tor-browser

The Tor Browser
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ScrollVelocityQueue.cpp (2937B)

      1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
      2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
      3 /* This Source Code Form is subject to the terms of the Mozilla Public
      4 * License, v. 2.0. If a copy of the MPL was not distributed with this
      5 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
      6 
      7 #include "ScrollVelocityQueue.h"
      8 
      9 #include "mozilla/StaticPrefs_apz.h"
     10 #include "mozilla/StaticPrefs_layout.h"
     11 #include "nsPresContext.h"
     12 #include "nsRefreshDriver.h"
     13 
     14 namespace mozilla {
     15 namespace layout {
     16 
     17 void ScrollVelocityQueue::Sample(const nsPoint& aScrollPosition) {
     18  float flingSensitivity =
     19      StaticPrefs::layout_css_scroll_snap_prediction_sensitivity();
     20  int maxVelocity =
     21      StaticPrefs::layout_css_scroll_snap_prediction_max_velocity();
     22  maxVelocity = nsPresContext::CSSPixelsToAppUnits(maxVelocity);
     23  int maxOffset = maxVelocity * flingSensitivity;
     24  TimeStamp currentRefreshTime =
     25      mPresContext->RefreshDriver()->MostRecentRefresh();
     26  if (mSampleTime.IsNull()) {
     27    mAccumulator = nsPoint();
     28  } else {
     29    uint32_t durationMs = (currentRefreshTime - mSampleTime).ToMilliseconds();
     30    if (durationMs > StaticPrefs::apz_velocity_relevance_time_ms()) {
     31      mAccumulator = nsPoint();
     32      mQueue.Clear();
     33    } else if (durationMs == 0) {
     34      mAccumulator += aScrollPosition - mLastPosition;
     35    } else {
     36      nsPoint velocity = mAccumulator * 1000 / durationMs;
     37      velocity.Clamp(maxVelocity);
     38      mQueue.AppendElement(std::make_pair(durationMs, velocity));
     39      mAccumulator = aScrollPosition - mLastPosition;
     40    }
     41  }
     42  mAccumulator.Clamp(maxOffset);
     43  mSampleTime = currentRefreshTime;
     44  mLastPosition = aScrollPosition;
     45  TrimQueue();
     46 }
     47 
     48 void ScrollVelocityQueue::TrimQueue() {
     49  if (mSampleTime.IsNull()) {
     50    // There are no samples, nothing to do here.
     51    return;
     52  }
     53 
     54  TimeStamp currentRefreshTime =
     55      mPresContext->RefreshDriver()->MostRecentRefresh();
     56  uint32_t timeDelta = (currentRefreshTime - mSampleTime).ToMilliseconds();
     57  for (int i = mQueue.Length() - 1; i >= 0; i--) {
     58    timeDelta += mQueue[i].first;
     59    if (timeDelta >= StaticPrefs::apz_velocity_relevance_time_ms()) {
     60      // The rest of the samples have expired and should be dropped
     61      for (; i >= 0; i--) {
     62        mQueue.RemoveElementAt(0);
     63      }
     64    }
     65  }
     66 }
     67 
     68 void ScrollVelocityQueue::Reset() {
     69  mAccumulator = nsPoint();
     70  mSampleTime = TimeStamp();
     71  mQueue.Clear();
     72 }
     73 
     74 /**
     75  Calculate the velocity of the scroll frame, in appunits / second.
     76 */
     77 nsPoint ScrollVelocityQueue::GetVelocity() {
     78  TrimQueue();
     79  if (mQueue.Length() == 0) {
     80    // If getting the scroll velocity before any scrolling has occurred,
     81    // the velocity must be (0, 0)
     82    return nsPoint();
     83  }
     84  nsPoint velocity;
     85  for (int i = mQueue.Length() - 1; i >= 0; i--) {
     86    velocity += mQueue[i].second;
     87  }
     88  return velocity / mQueue.Length();
     89  ;
     90 }
     91 
     92 }  // namespace layout
     93 }  // namespace mozilla