Create BrowserUserData.

This CL updates Browser to subclass base::SupportsUserData, and adds
BrowserUserData, which includes functionality analogous to
WebStateUserData.

BUG=none

Review-Url: https://codereview.chromium.org/2945713002
Cr-Commit-Position: refs/heads/master@{#481821}

ios/shared/chrome/browser/ui/browser_list/BUILD.gn

@@ -15,6 +15,7 @@ source_set("browser_list") {
     "browser_list_session_service_factory.mm",
     "browser_list_session_service_impl.h",
     "browser_list_session_service_impl.mm",
+    "browser_user_data.h",
     "browser_web_state_list_delegate.h",
     "browser_web_state_list_delegate.mm",
   ]

ios/shared/chrome/browser/ui/browser_list/browser.h

@@ -8,6 +8,7 @@
 #include <memory>
 
 #include "base/macros.h"
+#include "base/supports_user_data.h"
 
 class WebStateList;
 class WebStateListDelegate;
@@ -21,10 +22,10 @@ class ChromeBrowserState;
 
 // Browser holds the state backing a collection of Tabs and the attached
 // UI elements (Tab strip, ...).
-class Browser {
+class Browser : public base::SupportsUserData {
  public:
   explicit Browser(ios::ChromeBrowserState* browser_state);
-  ~Browser();
+  ~Browser() override;
 
   WebStateList& web_state_list() { return *web_state_list_.get(); }
   const WebStateList& web_state_list() const { return *web_state_list_.get(); }

ios/shared/chrome/browser/ui/browser_list/browser_user_data.h

+// Copyright 2017 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_SHARED_CHROME_BROWSER_UI_BROWSER_LIST_BROWSER_USER_DATA_H_
+#define IOS_SHARED_CHROME_BROWSER_UI_BROWSER_LIST_BROWSER_USER_DATA_H_
+
+#include "base/logging.h"
+#include "base/memory/ptr_util.h"
+#include "base/supports_user_data.h"
+#import "ios/shared/chrome/browser/ui/browser_list/browser.h"
+
+// A base class for classes attached to, and scoped to, the lifetime of a
+// Browser. For example:
+//
+// --- in foo.h ---
+// class Foo : public BrowserUserData<Foo> {
+//  public:
+//   ~Foo() override;
+//   // ... more public stuff here ...
+//  private:
+//   explicit Foo(Browser* browser);
+//   friend class BrowserUserData<Foo>;
+//   // ... more private stuff here ...
+// }
+// --- in foo.mm ---
+// DEFINE_BROWSER_USER_DATA_KEY(Foo);
+//
+template <typename T>
+class BrowserUserData : public base::SupportsUserData::Data {
+ public:
+  // Creates an object of type T, and attaches it to the specified Browser.
+  // If an instance is already attached, does nothing.
+  static void CreateForBrowser(Browser* browser) {
+    DCHECK(browser);
+    if (!FromBrowser(browser)) {
+      browser->SetUserData(UserDataKey(), base::WrapUnique(new T(browser)));
+    }
+  }
+
+  // Retrieves the instance of type T that was attached to the specified
+  // Browser (via CreateForBrowser above) and returns it. If no
+  // instance of the type was attached, returns nullptr.
+  static T* FromBrowser(Browser* browser) {
+    return static_cast<T*>(browser->GetUserData(UserDataKey()));
+  }
+  static const T* FromBrowser(const Browser* browser) {
+    return static_cast<const T*>(browser->GetUserData(UserDataKey()));
+  }
+  // Removes the instance attached to the specified WebState.
+  static void RemoveFromBrowser(Browser* browser) {
+    browser->RemoveUserData(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 should be defined in the
+// implementation file of the derived class.
+//
+// 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_BROWSER_USER_DATA_KEY(TYPE) \
+  template <>                              \
+  int BrowserUserData<TYPE>::kLocatorKey = 0
+
+#endif  // IOS_SHARED_CHROME_BROWSER_UI_BROWSER_LIST_BROWSER_USER_DATA_H_