[WIP] - trying out some state machine ideas.

media/base/state_machine.h

+// 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_MACHINE_H_
+#define MEDIA_BASE_STATE_MACHINE_H_
+
+#include <vector>
+
+#include "media/base/media_export.h"
+#include "media/base/state.h"
+
+namespace media {
+
+// Stateless implementation of a state machine.  The subclasses must handle
+// being given the State objects, since they'll probably want concrete
+// subclasses anyway.  Subclasses should call UsingState to record a ref to the
+// state, and automatically get updates for it.
+class MEDIA_EXPORT StateMachine : public State::Client {
+ public:
+  ~StateMachine() override;
+
+  // Subclasses will find out about State objects (or concrete subclasses of
+  // state) via methods on the StateMachine subclass / constructor / or
+  // whatever they like.
+
+  // Thing that owns / runs the state machine (e.g., StateMachineDriver).
+  class Owner {
+   public:
+    // Request that our owner run us, but not re-entrantly.  It is unclear
+    // if it may run before returning from this or not, assuming that |us|
+    // is not running currently.  It must not run immediately if this is
+    // called recursively from |us->Run()|.
+    // Probably this is never allowed to run before returning.
+    virtual void RequestRun(StateMachine* us) = 0;
+  };
+
+  // Set the owner that we'll use.
+  void SetOwner(Owner* owner);
+
+  // Run the state machine once.
+  virtual void Run() const = 0;
+
+  // State::Client
+  // If any state notifies us that it has changed.  We'll request that our
+  // owner runs us via owner_->RequestRun(this).
+  void OnStateChanged() override;
+
+ protected:
+  StateMachine();
+
+  // Adds |state| to the set that we're using.  This will cause us to register
+  // as a client with them, and probably keep a ref (which the caller likely
+  // also has too, to the subclass type).  By registering state, we will
+  // automatically request a run when it changes.  Alternatively, if it makes
+  // things cleaner, the state could keep a list of (machine, driver) pairs
+  // that it notifies, but that seems unlikely to improve it.
+  void UsingState(scoped_refptr<State> state);
+
+ private:
+  Owner* owner_;
+  std::vector<scoped_refptr<State>> states_;
+
+  DISALLOW_COPY_AND_ASSIGN(StateMachine);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_STATE_MACHINE_H_