[WIP] - trying out some state machine ideas.

media/base/simple_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_SIMPLE_STATE_MACHINE_H_
+#define MEDIA_BASE_SIMPLE_STATE_MACHINE_H_
+
+#include <tuple>
+#include <type_traits>
+
+#include "base/memory/ref_counted.h"
+#include "media/base/media_export.h"
+#include "media/base/state_machine.h"
+
+namespace media {
+
+// Convenient template for state machines that have all of their state provided
+// during construction.  Provides storage for each of the states, registers
+// them with StateMachine, and provides a getter for them.
+// |StateTypes| should be the raw type for a State subclass.  These must be
+// unique right now, unfortunately, for the provided Get template to work.  We
+// could work around this with an optional ordinal parameter in the getter.
+template <typename... StateTypes>
+class SimpleStateMachine : public StateMachine {
+ public:
+  SimpleStateMachine(scoped_refptr<StateTypes>... states) : states_(states...) {
+    // Register all states with StateMachine.
+    RegisterStates<0>();
+  }
+
+  // Return the state of type |T|.
+  template <typename T>
+  T* Get() const {
+    return FindElement<0, T>();
+  }
+
+ private:
+  typedef std::tuple<scoped_refptr<StateTypes>...> states_t;
+  states_t states_;
+
+  // Specialization to do nothing if we're asking for a state by index
+  // that's more than we have.
+  template <size_t pos>
+  typename std::enable_if<pos >= std::tuple_size<states_t>::value>::type
+  RegisterStates() {}
+
+  // Specialization to register the |pos|-th state, and recurse to the
+  // next one.
+  template <size_t pos>
+      typename std::enable_if <
+      pos<std::tuple_size<states_t>::value>::type RegisterStates() {
+    UsingState(std::get<pos>(states_));
+    RegisterStates<pos + 1>();
+  }
+
+  // Check for type equality.
+  // TODO: there has to be a better way.  rewrite.
+  template <typename T1, typename T2>
+  struct is_same : std::false_type {};
+  template <typename T>
+  struct is_same<T, T> : std::true_type {};
+
+  // When the element at |pos| is a scoped_refptr to |T|, return it as a
+  // raw pointer.  If we wanted to support duplicate types, then we should
+  // also take an ordinal.  If it's zero, then we return, else we recurse
+  // on |pos+1| and |ordinal-1|.
+  template <size_t pos, typename T>
+      typename std::enable_if <
+      pos<std::tuple_size<states_t>::value &&
+              is_same<typename std::tuple_element<pos, states_t>::type,
+                      scoped_refptr<T>>::value,
+          T*>::type
+      FindElement() const {
+    return std::get<pos>(states_).get();
+  }
+
+  // Continue searching for an element of type |scoped_refptr<T>| if the
+  // |pos|-th element doesn't match.
+  template <size_t pos, typename T>
+      typename std::enable_if <
+      pos<std::tuple_size<states_t>::value &&
+              !is_same<typename std::tuple_element<pos, states_t>::type,
+                       scoped_refptr<T>>::value,
+          T*>::type
+      FindElement() const {
+    return FindElement<pos + 1, T>();
+  }
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_SIMPLE_STATE_MACHINE_H_