[WIP] - trying out some state machine ideas.

media/base/state.h

Index: media/base/state.h
diff --git a/media/base/state.h b/media/base/state.h
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+++ b/media/base/state.h
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+// 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 MEDIA_BASE_STATE_H_
+#define MEDIA_BASE_STATE_H_
+
+#include <unordered_set>
+
+#include "base/memory/ref_counted.h"
+#include "media/base/media_export.h"
+
+namespace media {
+
+// Subclass this to include actual state + getters and setters.  Setters will
+// call StateChanged() as needed.
+// TODO(liberato): StatePartition?  StateGroup?
+class MEDIA_EXPORT State : public base::RefCountedThreadSafe<State> {
+ public:
+  // Client (state machine, probably) that's notified about state
+  // changes so that it can schedule state machine runs.
+  class Client {
+   public:
+    virtual ~Client() {}
+
+    // Notify the client that some property has changed.
+    virtual void OnStateChanged() = 0;
+  };
+
+ public:
+  // Add a client to us.  Must be valid until we're destroyed, or until
+  // it's unregistered.
+  void RegisterClient(Client* client);
+
+  // Unregister a previously-registered client.
+  void UnregisterClient(Client* client);
+
+ protected:
+  State();
+  virtual ~State();
+
+  friend class base::RefCountedThreadSafe<State>;
+
+  // Called by subclass to notify that a state has changed.  This can
+  // happen in response to a setter, or if the state changes by itself
+  // (such as state subclasses that provide access to external state).
+  void StateChanged();
+
+ private:
+  std::unordered_set<Client*> clients_;
+
+  DISALLOW_COPY_AND_ASSIGN(State);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_STATE_H_