Clean up threading model of external extension providers

chrome/browser/extensions/external_pref_extension_loader.h

+// Copyright (c) 2011 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 CHROME_BROWSER_EXTENSIONS_EXTERNAL_PREF_EXTENSION_LOADER_H_
+#define CHROME_BROWSER_EXTENSIONS_EXTERNAL_PREF_EXTENSION_LOADER_H_
+#pragma once
+
+#include "chrome/browser/extensions/external_extension_loader.h"
+
+#include <string>
+
+#include "base/scoped_ptr.h"
+#include "base/values.h"
+
+// A specialization of the ExternalExtensionLoader that uses a json file to
+// look up which external extensions are registered.
+// Instances of this class are expected to be created and destroyed on the UI
+// thread and they are expecting public method calls from the UI thread.
+class ExternalPrefExtensionLoader : public ExternalExtensionLoader {
+ public:
+  ExternalPrefExtensionLoader();
+
+ protected:
+  virtual void StartLoading();
+
+ private:
+  friend class base::RefCountedThreadSafe<ExternalExtensionLoader>;
+
+  virtual ~ExternalPrefExtensionLoader() {}
+
+  void LoadOnFileThread();
+
+  DISALLOW_COPY_AND_ASSIGN(ExternalPrefExtensionLoader);
+};
+
+// A simplified version of ExternalPrefExtensionLoader that loads the dictionary
+// from json data specified in a string.
+class ExternalTestingExtensionLoader : public ExternalExtensionLoader {
+ public:
+  explicit ExternalTestingExtensionLoader(const std::string& json_data);
+
+ protected:
+  virtual void StartLoading();
+
+ private:
+  friend class base::RefCountedThreadSafe<ExternalExtensionLoader>;
+
+  virtual ~ExternalTestingExtensionLoader() {}
+
+  scoped_ptr<DictionaryValue> testing_prefs_;
+
+  DISALLOW_COPY_AND_ASSIGN(ExternalTestingExtensionLoader);
+};
+
+#endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_PREF_EXTENSION_LOADER_H_