[Sync] Add sync api support for context changes

sync/api/sync_data.h

Index: sync/api/sync_data.h
diff --git a/sync/api/sync_data.h b/sync/api/sync_data.h
index 43f64f9185b10f798abe3086aec60fd85310d11e..d4072db298b87dc8e1295e41ef4989bf48a9ab8a 100644
--- a/sync/api/sync_data.h
+++ b/sync/api/sync_data.h
@@ -17,11 +17,12 @@
 
 namespace sync_pb {
 class EntitySpecifics;
-class SyncEntity;
 }  // namespace sync_pb
 
 namespace syncer {
 
+struct SyncDataCore;
+
 // A light-weight container for immutable sync data. Pass-by-value and storage
 // in STL containers are supported and encouraged if helpful.
 class SYNC_EXPORT SyncData {
@@ -55,6 +56,10 @@ class SYNC_EXPORT SyncData {
       const sync_pb::EntitySpecifics& specifics,
       const base::Time& last_modified_time);
 
+  // For datatype context changes: |datatype| specifies the type whose context
+  // is being updated.
+  static SyncData CreateContext(ModelType datatype, const std::string& context);

[maniscalco, 2014/03/31 23:59:16]

Any reason not to pass datatype by const ref?  The one advantage I see to
passing via const ref is that if ModelType ever becomes a big class, we don't
have to worry about unnecessary temporaries.

[maniscalco, 2014/03/31 23:59:16]

Are SyncData created by this method "local" or "remote"?  Or neither?

[Nicolas Zea, 2014/04/02 18:34:36]

ModelType is just an enum, so passing by ref is actually more expensive (passing
full pointer vs small value). And I don't think there are any plans to change
the ModelType's class in the near future, so we typically prefer to pass by
value.

[Nicolas Zea, 2014/04/02 18:34:36]

Neither. I was actually planning to get rid of the whole IsLocal() method, which
is only used in one place (generic_change_processor).

[maniscalco, 2014/04/02 21:13:09]

Pass by value SGTM.

+
   // Whether this SyncData holds valid data. The only way to have a SyncData
   // without valid data is to use the default constructor.
   bool IsValid() const;
@@ -74,6 +79,10 @@ class SYNC_EXPORT SyncData {
   // going TO the syncer, not from.
   const std::string& GetTitle() const;
 
+  // Returns the datatype context, if one exists. Returns empty string
+  // otherwise.
+  const std::string& GetContext() const;
+
   // Returns the last motification time according to the server. This is
   // only valid if IsLocal() is false, and may be null if the SyncData
   // represents a deleted item.
@@ -83,6 +92,7 @@ class SYNC_EXPORT SyncData {
   int64 GetRemoteId() const;
 
   // Whether this sync data is for local data or data coming from the syncer.
+  // TODO(zea): remove this.
   bool IsLocal() const;
 
   std::string ToString() const;
@@ -90,42 +100,33 @@ class SYNC_EXPORT SyncData {
   // TODO(zea): Query methods for other sync properties: parent, successor, etc.
 
  private:
-  // Necessary since we forward-declare sync_pb::SyncEntity; see
+  // Necessary since we forward-declare SyncDataCore; see
   // comments in immutable.h.
-  struct ImmutableSyncEntityTraits {
-    typedef sync_pb::SyncEntity* Wrapper;
+  struct ImmutableSyncDataCoreTraits {
+    typedef SyncDataCore* Wrapper;
 
     static void InitializeWrapper(Wrapper* wrapper);
 
     static void DestroyWrapper(Wrapper* wrapper);
 
-    static const sync_pb::SyncEntity& Unwrap(const Wrapper& wrapper);
+    static const SyncDataCore& Unwrap(const Wrapper& wrapper);
 
-    static sync_pb::SyncEntity* UnwrapMutable(Wrapper* wrapper);
+    static SyncDataCore* UnwrapMutable(Wrapper* wrapper);
 
-    static void Swap(sync_pb::SyncEntity* t1, sync_pb::SyncEntity* t2);
+    static void Swap(SyncDataCore* t1, SyncDataCore* t2);
   };
 
-  typedef Immutable<sync_pb::SyncEntity, ImmutableSyncEntityTraits>
-      ImmutableSyncEntity;
+  typedef Immutable<SyncDataCore, ImmutableSyncDataCoreTraits>
+      ImmutableSyncDataCore;
 
-  // Clears |entity|.
-  SyncData(int64 id,
-           sync_pb::SyncEntity* entity,
-           const base::Time& remote_modification_time);
+  // Clears |data|.
+  SyncData(SyncDataCore* data);
 
   // Whether this SyncData holds valid data.
   bool is_valid_;
 
-  // Equal to kInvalidId iff this is local.
-  int64 id_;
-
-  // This is only valid if IsLocal() is false, and may be null if the
-  // SyncData represents a deleted item.
-  base::Time remote_modification_time_;
-
-  // The actual shared sync entity being held.
-  ImmutableSyncEntity immutable_entity_;
+  // The actual sync data being held.
+  ImmutableSyncDataCore immutable_data_;
 };
 
 // gmock printer helper.