Observe input metrics from web content area.

chrome/browser/metrics/desktop_engagement_observer.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/metrics/desktop_engagement_observer.h"
+
+#include "chrome/browser/metrics/desktop_engagement_service.h"
+#include "content/public/browser/render_view_host.h"
+
+DEFINE_WEB_CONTENTS_USER_DATA_KEY(DesktopEngagementObserver);
+
+DesktopEngagementObserver::DesktopEngagementObserver(
+    content::WebContents* web_contents,
+    DesktopEngagementService* service)
+    : content::WebContentsObserver(web_contents), service_(service) {
+  RegisterInputEventObserver(web_contents->GetRenderViewHost());
+}
+
+DesktopEngagementObserver::~DesktopEngagementObserver() {}
+
+// static
+DesktopEngagementObserver* DesktopEngagementObserver::CreateForWebContents(
+    content::WebContents* web_contents) {
+  DCHECK(web_contents);
+
+  DesktopEngagementObserver* observer = FromWebContents(web_contents);
+  if (!observer) {
+    observer = new DesktopEngagementObserver(web_contents,
+                                             DesktopEngagementService::Get());
+    web_contents->SetUserData(UserDataKey(), observer);
+  }
+  return observer;
+}
+
+void DesktopEngagementObserver::RegisterInputEventObserver(
+    content::RenderViewHost* host) {
+  if (host != nullptr)
+    host->GetWidget()->AddInputEventObserver(this);
+}
+
+void DesktopEngagementObserver::UnregisterInputEventObserver(
+    content::RenderViewHost* host) {
+  if (host != nullptr)
+    host->GetWidget()->RemoveInputEventObserver(this);
+}
+
+void DesktopEngagementObserver::OnInputEvent(
+    const blink::WebInputEvent& event) {
+  service_->OnUserEvent();
+}
+
+void DesktopEngagementObserver::RenderViewHostChanged(
+    content::RenderViewHost* old_host,
+    content::RenderViewHost* new_host) {
+  UnregisterInputEventObserver(old_host);
+  RegisterInputEventObserver(new_host);
+}