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-rw-r--r--layout/generic/ScrollVelocityQueue.cpp93
1 files changed, 93 insertions, 0 deletions
diff --git a/layout/generic/ScrollVelocityQueue.cpp b/layout/generic/ScrollVelocityQueue.cpp
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+++ b/layout/generic/ScrollVelocityQueue.cpp
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScrollVelocityQueue.h"
+
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "nsPresContext.h"
+#include "nsRefreshDriver.h"
+
+namespace mozilla {
+namespace layout {
+
+void ScrollVelocityQueue::Sample(const nsPoint& aScrollPosition) {
+ float flingSensitivity =
+ StaticPrefs::layout_css_scroll_snap_prediction_sensitivity();
+ int maxVelocity =
+ StaticPrefs::layout_css_scroll_snap_prediction_max_velocity();
+ maxVelocity = nsPresContext::CSSPixelsToAppUnits(maxVelocity);
+ int maxOffset = maxVelocity * flingSensitivity;
+ TimeStamp currentRefreshTime =
+ mPresContext->RefreshDriver()->MostRecentRefresh();
+ if (mSampleTime.IsNull()) {
+ mAccumulator = nsPoint();
+ } else {
+ uint32_t durationMs = (currentRefreshTime - mSampleTime).ToMilliseconds();
+ if (durationMs > StaticPrefs::apz_velocity_relevance_time_ms()) {
+ mAccumulator = nsPoint();
+ mQueue.Clear();
+ } else if (durationMs == 0) {
+ mAccumulator += aScrollPosition - mLastPosition;
+ } else {
+ nsPoint velocity = mAccumulator * 1000 / durationMs;
+ velocity.Clamp(maxVelocity);
+ mQueue.AppendElement(std::make_pair(durationMs, velocity));
+ mAccumulator = aScrollPosition - mLastPosition;
+ }
+ }
+ mAccumulator.Clamp(maxOffset);
+ mSampleTime = currentRefreshTime;
+ mLastPosition = aScrollPosition;
+ TrimQueue();
+}
+
+void ScrollVelocityQueue::TrimQueue() {
+ if (mSampleTime.IsNull()) {
+ // There are no samples, nothing to do here.
+ return;
+ }
+
+ TimeStamp currentRefreshTime =
+ mPresContext->RefreshDriver()->MostRecentRefresh();
+ uint32_t timeDelta = (currentRefreshTime - mSampleTime).ToMilliseconds();
+ for (int i = mQueue.Length() - 1; i >= 0; i--) {
+ timeDelta += mQueue[i].first;
+ if (timeDelta >= StaticPrefs::apz_velocity_relevance_time_ms()) {
+ // The rest of the samples have expired and should be dropped
+ for (; i >= 0; i--) {
+ mQueue.RemoveElementAt(0);
+ }
+ }
+ }
+}
+
+void ScrollVelocityQueue::Reset() {
+ mAccumulator = nsPoint();
+ mSampleTime = TimeStamp();
+ mQueue.Clear();
+}
+
+/**
+ Calculate the velocity of the scroll frame, in appunits / second.
+*/
+nsPoint ScrollVelocityQueue::GetVelocity() {
+ TrimQueue();
+ if (mQueue.Length() == 0) {
+ // If getting the scroll velocity before any scrolling has occurred,
+ // the velocity must be (0, 0)
+ return nsPoint();
+ }
+ nsPoint velocity;
+ for (int i = mQueue.Length() - 1; i >= 0; i--) {
+ velocity += mQueue[i].second;
+ }
+ return velocity / mQueue.Length();
+ ;
+}
+
+} // namespace layout
+} // namespace mozilla