[Invalidations] Add GetInvalidatorState() to Invalidator{,Frontend}

chrome/browser/sync/invalidation_frontend.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_SYNC_INVALIDATION_FRONTEND_H_
 #define CHROME_BROWSER_SYNC_INVALIDATION_FRONTEND_H_
 
 #include "sync/notifier/invalidation_util.h"
 
 namespace syncer {
 class InvalidationHandler;
 }  // namespace syncer
 
 // Interface for classes that handle invalidation registrations and send out
 // invalidations to register handlers.
+//
+// Invalidation clients should follow the pattern below:
+//
+// When starting the client:
+//
+//   frontend->RegisterInvalidationHandler(client_handler);
+//
+// When the set of IDs to register changes for the client during its lifetime
+// (i.e., between calls to RegisterInvalidationHandler(client_handler) and
+// UnregisterInvalidationHandler(client_handler):
+//
+//   frontend->UpdateRegisteredInvalidationIds(client_handler, client_ids);
+//
+// When shutting down the client for browser shutdown:
+//
+//   frontend->UnregisterInvalidationHandler(client_handler);
+//
+// Note that there's no call to UpdateRegisteredIds() -- this is because the
+// invalidation API persists registrations across browser restarts.
+//
+// When permanently shutting down the client, e.g. when disabling the related
+// feature:
+//
+//   frontend->UpdateRegisteredInvalidationIds(client_handler, ObjectIdSet());
+//   frontend->UnregisterInvalidationHandler(client_handler);
+//
+// If an invalidation handler cares about the invalidator state, it should also
+// do the following when starting the client:
+//
+//   invalidator_state = frontend->GetInvalidatorState();
+//
+// It can also do the above in OnInvalidatorStateChange(), or it can use the
+// argument to OnInvalidatorStateChange().
+//
+// NOTE(akalin): Invalidations that come in during browser shutdown may get
+// dropped.  This won't matter once we have an Acknowledge API, though: see
+// http://crbug.com/78462 and http://crbug.com/124149.
 class InvalidationFrontend {
  public:
   // Starts sending notifications to |handler|.  |handler| must not be NULL,
   // and it must not already be registered.
   //
   // Handler registrations are persisted across restarts of sync.
   virtual void RegisterInvalidationHandler(
       syncer::InvalidationHandler* handler) = 0;
 
   // Updates the set of ObjectIds associated with |handler|.  |handler| must
   // not be NULL, and must already be registered.  An ID must be registered for
   // at most one handler.
   //
   // Registered IDs are persisted across restarts of sync.
   virtual void UpdateRegisteredInvalidationIds(
       syncer::InvalidationHandler* handler,
       const syncer::ObjectIdSet& ids) = 0;
 
   // Stops sending notifications to |handler|.  |handler| must not be NULL, and
   // it must already be registered.  Note that this doesn't unregister the IDs
   // associated with |handler|.
   //
   // Handler registrations are persisted across restarts of sync.
   virtual void UnregisterInvalidationHandler(
       syncer::InvalidationHandler* handler) = 0;
 
+  // Returns the current invalidator state.  When called from within
+  // InvalidationHandler::OnInvalidatorStateChange(), this must return
+  // the updated state.
+  virtual syncer::InvalidatorState GetInvalidatorState() const = 0;
+
  protected:
   virtual ~InvalidationFrontend() { }
 };
 
 #endif  // CHROME_BROWSER_SYNC_INVALIDATION_FRONTEND_H_