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

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

+// 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.
+
+#include "chrome/browser/chromeos/login/ui/preloaded_web_view.h"
+
+#include "base/callback_helpers.h"
+#include "base/memory/ptr_util.h"
+#include "chrome/browser/chrome_notification_types.h"
+#include "chrome/browser/chromeos/idle_detector.h"
+#include "chrome/browser/profiles/profile.h"
+#include "content/public/browser/notification_service.h"
+#include "content/public/browser/render_view_host.h"
+#include "content/public/browser/render_widget_host.h"
+#include "content/public/browser/web_contents.h"
+#include "ui/views/controls/webview/webview.h"
+
+namespace chromeos {
+
+namespace {
+// Duration of user inactivity before running the preload function.
+constexpr int kIdleSecondsBeforePreloadingLockScreen = 8;
+}
+
+PreloadedWebView::PreloadedWebView(Profile* profile)
+    : profile_(profile), weak_factory_(this) {
+  registrar_.Add(this, chrome::NOTIFICATION_APP_TERMINATING,
+                 content::NotificationService::AllSources());
+  memory_pressure_listener_ =
+      base::MakeUnique<base::MemoryPressureListener>(base::Bind(
+          &PreloadedWebView::OnMemoryPressure, weak_factory_.GetWeakPtr()));
+}
+
+PreloadedWebView::~PreloadedWebView() {}
+
+void PreloadedWebView::PreloadOnIdle(PreloadCallback preload) {
+  preload_function_ = std::move(preload);
+  idle_detector_ = base::MakeUnique<chromeos::IdleDetector>(
+      base::Bind(&PreloadedWebView::RunPreloader, weak_factory_.GetWeakPtr()));
+  idle_detector_->Start(
+      base::TimeDelta::FromSeconds(kIdleSecondsBeforePreloadingLockScreen));
+}
+
+std::unique_ptr<views::WebView> PreloadedWebView::TryTake() {
+  idle_detector_.reset();
+  return std::move(preloaded_instance_);

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

We should not use std::move on return statement unless the var type and return
type do not exactly match.

https://www.chromium.org/rvalue-references#TOC-11.-Don-t-return-references.-D....

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

Unfortunately to elide the std::move the data needs to live on the stack. So for
this case, without the std::move the return invokes the copy ctor which is
deleted inside of unique_ptr.

[xiyuan, 2017/01/13 20:14:18]

Acknowledged.

+}
+
+void PreloadedWebView::Shutdown() {
+  preloaded_instance_.reset();
+}
+
+void PreloadedWebView::Observe(int type,
+                               const content::NotificationSource& source,
+                               const content::NotificationDetails& details) {
+  DCHECK_EQ(chrome::NOTIFICATION_APP_TERMINATING, type);
+  preloaded_instance_.reset();
+}
+
+void PreloadedWebView::OnMemoryPressure(
+    base::MemoryPressureListener::MemoryPressureLevel level) {
+  switch (level) {
+    case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE:
+      break;
+    case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE:
+    case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL:
+      preloaded_instance_.reset();
+      break;
+  }
+}
+
+void PreloadedWebView::RunPreloader() {
+  idle_detector_.reset();
+  preloaded_instance_ = std::move(preload_function_).Run(profile_);
+}
+
+}  // namespace chromeos