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diff --git a/components/update_client/action.h b/components/update_client/action.h
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+++ b/components/update_client/action.h
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+// Copyright 2015 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 COMPONENTS_UPDATE_CLIENT_ACTION_H_
+#define COMPONENTS_UPDATE_CLIENT_ACTION_H_
+
+#include <string>
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/threading/thread_checker.h"
+#include "components/update_client/crx_update_item.h"
+#include "components/update_client/update_client.h"
+
+namespace update_client {
+
+class Configurator;
+struct CrxUpdateItem;
+struct UpdateContext;
+
+// Any update can be broken down as a sequence of discrete steps, such as
+// checking for updates, downloading patches, updating, and waiting between
+// successive updates. An action is the smallest unit of work executed by
+// the update engine.
+//
+// Defines an abstract interface for a unit of work, executed by the
+// update engine as part of an update.
+class Action {
+ public:
+  enum class ErrorCategory {
+    kErrorNone = 0,
+    kNetworkError,
+    kUnpackError,
+    kInstallError,
+    kServiceError,  // Runtime errors which occur in the service itself.
+  };
+
+  enum class ServiceError {
+    ERROR_WAIT = 1,
+  };
+
+  using Callback = base::Callback<void(int error)>;
+  virtual ~Action() {}
+
+  // Runs the code encapsulated by the action. When an action completes, it can
+  // chain up and transfer the execution flow to another action or it can
+  // invoke the |callback| when this function has completed and there is nothing
+  // else to do.
+  virtual void Run(UpdateContext* update_context, Callback callback) = 0;
+};
+
+// Provides a reusable implementation of common functions needed by actions.
+class ActionImpl {
+ protected:
+  ActionImpl();
+  ~ActionImpl();
+
+  void Run(UpdateContext* update_context, Action::Callback callback);
+
+  // Changes the current state of the |item| to the new state |to|.
+  void ChangeItemState(CrxUpdateItem* item, CrxUpdateItem::State to);
+
+  // Changes the state of all items in |update_context_|. Returns the count
+  // of items affected by the call.
+  size_t ChangeAllItemsState(CrxUpdateItem::State from,
+                             CrxUpdateItem::State to);
+
+  // Returns the item associated with the component |id| or nullptr in case
+  // of errors.
+  CrxUpdateItem* FindUpdateItemById(const std::string& id) const;
+
+  void NotifyObservers(UpdateClient::Observer::Events event,
+                       const std::string& id);
+
+  // Updates the CRX at the front of the CRX queue in this update context.
+  void UpdateCrx();
+
+  // Completes updating the CRX at the front of the queue, and initiates
+  // the update for the next CRX in the queue, if the queue is not empty.
+  void UpdateCrxComplete(CrxUpdateItem* item);
+
+  // Called when the updates for all CRXs have finished and the execution
+  // flow must return back to the update engine.
+  void UpdateComplete(int error);
+
+  base::ThreadChecker thread_checker_;
+
+  UpdateContext* update_context_;  // Not owned by this class.
+  Action::Callback callback_;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ActionImpl);
+};
+
+}  // namespace update_client
+
+#endif  // COMPONENTS_UPDATE_CLIENT_ACTION_H_