content/public/browser/web_contents_user_data.h

   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.
   4
   5 #ifndef CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_USER_DATA_H_
   6 #define CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_USER_DATA_H_
   7
   8 #include "base/logging.h"
   9 #include "base/supports_user_data.h"
  10 #include "content/public/browser/web_contents.h"
  11
  12 namespace content {
  13
  14 // A base class for classes attached to, and scoped to, the lifetime of a
  15 // WebContents. For example:
  16 //
  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);
  29 //
  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));
  39   }
  40
  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()));
  47   }
  48   static const T* FromWebContents(const WebContents* contents) {
  49     DCHECK(contents);
  50     return static_cast<const T*>(contents->GetUserData(UserDataKey()));
  51   }
  52
  53  protected:
  54   static inline void* UserDataKey() {
  55     return &kLocatorKey;
  56   }
  57
  58  private:
  59   // The user data key.
  60   static int kLocatorKey;
  61 };
  62
  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.
  66 //
  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)
  70 //
  71 #define DEFINE_WEB_CONTENTS_USER_DATA_KEY(TYPE) \
  72 template<>                                      \
  73 int content::WebContentsUserData<TYPE>::kLocatorKey = 0
  74
  75 }  // namespace content
  76
  77 #endif  // CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_USER_DATA_H_