Initial definition of HeliumSyncManager and SyncRegistration objects.

blimp/net/helium/helium_sync_manager.h

Index: blimp/net/helium/helium_sync_manager.h
diff --git a/blimp/net/helium/helium_sync_manager.h b/blimp/net/helium/helium_sync_manager.h
new file mode 100644
index 0000000000000000000000000000000000000000..bb2155acf0b037fda3ec8d87848537856809b2cb
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+++ b/blimp/net/helium/helium_sync_manager.h
@@ -0,0 +1,94 @@
+// Copyright 2016 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 BLIMP_NET_HELIUM_HELIUM_SYNC_MANAGER_H_
+#define BLIMP_NET_HELIUM_HELIUM_SYNC_MANAGER_H_
+
+#include <stdint.h>
+#include <memory>
+
+#include "base/macros.h"
+
+namespace blimp {
+
+class HeliumObject;
+class HeliumTransport;
+class Syncable;
+
+using HeliumObjectId = uint32_t;
+
+// TODO(kmarshall): Define this type.

[scf, 2016/09/28 17:18:00]

put bug#?

+class HeliumTransport {};
+
+class HeliumSyncManager {
+ public:
+  // RAII object for managing the sync control and registration status of a
+  // Syncable. When a Syncable is registered, it should hold onto the resulting
+  // SyncRegistration object.
+  //
+  // When the Syncable is destroyed, it will delete the
+  // SyncRegistration along with it, which will automatically unregister the
+  // object from the SyncManager.
+  //
+  // If a Syncable wishes to halt synchronization for any reason (e.g. a tab
+  // is hidden on the Client), it may call SyncRegistration::Pause(), which will
+  // tell the SyncManager to exclude or include the Syncable in state sync.
+  class SyncRegistration {
+   public:
+    explicit SyncRegistration(HeliumSyncManager* sync_manager,
+                              HeliumObjectId id);
+    ~SyncRegistration();
+
+    // Tells the HeliumSyncManager to pause or unpause synchronization for the
+    // HeliumObject associated with |this|.
+    void Pause(bool paused);
+
+    HeliumObjectId id() { return id_; }
+
+   private:
+    HeliumObjectId id_;
+    HeliumSyncManager* sync_manager_;
+
+    DISALLOW_COPY_AND_ASSIGN(SyncRegistration);
+  };
+
+  virtual ~HeliumSyncManager() {}
+
+  // Returns a concrete implementation of HeliumSyncManager.
+  static std::unique_ptr<HeliumSyncManager> Create(

[perumaal, 2016/09/28 23:31:30]

Consider: A member function such as HeliumTransport::CreateSyncManager() instead
of a static function in a class that is being created?

[Kevin M, 2016/09/29 00:06:28]

I think that would invert the dependency order. SyncManager will depend on
HeliumTransport; HeliumTransport has no idea what SyncManagers are.

Having a static Create() method within an interface when there is only one
actual impl class is not an uncommon pattern.

+      std::unique_ptr<HeliumTransport> transport);

[scf, 2016/09/28 17:18:00]

don't we need an api to get the current HeliumSyncManager? The same way
notifications services has Create()/current() methods?

[Kevin M, 2016/09/29 00:06:28]

Which classes would rely on such an accessor? Is there a reason they couldn't be
passed a pointer to it directly?

[scf, 2016/10/03 17:59:33]

make sense

+
+  // Registers a new Syncable for synchronization. The Sync layer allocates a
+  // unique identifier to the Object (including guaranteeing no clashes between
+  // Client & Engine).
+  // The identifier may be used by other Objects to refer to |object|. E.g. the
+  // tab-list Object might include the Id for each tab’s Object.
+  virtual std::unique_ptr<SyncRegistration> Register(Syncable* syncable) = 0;
+
+  // Registers a Syncable for synchronization with a pre-existing ID.
+  // This form is used when the remote peer has created a new Syncable, to
+  // register our local equivalent.
+  virtual std::unique_ptr<SyncRegistration> RegisterExisting(
+      HeliumObjectId id,
+      Syncable* syncable) = 0;
+
+  // Tells the HeliumSyncManager to pause or unpause synchronization for the
+  // HeliumObject associated with |this|.
+  virtual void Pause(HeliumObjectId id, bool paused) = 0;

[scf, 2016/09/28 17:18:00]

move this under the protected section too?

[Kevin M, 2016/09/29 00:06:28]

OK. In the future we might want to make it public so that Objects can control
the synchronization of other objects, too - e.g. the Tablist knows that a tab
became invisible, so it might want to pause synchronization of said tab. But I
can see the wisdom in keeping the interface narrow for now and widening it only
as needed.

+
+ protected:
+  friend class HeliumSyncManager::SyncRegistration;
+
+  // Indicates that a Syncable is being destroyed, and should no longer be
+  // synchronized.
+  // Because the Syncable is being destroyed, it is assumed that in-flight
+  // messages
+  // about it are now irrelevant, and the Sync layer should tear down underlying
+  // resources, such as the HeliumStream, immediately.
+  virtual void Unregister(HeliumObjectId id) = 0;
+};
+
+}  // namespace blimp
+
+#endif  // BLIMP_NET_HELIUM_HELIUM_SYNC_MANAGER_H_