sync: Inject sync/'s dependency on invalidations

sync/internal_api/public/base/invalidation_interface.h

+// Copyright 2014 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_INTERNAL_API_PUBLIC_BASE_INVALIDATION_INTERFACE_H_
+#define SYNC_INTERNAL_API_PUBLIC_BASE_INVALIDATION_INTERFACE_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "sync/base/sync_export.h"
+
+namespace syncer {
+
+// An interface that wraps sync's interactions with the component that provides
+// it with invalidations.
+class SYNC_EXPORT InvalidationInterface {
+ public:
+  // Orders invalidations based on version number and IsUnknownVersion().
+  static bool LessThan(const InvalidationInterface& a,
+                       const InvalidationInterface& b);
+
+  InvalidationInterface();
+  virtual ~InvalidationInterface();
+
+  // Returns true if this is an 'unknown version' invalidation.
+  // Such invalidations have no valid payload or version number.
+  virtual bool IsUnknownVersion() const = 0;
+
+  // Returns the payload of this item.
+  // DCHECKs if this is an unknown version invalidation.
+  virtual const std::string& GetPayload() const = 0;
+
+  // Retursn the version of this item.
+  // DCHECKs if this is an unknown version invalidation.
+  //
+  // It is preferable to use the LessThan() function, which handles unknown
+  // versions properly, rather than this function.
+  virtual int64 GetVersion() const = 0;
+
+  // This function will be called when the invalidation has been handled
+  // successfully.
+  virtual void Acknowledge() = 0;
+
+  // This function should be called if a lack of buffer space required that we
+  // drop this invalidation.
+  //
+  // To indicate recovery from a drop event, the receiver of this invalidation
+  // will call Acknowledge() on the most recently dropped invalidation.
+  virtual void Drop() = 0;
+};
+
+}  // namespace syncer
+
+#endif

[tim (not reviewing), 2014/06/18 22:41:11]

nit - // SYNC_INTERNAL_API_PUBLIC_BASE...