Add UserInputTracker, which keeps track of recent user input events.

chrome/browser/page_load_metrics/user_input_tracker.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/page_load_metrics/user_input_tracker.h"
+
+#include <algorithm>
+
+#include "third_party/WebKit/public/platform/WebInputEvent.h"
+
+namespace page_load_metrics {
+
+namespace {
+
+// Blink's UserGestureIndicator allows events to be associated with gestures
+// that are up to 1 second old, based on guidance in the HTML spec:
+// https://html.spec.whatwg.org/multipage/interaction.html#triggered-by-user-activation.
+const int kMaxEventAgeSeconds = 1;
+
+bool IsInterestingInputEvent(const blink::WebInputEvent& event) {
+  // Ignore synthesized auto repeat events.
+  if (event.modifiers & blink::WebInputEvent::IsAutoRepeat)
+    return false;
+
+  switch (event.type) {
+    case blink::WebInputEvent::MouseDown:
+    case blink::WebInputEvent::MouseUp:
+    case blink::WebInputEvent::RawKeyDown:
+    case blink::WebInputEvent::KeyDown:
+    case blink::WebInputEvent::Char:
+    case blink::WebInputEvent::TouchStart:
+    case blink::WebInputEvent::TouchEnd:
+      return true;
+    default:
+      return false;
+  }
+}
+
+}  // namespace
+
+UserInputTracker::UserInputTracker() {}
+UserInputTracker::~UserInputTracker() {}
+
+// static
+base::TimeTicks UserInputTracker::GetEventTime(
+    const blink::WebInputEvent& event) {
+  return base::TimeTicks() +

[Charlie Harrison, 2016/12/02 02:58:44]

Can you document why this conversion is correct?

[Bryan McQuade, 2016/12/02 20:43:01]

Done

+         base::TimeDelta::FromSecondsD(event.timeStampSeconds);
+}
+
+void UserInputTracker::OnInputEvent(const blink::WebInputEvent& event) {
+  RemoveOldInputEvents();
+
+  if (!IsInterestingInputEvent(event))
+    return;
+
+  // TODO(bmcquade): ideally we'd limit tracking to events generated by a user
+  // action, as opposed to those generated from JavaScript. The JS API isTrusted
+  // can be used to distinguish these cases. isTrusted isn't yet a property of
+  // WebInputEvent. We should consider adding it.
+

[Charlie Harrison, 2016/12/02 02:58:44]

Should we trim event_times_ here too, to avoid it growing too big?

[Bryan McQuade, 2016/12/02 20:43:01]

Yep - moved to the rate limiting model we talked about, with a DCHECK to verify
we don't exceed the allowed capacity

+  event_times_.insert(GetEventTime(event));
+}
+
+base::TimeTicks UserInputTracker::FindMostRecentUserInputEventBefore(
+    base::TimeTicks time) {
+  RemoveOldInputEvents();
+
+  if (event_times_.empty())
+    return base::TimeTicks();
+
+  // lower_bound finds the first element >= |time|.
+  auto it = event_times_.lower_bound(time);
+
+  // If all times are after the requested time, then we don't have a time to
+  // return.
+  if (it == event_times_.begin())
+    return base::TimeTicks();
+
+  // |it| points to the first event >= the specified time, so decrement once to
+  // find the greatest event before the specified time.
+  --it;
+  base::TimeTicks candidate = *it;
+  DCHECK(candidate < time);
+
+  // If the most recent event is too old, then don't return it.
+  if (candidate < time - base::TimeDelta::FromSeconds(kMaxEventAgeSeconds))
+    return base::TimeTicks();
+
+  return candidate;
+}
+
+void UserInputTracker::ConsumeUserInputEventsUpTo(base::TimeTicks time) {
+  RemoveInputEventsUpTo(std::max(time, GetOldestAllowedEventTime()));
+}
+
+void UserInputTracker::RemoveOldInputEvents() {
+  RemoveInputEventsUpTo(GetOldestAllowedEventTime());
+}
+
+void UserInputTracker::RemoveInputEventsUpTo(base::TimeTicks cutoff) {
+  event_times_.erase(event_times_.begin(), event_times_.upper_bound(cutoff));
+}
+
+// static
+base::TimeTicks UserInputTracker::GetOldestAllowedEventTime() {

[Charlie Harrison, 2016/12/02 02:58:44]

Why don't we limit to 1s old instead of 2s old events?

[Bryan McQuade, 2016/12/02 20:43:01]

The idea here is that if someone calls FindMostRecentUserInputEventBefore with a
time slightly in the past, we'd like to be able to provide data for times at
least kMaxEventAgeSeconds before any reasonable time they might provide. So we
use a time that's 2x the period, effectively allowing times passed to
FindMostRecentUserInputEventBefore up to 1 second in the past.

+  // Allow for up to 2x the oldest time. This allows consumers to continue to
+  // find events for timestamps up to 1 second in the past.
+  return base::TimeTicks::Now() -
+         base::TimeDelta::FromSeconds(kMaxEventAgeSeconds * 2);
+}
+
+}  // namespace page_load_metrics