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[chromium-blink-merge.git] / content / public / browser / web_contents_user_data.h
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1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #ifndef CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_USER_DATA_H_
6 #define CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_USER_DATA_H_
8 #include "base/logging.h"
9 #include "base/supports_user_data.h"
10 #include "content/public/browser/web_contents.h"
12 namespace content {
14 // A base class for classes attached to, and scoped to, the lifetime of a
15 // WebContents. For example:
17 // --- in foo_tab_helper.h ---
18 // class FooTabHelper : public content::WebContentsUserData<FooTabHelper> {
19 // public:
20 // virtual ~FooTabHelper();
21 // // ... more public stuff here ...
22 // private:
23 // explicit FooTabHelper(content::WebContents* contents);
24 // friend class content::WebContentsUserData<FooTabHelper>;
25 // // ... more private stuff here ...
26 // }
27 // --- in foo_tab_helper.cc ---
28 // DEFINE_WEB_CONTENTS_USER_DATA_KEY(FooTabHelper);
30 template <typename T>
31 class WebContentsUserData : public base::SupportsUserData::Data {
32 public:
33 // Creates an object of type T, and attaches it to the specified WebContents.
34 // If an instance is already attached, does nothing.
35 static void CreateForWebContents(WebContents* contents) {
36 DCHECK(contents);
37 if (!FromWebContents(contents))
38 contents->SetUserData(UserDataKey(), new T(contents));
41 // Retrieves the instance of type T that was attached to the specified
42 // WebContents (via CreateForWebContents above) and returns it. If no instance
43 // of the type was attached, returns nullptr.
44 static T* FromWebContents(WebContents* contents) {
45 DCHECK(contents);
46 return static_cast<T*>(contents->GetUserData(UserDataKey()));
48 static const T* FromWebContents(const WebContents* contents) {
49 DCHECK(contents);
50 return static_cast<const T*>(contents->GetUserData(UserDataKey()));
53 protected:
54 static inline void* UserDataKey() {
55 return &kLocatorKey;
58 private:
59 // The user data key.
60 static int kLocatorKey;
63 // The macro to define the locator key. This key must be defined in the .cc file
64 // of the tab helper otherwise different instances for different template types
65 // will be collapsed by the Visual Studio linker.
67 // The "= 0" is surprising, but is required to effect a definition rather than
68 // a declaration. Without it, this would be merely a declaration of a template
69 // specialization. (C++98: 14.7.3.15; C++11: 14.7.3.13)
71 #define DEFINE_WEB_CONTENTS_USER_DATA_KEY(TYPE) \
72 template<> \
73 int content::WebContentsUserData<TYPE>::kLocatorKey = 0
75 } // namespace content
77 #endif // CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_USER_DATA_H_