sync: Implement per-type commit interface

sync/engine/sync_directory_commit_contributor.h

Index: sync/engine/sync_directory_commit_contributor.h
diff --git a/sync/engine/sync_directory_commit_contributor.h b/sync/engine/sync_directory_commit_contributor.h
new file mode 100644
index 0000000000000000000000000000000000000000..527644cdaf2e57d0973ba6b662d71989eea06afe
--- /dev/null
+++ b/sync/engine/sync_directory_commit_contributor.h
@@ -0,0 +1,38 @@
+// Copyright (c) 2013 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_ENGINE_SYNC_DIRECTORY_COMMIT_CONTRIBUTOR_H_
+#define SYNC_ENGINE_SYNC_DIRECTORY_COMMIT_CONTRIBUTOR_H_
+
+#include <map>
+
+#include "sync/internal_api/public/base/model_type.h"
+
+namespace syncer {
+
+class SyncDirectoryCommitContribution;
+
+namespace syncable {
+class Directory;
+}
+
+class SyncDirectoryCommitContributor {

[Nicolas Zea, 2013/10/05 00:06:03]

what is the purpose of this class? It just seems like a wrapper around
BuildContribution.

[rlarocque, 2013/10/07 23:00:05]

Yep, that's exactly what it is.  

It will be more important when I get around to defining the class hierarchy that
supports different kinds of CommitContributors.  The
SyncDirectoryCommitContributor is the first concrete implementation of the
CommitContributor interface (which I haven't bothered to explicitly define yet).
 In the future there will be another implementation, BismarckCommitContributor*
that creates BismarckCommitContribution* objects.  Having different factories is
necessary because the different kinds of contributions will be build using
different kinds of inputs.  For example, the BismarckCommitContribution will not
need a syncable::Directory.  

* Hopefully I'll have though of better names by the time I start writing those
CLs.  

+ public:
+  SyncDirectoryCommitContributor(syncable::Directory* dir, ModelType type);
+  ~SyncDirectoryCommitContributor();
+
+  SyncDirectoryCommitContribution* GetContribution(size_t max_entries);
+
+ private:
+  syncable::Directory* dir_;
+  ModelType type_;
+};
+
+// TODO(rlarocque): Find a better place for this definition.
+typedef std::map<ModelType, SyncDirectoryCommitContributor*>
+    CommitContributorMap;
+
+}  // namespace
+
+#endif  // SYNC_ENGINE_SYNC_DIRECTORY_COMMIT_CONTRIBUTOR_H_