cros: Enable WebUILoginView reuse.

chrome/browser/chromeos/login/ui/shared_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_SHARED_WEB_VIEW_H_
+#define CHROME_BROWSER_CHROMEOS_LOGIN_UI_SHARED_WEB_VIEW_H_
+
+#include <memory>
+
+#include "base/macros.h"
+#include "base/memory/memory_pressure_listener.h"
+#include "components/keyed_service/core/keyed_service.h"
+#include "content/public/browser/notification_observer.h"
+#include "content/public/browser/notification_registrar.h"
+#include "url/gurl.h"
+
+class Profile;
+
+namespace views {
+class WebView;
+}
+
+namespace chromeos {
+
+class SharedWebView : public KeyedService,

[xiyuan, 2016/12/09 22:37:30]

nit: Document the class.

[jdufault, 2016/12/12 18:23:09]

Done.

+                      public content::NotificationObserver {
+ public:
+  explicit SharedWebView(Profile* profile);
+  ~SharedWebView() override;
+  void Shutdown() override;
+
+  // Stores a webview instance inside of |out_web_view|. |url| is the initial
+  // URL that the webview stores (note: this function does not perform the
+  // navigation). This returns true if the webview was reused, false if it
+  // was freshly created.
+  bool Get(const GURL& url, views::WebView** out_web_view);
+
+ private:
+  friend class SharedWebViewUsageHandle;
+
+  // Mark the webview as in-use.
+  void MarkUsed();
+  // Mark the webview as no-longer in use.

[xiyuan, 2016/12/09 22:37:30]

nit: insert an empty line before

[jdufault, 2016/12/12 18:23:09]

Removed

+  void MarkUnused();
+
+  // 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);
+
+  content::NotificationRegistrar registrar_;
+
+  // Profile used for creating the views::WebView instance.
+  Profile* profile_;

[xiyuan, 2016/12/09 22:37:30]

nit: Profile* const since |profile_| is not going to change during the lifetime
of the class.

[jdufault, 2016/12/12 18:23:09]

Done.

+
+  // Cached URL that the |web_view_| instance should point to used for
+  // validation.
+  GURL web_view_url_;
+
+  // Number of SharedWebViewUsageHandle instances associated with this type. We
+  // do not destroy the WebView on low-memory if this is >= 1.
+  int handle_count_ = 0;
+
+  // Shared web-view instance; owned by this class.
+  std::unique_ptr<views::WebView> web_view_;
+
+  std::unique_ptr<base::MemoryPressureListener> memory_pressure_listener_;
+
+  DISALLOW_COPY_AND_ASSIGN(SharedWebView);
+};
+
+// Ensures that the WebView instance stored inside of SharedWebView will
+// not be destroyed during the lifetime of the SharedWebViewUsageHandle
+// instance.
+class SharedWebViewUsageHandle {

[xiyuan, 2016/12/09 22:37:30]

This looks like to be used as a ref count for |web_view_| in SharedWebView. I
wonder if we could leverage existing infra to achieve this.

e.g.

Make SharedWebViewUsageHandle a RefCounted and own the web_view_. SharedWebView
can hold a ref to keep it alive. And we can use RefCounted::HasOneRef to tell
whether there is only one ref on it.

class SharedWebViewUsageHandle : RefCounted<SharedWebViewUsageHandle> {
 ...
 private:
  std::unique_ptr<views::WebView> web_view_;
 ...
};

class SharedWebView : ... {
 public:
  ...
  bool Get(const GURL& url,
           scoped_refptr<SharedWebViewUsageHandle>* out_handle);
 private:
  ...
  scoped_refptr<SharedWebViewUsageHandle> web_view_handle_;
  ...
};

[jdufault, 2016/12/12 18:23:09]

Done.

+ public:
+  explicit SharedWebViewUsageHandle(SharedWebView* shared_web_view);
+  ~SharedWebViewUsageHandle();
+
+ private:
+  SharedWebView* shared_web_view_;
+
+  DISALLOW_COPY_AND_ASSIGN(SharedWebViewUsageHandle);
+};
+
+}  // namespace chromeos
+
+#endif  // CHROME_BROWSER_CHROMEOS_LOGIN_UI_SHARED_WEB_VIEW_H_