Add onChanged events to the extension settings API, both from sync and between

chrome/browser/extensions/extension_setting_changes.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_EXTENSION_SETTING_CHANGES_H_
+#define CHROME_BROWSER_EXTENSIONS_EXTENSION_SETTING_CHANGES_H_
+#pragma once
+
+#include "base/memory/ref_counted.h"
+#include "base/values.h"
+
+// A list of changes to extension settings.  Create using the Builder class.
+// The Changes object is thread-safe and efficient to copy, while the Builder
+// object isn't.
+class ExtensionSettingChanges {
+ public:
+  // Non-thread-safe builder for ExtensionSettingChanges.
+  class Builder {
+   public:
+    Builder() {}
+
+    // Appends a change for a setting.
+    void AppendChange(
+        const std::string& key,
+        // Ownership taken.  May be NULL to imply there is no old value.
+        Value* old_value,
+        // Ownership taken.  May be NULL to imply there is no new value.
+        Value* new_value);
+
+    // Creates an ExtensionSettingChanges object.  The Builder should not be
+    // used after calling this.
+    ExtensionSettingChanges Build();
+
+   private:
+    ListValue changes_;
+
+    DISALLOW_COPY_AND_ASSIGN(Builder);
+  };
+
+  ~ExtensionSettingChanges();
+
+  // Gets the JSON serialized form of the changes, for example:
+  // [ { "key": "foo", "oldValue": "bar", "newValue": "baz" } ]
+  std::string ToJson() const;
+
+ private:
+  // Create using the Builder class.  |changes| will be cleared.
+  explicit ExtensionSettingChanges(ListValue* changes);
+
+  // Ref-counted inner class, a wrapper over a non-thread-safe string.
+  class Inner : public base::RefCountedThreadSafe<Inner> {
+   public:
+    Inner() {}
+
+    // swap() this with the changes from the Builder after construction.
+    ListValue changes_;
+
+   private:
+    virtual ~Inner() {}
+    friend class base::RefCountedThreadSafe<Inner>;
+  };
+
+  scoped_refptr<Inner> inner_;
+};
+
+#endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_SETTING_CHANGES_H_