()) {
@@ -239,12 +239,12 @@ void executeStopHook
     util::executeHookByOptName(group, option, PREF_ON_DOWNLOAD_STOP);
   }
   if(result == error_code::FINISHED) {
-    notifyDownloadEvent(Notifier::ON_DOWNLOAD_COMPLETE, group);
+    notifyDownloadEvent(EVENT_ON_DOWNLOAD_COMPLETE, group);
   } else if(result != error_code::IN_PROGRESS &&
             result != error_code::REMOVED) {
-    notifyDownloadEvent(Notifier::ON_DOWNLOAD_ERROR, group);
+    notifyDownloadEvent(EVENT_ON_DOWNLOAD_ERROR, group);
   } else {
-    notifyDownloadEvent(Notifier::ON_DOWNLOAD_STOP, group);
+    notifyDownloadEvent(EVENT_ON_DOWNLOAD_STOP, group);
   }
 }
 
@@ -392,7 +392,7 @@ public:
         group->setForceHaltRequested(false);
         util::executeHookByOptName(group, e_->getOption(),
                                    PREF_ON_DOWNLOAD_PAUSE);
-        notifyDownloadEvent(Notifier::ON_DOWNLOAD_PAUSE, group);
+        notifyDownloadEvent(EVENT_ON_DOWNLOAD_PAUSE, group);
         // TODO Should we have to prepend spend uris to remaining uris
         // in case PREF_REUSE_URI is disabed?
       } else {
@@ -508,7 +508,7 @@ void RequestGroupMan::fillRequestGroupFromReserver(DownloadEngine* e)
 
     util::executeHookByOptName(groupToAdd, e->getOption(),
                                PREF_ON_DOWNLOAD_START);
-    notifyDownloadEvent(Notifier::ON_DOWNLOAD_START, groupToAdd);
+    notifyDownloadEvent(EVENT_ON_DOWNLOAD_START, groupToAdd);
   }
   if(!pending.empty()) {
     reservedGroups_.insert(reservedGroups_.begin(), RequestGroupKeyFunc(),
@@ -577,7 +577,7 @@ RequestGroupMan::DownloadStat RequestGroupMan::getDownloadStat() const
                       lastError);
 }
 
-enum DownloadStatus {
+enum DownloadResultStatus {
   A2_STATUS_OK,
   A2_STATUS_INPR,
   A2_STATUS_RM,
@@ -585,7 +585,7 @@ enum DownloadStatus {
 };
 
 namespace {
-const char* getStatusStr(DownloadStatus status, bool useColor)
+const char* getStatusStr(DownloadResultStatus status, bool useColor)
 {
   // status string is formatted in 4 characters wide.
   switch(status) {
diff --git a/src/WebSocketInteractionCommand.cc b/src/WebSocketInteractionCommand.cc
index 9dccc3c3..a21db84d 100644
--- a/src/WebSocketInteractionCommand.cc
+++ b/src/WebSocketInteractionCommand.cc
@@ -42,6 +42,7 @@
 #include "fmt.h"
 #include "SingletonHolder.h"
 #include "Notifier.h"
+#include "WebSocketSessionMan.h"
 
 namespace aria2 {
 
@@ -58,7 +59,7 @@ WebSocketInteractionCommand::WebSocketInteractionCommand
    writeCheck_(false),
    wsSession_(wsSession)
 {
-  SingletonHolder<Notifier>::instance()->addWebSocketSession(wsSession_);
+  e_->getWebSocketSessionMan()->addSession(wsSession_);
   e_->addSocketForReadCheck(socket_, this);
 }
 
@@ -68,7 +69,7 @@ WebSocketInteractionCommand::~WebSocketInteractionCommand()
   if(writeCheck_) {
     e_->deleteSocketForWriteCheck(socket_, this);
   }
-  SingletonHolder<Notifier>::instance()->removeWebSocketSession(wsSession_);
+  e_->getWebSocketSessionMan()->removeSession(wsSession_);
 }
 
 void WebSocketInteractionCommand::updateWriteCheck()
diff --git a/src/WebSocketSessionMan.cc b/src/WebSocketSessionMan.cc
index 26771bc4..54c96fac 100644
--- a/src/WebSocketSessionMan.cc
+++ b/src/WebSocketSessionMan.cc
@@ -38,6 +38,7 @@
 #include "json.h"
 #include "util.h"
 #include "WebSocketInteractionCommand.h"
+#include "LogFactory.h"
 
 namespace aria2 {
 
@@ -50,12 +51,14 @@ WebSocketSessionMan::~WebSocketSessionMan() {}
 void WebSocketSessionMan::addSession
 (const SharedHandle<WebSocketSession>& wsSession)
 {
+  A2_LOG_DEBUG("WebSocket session added.");
   sessions_.insert(wsSession);
 }
 
 void WebSocketSessionMan::removeSession
 (const SharedHandle<WebSocketSession>& wsSession)
 {
+  A2_LOG_DEBUG("WebSocket session removed.");
   sessions_.erase(wsSession);
 }
 
@@ -78,6 +81,48 @@ void WebSocketSessionMan::addNotification
   }
 }
 
+
+namespace {
+// The string constants for download events.
+const std::string ON_DOWNLOAD_START = "aria2.onDownloadStart";
+const std::string ON_DOWNLOAD_PAUSE = "aria2.onDownloadPause";
+const std::string ON_DOWNLOAD_STOP = "aria2.onDownloadStop";
+const std::string ON_DOWNLOAD_COMPLETE = "aria2.onDownloadComplete";
+const std::string ON_DOWNLOAD_ERROR = "aria2.onDownloadError";
+const std::string ON_BT_DOWNLOAD_COMPLETE = "aria2.onBtDownloadComplete";
+} // namespace
+
+namespace {
+const std::string& getMethodName(DownloadEvent event)
+{
+  switch(event) {
+  case EVENT_ON_DOWNLOAD_START:
+    return ON_DOWNLOAD_START;
+  case EVENT_ON_DOWNLOAD_PAUSE:
+    return ON_DOWNLOAD_PAUSE;
+  case EVENT_ON_DOWNLOAD_STOP:
+    return ON_DOWNLOAD_STOP;
+  case EVENT_ON_DOWNLOAD_COMPLETE:
+    return ON_DOWNLOAD_COMPLETE;
+  case EVENT_ON_DOWNLOAD_ERROR:
+    return ON_DOWNLOAD_ERROR;
+  case EVENT_ON_BT_DOWNLOAD_COMPLETE:
+    return ON_BT_DOWNLOAD_COMPLETE;
+  default:
+    // Not reachable
+    assert(0);
+    // For suppress compiler warning
+    return A2STR::NIL;
+  }
+}
+} // namespace
+
+void WebSocketSessionMan::onEvent(DownloadEvent event,
+                                  const RequestGroup* group)
+{
+  addNotification(getMethodName(event), group);
+}
+
 } // namespace rpc
 
 } // namespace aria2
diff --git a/src/WebSocketSessionMan.h b/src/WebSocketSessionMan.h
index 8004b485..4b2b5acc 100644
--- a/src/WebSocketSessionMan.h
+++ b/src/WebSocketSessionMan.h
@@ -35,7 +35,7 @@
 #ifndef D_WEB_SOCKET_SESSION_MAN_H
 #define D_WEB_SOCKET_SESSION_MAN_H
 
-#include "common.h"
+#include "Notifier.h"
 
 #include <set>
 #include <string>
@@ -51,7 +51,7 @@ namespace rpc {
 
 class WebSocketSession;
 
-class WebSocketSessionMan {
+class WebSocketSessionMan : public DownloadEventListener {
 public:
   typedef std::set<SharedHandle<WebSocketSession>,
                    RefLess<WebSocketSession> > WebSocketSessions;
@@ -60,6 +60,7 @@ public:
   void addSession(const SharedHandle<WebSocketSession>& wsSession);
   void removeSession(const SharedHandle<WebSocketSession>& wsSession);
   void addNotification(const std::string& method, const RequestGroup* group);
+  virtual void onEvent(DownloadEvent event, const RequestGroup* group);
 private:
   WebSocketSessions sessions_;
 };
diff --git a/src/includes/aria2/aria2.h b/src/includes/aria2/aria2.h
index d0edb520..5671c4ee 100644
--- a/src/includes/aria2/aria2.h
+++ b/src/includes/aria2/aria2.h
@@ -97,6 +97,39 @@ typedef uint64_t A2Gid;
  */
 typedef std::vector<std::pair<std::string, std::string> > KeyVals;
 
+/**
+ * @enum
+ *
+ * Download event constants
+ */
+enum DownloadEvent {
+  /**
+   * Indicating download has started.
+   */
+  EVENT_ON_DOWNLOAD_START = 1,
+  /**
+   * Indicating download has paused.
+   */
+  EVENT_ON_DOWNLOAD_PAUSE,
+  /**
+   * Indicating download has stopped.
+   */
+  EVENT_ON_DOWNLOAD_STOP,
+  /**
+   * Indicating download has completed.
+   */
+  EVENT_ON_DOWNLOAD_COMPLETE,
+  /**
+   * Indicating download has stopped becauseof the error.
+   */
+  EVENT_ON_DOWNLOAD_ERROR,
+  /**
+   * Indicating BitTorrent download has completed, but it may still
+   * continue to perform seeding.
+   */
+  EVENT_ON_BT_DOWNLOAD_COMPLETE
+};
+
 /**
  * @struct
  *