sync: SyncableService support for starting sync

sync/api/syncable_service.h

 // Copyright 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 SYNC_API_SYNCABLE_SERVICE_H_
 #define SYNC_API_SYNCABLE_SERVICE_H_
 
 #include <vector>
 
+#include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "sync/api/sync_change_processor.h"
 #include "sync/api/sync_data.h"
 #include "sync/api/sync_error.h"
 #include "sync/api/sync_merge_result.h"
 #include "sync/base/sync_export.h"
 #include "sync/internal_api/public/base/model_type.h"
 
 namespace syncer {
 
 class SyncErrorFactory;
 
 // TODO(zea): remove SupportsWeakPtr in favor of having all SyncableService
 // implementers provide a way of getting a weak pointer to themselves.
 // See crbug.com/100114.
 class SYNC_EXPORT SyncableService
     : public SyncChangeProcessor,
       public base::SupportsWeakPtr<SyncableService> {
  public:
+  // A StartSyncFlare is useful when your SyncableService has a need for sync
+  // to start ASAP, typically because a local change event has occurred but
+  // MergeDataAndStartSyncing hasn't been called yet, meaning you don't have a
+  // SyncChangeProcessor. The sync subsystem will respond soon after invoking
+  // Run() on your flare by calling MergeDataAndStartSyncing. The ModelType
+  // parameter is included so that the recieving end can track usage and timing
+  // statistics, make optimizations or tradeoffs by type, etc.
+  typedef base::Callback<void(ModelType)> StartSyncFlare;
+
   // Informs the service to begin syncing the specified synced datatype |type|.
   // The service should then merge |initial_sync_data| into it's local data,
   // calling |sync_processor|'s ProcessSyncChanges as necessary to reconcile the
   // two. After this, the SyncableService's local data should match the server
   // data, and the service should be ready to receive and process any further
   // SyncChange's as they occur.
   // Returns: a SyncMergeResult whose error field reflects whether an error
   //          was encountered while merging the two models. The merge result
   //          may also contain optional merge statistics.
   virtual SyncMergeResult MergeDataAndStartSyncing(
@@ skipping 19 matching lines @@
       const tracked_objects::Location& from_here,
       const SyncChangeList& change_list) OVERRIDE = 0;
 
  protected:
   virtual ~SyncableService();
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_SYNCABLE_SERVICE_H_