cros: Only preload the lock screen; do not reuse it.

chrome/browser/chromeos/login/ui/preloaded_web_view.h

+// Copyright 2016 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_CHROMEOS_LOGIN_UI_PRELOADED_WEB_VIEW_H_
+#define CHROME_BROWSER_CHROMEOS_LOGIN_UI_PRELOADED_WEB_VIEW_H_
+
+#include <memory>
+
+#include "base/callback_forward.h"
+#include "base/macros.h"
+#include "base/memory/memory_pressure_listener.h"
+#include "base/memory/weak_ptr.h"
+#include "components/keyed_service/core/keyed_service.h"
+#include "content/public/browser/notification_observer.h"
+#include "content/public/browser/notification_registrar.h"
+
+class Profile;
+
+namespace views {
+class WebView;
+}
+
+namespace chromeos {
+
+class IdleDetector;
+
+// Stores and fetches a views::WebView instance that is ulimately owned by the
+// signin profile. This allows for a WebView to be reused over time or
+// preloaded. Use PreloadedWebViewFactory to get an instance of this class.
+class PreloadedWebView : public KeyedService,
+                         public content::NotificationObserver {
+ public:
+  explicit PreloadedWebView(Profile* profile);
+  ~PreloadedWebView() override;
+
+  using PreloadCallback =
+      base::OnceCallback<std::unique_ptr<views::WebView>(Profile*)>;

[xiyuan, 2017/01/06 17:14:25]

nit: move this to the beginning of all declarations (before ctor). 

Style guide prefers the following order: types (including typedef, using, and
nested structs and classes), constants, factory functions, constructors,
assignment operators, destructor, all other methods, data members.

https://google.github.io/styleguide/cppguide.html#Declaration_Order

[jdufault, 2017/01/13 19:46:58]

Done.

+
+  // Executes the given |preload| function when the device is idle.
+  void PreloadOnIdle(PreloadCallback preload);
+
+  // Try to fetch a preloaded instance. Returns nullptr if no instance has been
+  // preloaded. Calling this function will cancel any pending preload.
+  std::unique_ptr<views::WebView> TryTake();
+
+ private:
+  // KeyedSerivce:
+  void Shutdown() override;
+
+  // content::NotificationObserver:
+  void Observe(int type,
+               const content::NotificationSource& source,
+               const content::NotificationDetails& details) override;
+
+  // Called when there is a memory pressure event.
+  void OnMemoryPressure(
+      base::MemoryPressureListener::MemoryPressureLevel level);
+
+  // Runs the preload function. Called only by |idle_detector_|.
+  void RunPreloader();
+
+  // Used to execute the preload function when the user is idle.
+  std::unique_ptr<IdleDetector> idle_detector_;
+  // The preload function. Can only be called once.
+  PreloadCallback preload_function_;
+  // The result of the preload function.
+  std::unique_ptr<views::WebView> preloaded_instance_;
+  // Profile passed into the preload function.
+  Profile* profile_;
+
+  // Used to destroy a preloaded but not used WebView on shutdown.
+  content::NotificationRegistrar registrar_;
+  // Used to destroy a preloaded but not used WebView on low-memory.
+  std::unique_ptr<base::MemoryPressureListener> memory_pressure_listener_;
+
+  base::WeakPtrFactory<PreloadedWebView> weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(PreloadedWebView);
+};
+
+}  // namespace chromeos
+
+#endif  // CHROME_BROWSER_CHROMEOS_LOGIN_UI_PRELOADED_WEB_VIEW_H_