Introduce HostID and de-couple Extensions from "script injection System" [browser side]

extensions/common/consumer.h

+// Copyright 2014 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 EXTENSIONS_COMMON_CONSUMER_H_
+#define EXTENSIONS_COMMON_CONSUMER_H_
+
+#include <string>
+
+#include "base/macros.h"
+
+// IDs of API consumers who own user scripts.
+// For example: an extension, a <webview> created by an extension,
+// a <webview> created by WebUI, ...
+class ConsumerID {
+ public:
+  // The type of a host who contains regular tabs or <webview>s.
+  enum HostType { EXTENSIONS, WEBUI };
+
+  enum InstanceType { TAB, WEBVIEW };
+
+  // Default instance ID of a consumer who owns staticlly-defined user scripts.
+  static const int kDefaultInstanceID;
+
+  ConsumerID();

[Fady Samuel, 2015/01/07 19:19:40]

Is this necessary?

[Xi Han, 2015/01/07 20:28:42]

Yes, the ConsumerID() creates an empty ID for extensions with a empty
extension_id. It is used by
SharedUserScriptMaster(https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/extensions/shared_user_script_master.cc&rcl=1420565673&l=16).

+  explicit ConsumerID(const ConsumerID& other);
+  ConsumerID(HostType host_type,
+             const std::string& host_id,
+             InstanceType instance_type,
+             int instance_id);
+  ~ConsumerID();
+
+  bool operator<(const ConsumerID& id) const;
+
+  HostType host_type() const { return host_type_; }
+  const std::string& host_id() const { return host_id_; }
+  InstanceType instance_type() const { return instance_type_; }
+  int instance_id() const { return instance_id_; }
+
+ private:
+  // Extension, or WebUI, ...
+  HostType host_type_;

[Fady Samuel, 2015/01/07 19:19:40]

Mark as const HostType host_type_.

This tells developers and compilers that this field will not change after
construction.

[Xi Han, 2015/01/07 20:28:42]

I tried the "const" keyword, but a "const" member variable has its own troubles:
the compiler generated assignment operator is disabled. Without an assignment
operator a class won't work with STL. 

+
+  // Similar to extension_id, host_id is a uniquely indentifier for a host,
+  // e.g., an Extension or WebUI.
+  std::string host_id_;

[Fady Samuel, 2015/01/07 19:19:40]

Mark as const.

[Xi Han, 2015/01/07 20:28:42]

Same as HostType.

+
+  // A tab or a <webiew>.
+  InstanceType instance_type_;

[Fady Samuel, 2015/01/07 19:19:40]

Mark as const.

[Xi Han, 2015/01/07 20:28:42]

Same as HostType.

+
+  // For a consumer that owns staticlly defined user scripts, the
+  // |instance_id| is 0;
+  // For a consumer that owns declarative user scripts, the |instance_id|
+  // is assigned by DeclarativeUserScriptManager when the consumer requests a
+  // master object.
+  int instance_id_;

[Fady Samuel, 2015/01/07 19:19:40]

Mark as const.

[Xi Han, 2015/01/07 20:28:42]

Same as HostType.

+};
+
+// Represents an API consumer.
+class Consumer {
+ public:
+  Consumer();
+  virtual ~Consumer();
+
+  DISALLOW_COPY_AND_ASSIGN(Consumer);
+};
+
+#endif  // EXTENSIONS_COMMON_CONSUMER_H_