layout/generic/ScrollVelocityQueue.cpp

   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