[Sensors] Align sensor reading updates and 'onchange' notification with rAF.

third_party/WebKit/Source/modules/sensor/SensorUpdateNotifier.cpp

+// 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 "modules/sensor/SensorUpdateNotifier.h"
+
+#include "device/generic_sensor/public/interfaces/sensor.mojom-blink.h"
+#include "platform/Timer.h"
+#include "wtf/CurrentTime.h"
+
+namespace blink {
+
+SensorUpdateNotifier::SensorUpdateNotifier(
+    double frequency,
+    std::unique_ptr<Function<void()>> sensorUpdatingFunction)
+    : m_period(frequency),
+      m_sensorUpdatingFunction(std::move(sensorUpdatingFunction)),
+      m_lastTimestamp(0.0) {
+  DCHECK_GT(frequency, 0.0);
+  DCHECK(m_sensorUpdatingFunction);
+}
+
+// Implementation for sensors with 'onchange' reporting mode.
+// Uses a "single-shot" timer to make sure notification is not invoked
+// more frequently than expected.
+class SensorUpdateNotifierOnChange : public SensorUpdateNotifier {
+ public:
+  SensorUpdateNotifierOnChange(
+      double period,
+      std::unique_ptr<Function<void()>> sensorUpdatingFunction)
+      : SensorUpdateNotifier(period, std::move(sensorUpdatingFunction)),
+        m_timer(this, &SensorUpdateNotifierOnChange::onTimer) {}
+
+ private:
+  // SensorUpdateNotifier overrides.
+  void onSensorReadingChanged(double timestamp) override;
+  void cancelPendingNotifications() override;
+
+  void onTimer(TimerBase*);
+
+  Timer<SensorUpdateNotifierOnChange> m_timer;
+};
+
+void SensorUpdateNotifierOnChange::onSensorReadingChanged(double timestamp) {
+  if (m_timer.isActive())
+    return;  // Skipping changes if update notification was already sheduled.
+
+  double elapsedTime = timestamp - m_lastTimestamp;
+  const double kMinInterval =
+      1 / device::mojom::blink::SensorConfiguration::kMaxAllowedFrequency;
+  double waitingTime = m_period - elapsedTime;
+
+  // Negative or zero 'waitingTime' means that polling period has elapsed.
+  // We also avoid scheduling if the elapsed time is slightly behind the
+  // polling period. This is done to reduce using of timer.
+  if (waitingTime < kMinInterval) {
+    m_lastTimestamp = timestamp;
+    (*m_sensorUpdatingFunction)();
+  } else {
+    m_timer.startOneShot(waitingTime, BLINK_FROM_HERE);

[Reilly Grant (use Gerrit), 2016/12/08 00:56:57]

Can we also use RAF here to deliver events no more frequently than once per
frame and avoid the timer?

+  }
+}
+
+void SensorUpdateNotifierOnChange::cancelPendingNotifications() {
+  m_timer.stop();
+}
+
+void SensorUpdateNotifierOnChange::onTimer(TimerBase*) {
+  m_lastTimestamp = WTF::monotonicallyIncreasingTime();
+  (*m_sensorUpdatingFunction)();
+}
+
+// Implementation for sensors with 'continuous' reporting mode.
+class SensorUpdateNotifierContinuous : public SensorUpdateNotifier {
+ public:
+  SensorUpdateNotifierContinuous(
+      double period,
+      std::unique_ptr<Function<void()>> sensorUpdatingFunction)
+      : SensorUpdateNotifier(period, std::move(sensorUpdatingFunction)) {}
+
+ private:
+  // SensorUpdateNotifier overrides.
+  void onSensorReadingChanged(double timestamp) override {
+    if (timestamp - m_lastTimestamp >= m_period) {
+      m_lastTimestamp = timestamp;
+      (*m_sensorUpdatingFunction)();
+    }
+  }
+
+  void cancelPendingNotifications() override {}
+};
+
+// static
+std::unique_ptr<SensorUpdateNotifier> SensorUpdateNotifier::create(
+    double frequency,
+    std::unique_ptr<Function<void()>> sensorUpdatingFunction,
+    device::mojom::blink::ReportingMode mode) {
+  double period = 1 / frequency;
+  if (mode == device::mojom::blink::ReportingMode::CONTINUOUS) {
+    return std::unique_ptr<SensorUpdateNotifier>(
+        new SensorUpdateNotifierContinuous(period,
+                                           std::move(sensorUpdatingFunction)));
+  }
+  return std::unique_ptr<SensorUpdateNotifier>(new SensorUpdateNotifierOnChange(
+      period, std::move(sensorUpdatingFunction)));
+}
+
+}  // namespace blink