Optimize for TouchStart responsiveness

components/scheduler/renderer/user_model.h

+// Copyright 2015 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 COMPONENTS_SCHEDULER_RENDERER_USER_MODEL_H_
+#define COMPONENTS_SCHEDULER_RENDERER_USER_MODEL_H_
+
+#include "base/macros.h"
+#include "base/trace_event/trace_event.h"
+#include "base/trace_event/trace_event_argument.h"
+#include "components/scheduler/renderer/renderer_scheduler.h"
+#include "components/scheduler/scheduler_export.h"
+#include "third_party/WebKit/public/web/WebInputEvent.h"
+
+namespace scheduler {
+
+class SCHEDULER_EXPORT UserModel {
+ public:
+  UserModel();
+  ~UserModel();
+
+  // Tells us that the system started processing an input event. Must be paired
+  // with a call to DidFinishProcessingInputEvent.
+  void DidStartProcessingInputEvent(blink::WebInputEvent::Type type,
+                                    const base::TimeTicks now);
+
+  // Tells us that the system finished processing an input event.
+  void DidFinishProcessingInputEvent(const base::TimeTicks now);
+
+  // Returns the amount of time left in the current input escalated priority
+  // policy.
+  base::TimeDelta TimeLeftInInputEscalatedPolicy(base::TimeTicks now) const;

[Sami, 2015/09/04 10:58:21]

It feels like the user model shouldn't know anything about priorities or
policies. How about calling this TimeLeftInUserGesture, or
ExpectedUserGestureDuration?

[alex clarke (OOO till 29th), 2015/09/07 11:09:27]

Done.

+
+  // Tries to guess if a Touchstart is expected soon. Currently this is
+  // very simple, but one day I hope to do something more sophisticated here.
+  // The prediction may change after |prediction_valid_duration| has elapsed.
+  bool IsTouchStartExpectedSoon(

[Sami, 2015/09/04 10:58:21]

Similarly the reason why TouchStart is special is a little outside this class's
jurisdiction. Maybe this should just be IsUserGestureExpectedSoon() and in
addition to touchstart we track GestureScrollBegin and GesturePinchBegin (i.e.,
all the continuous gestures). |last_touchstart_time_| could then be
|last_gesture_start_time_|. WDYT?

[alex clarke (OOO till 29th), 2015/09/07 11:09:27]

Done.

+      RendererScheduler::UseCase use_case,
+      const base::TimeTicks now,
+      base::TimeDelta* prediction_valid_duration) const;
+
+  void AsValueInto(base::trace_event::TracedValue* state) const;
+
+  // The time we should stay in a priority-escalated mode after an input event.
+  static const int kPriorityEscalationAfterInputMillis = 100;
+
+  // TODO(alexclarke): Get a real number on actual data.
+  static const int kMinimumTypicalScrollDurationMillis = 500;
+
+  // We consider further input events to be likely if the user has interacted
+  // with the device in the past two seconds.
+  // TODO(alexclarke): Get a real number based on actual data.
+  static const int kExpectSubsequentInputMillis = 2000;
+
+ private:
+  int pending_input_event_count_;
+  base::TimeTicks last_input_signal_time_;
+  base::TimeTicks last_touchstart_time_;
+
+  DISALLOW_COPY_AND_ASSIGN(UserModel);
+};
+
+}  // namespace scheduler
+
+#endif  // COMPONENTS_SCHEDULER_RENDERER_USER_MODEL_H_