Introduce the WebState iOS provider interface.

ios/public/provider/web/web_state_user_data.h

+// Copyright 2013 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 IOS_PUBLIC_PROVIDER_WEB_WEB_STATE_USER_DATA_H_
+#define IOS_PUBLIC_PROVIDER_WEB_WEB_STATE_USER_DATA_H_
+
+#include "ios/public/consumer/base/supports_user_data.h"
+#include "ios/public/provider/web/web_state.h"
+
+namespace ios {
+
+// A base class for classes attached to, and scoped to, the lifetime of a
+// WebState. For example:
+//
+// --- in foo_tab_helper.h ---

[stuartmorgan, 2013/07/02 15:24:26]

Let's avoid tab, per the previous comment.

[blundell, 2013/07/02 16:16:37]

Done.

+// class FooTabHelper : public content::WebStateUserData<FooTabHelper> {
+//  public:
+//   virtual ~FooTabHelper();
+//   // ... more public stuff here ...
+//  private:
+//   explicit FooTabHelper(content::WebState* web_state);
+//   friend class content::WebStateUserData<FooTabHelper>;
+//   // ... more private stuff here ...
+// }
+// --- in foo_tab_helper.cc ---
+// DEFINE_WEB_CONTENTS_USER_DATA_KEY(FooTabHelper);
+//
+template <typename T>
+class WebStateUserData : public ios::SupportsUserData::Data {
+ public:
+  // Creates an object of type T, and attaches it to the specified WebState.
+  // If an instance is already attached, does nothing.
+  static void CreateForWebState(WebState* web_state) {
+    if (!FromWebState(web_state))
+      web_state->SetUserData(UserDataKey(), new T(web_state));
+  }
+
+  // Retrieves the instance of type T that was attached to the specified
+  // WebState (via CreateForWebState above) and returns it. If no instance
+  // of the type was attached, returns NULL.
+  static T* FromWebState(WebState* web_state) {
+    return static_cast<T*>(web_state->GetUserData(UserDataKey()));
+  }
+  static const T* FromWebState(const WebState* web_state) {
+    return static_cast<const T*>(web_state->GetUserData(UserDataKey()));
+  }
+
+ protected:
+  static inline void* UserDataKey() {
+    return &kLocatorKey;
+  }
+
+ private:
+  // The user data key.
+  static int kLocatorKey;
+};
+
+// The macro to define the locator key. This key must be defined in the .cc file
+// of the tab helper otherwise different instances for different template types

[stuartmorgan, 2013/07/02 15:24:26]

Fix "tab helper"

[blundell, 2013/07/02 16:16:37]

Done.

+// will be collapsed by the Visual Studio linker.

[stuartmorgan, 2013/07/02 15:24:26]

Pretty sure this isn't a concern for iOS :) Just say "This key should be defined
in the .cc file of each WebStateUserData" or similar.

[blundell, 2013/07/02 16:16:37]

Done.

+//
+// The "= 0" is surprising, but is required to effect a definition rather than
+// a declaration. Without it, this would be merely a declaration of a template
+// specialization. (C++98: 14.7.3.15; C++11: 14.7.3.13)
+//
+#define DEFINE_WEB_STATE_USER_DATA_KEY(TYPE) \
+template<>                                      \
+int ios::WebStateUserData<TYPE>::kLocatorKey = 0
+
+}  // namespace ios
+
+#endif  // IOS_PUBLIC_PROVIDER_WEB_WEB_STATE_USER_DATA_H_