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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 */
#ifndef mozilla_widget_TouchResampler_h
#define mozilla_widget_TouchResampler_h
#include <queue>
#include <unordered_map>
#include "mozilla/Maybe.h"
#include "mozilla/TimeStamp.h"
#include "InputData.h"
namespace mozilla {
namespace widget {
* De-jitters touch motions by resampling (interpolating or extrapolating) touch
* positions for the vsync timestamp.
* Touch resampling improves the touch panning experience on devices where touch
* positions are sampled at a rate that's not an integer multiple of the display
* refresh rate, for example 100Hz touch sampling on a 60Hz display: Without
* resampling, we would alternate between taking one touch sample or two touch
* samples into account each frame, creating a jittery motion ("small step, big
* step, small step, big step").
* Intended for use on Android, where both touch events and vsync notifications
* arrive on the same thread, the Java UI thread.
* This class is not thread safe.
* TouchResampler operates in the following way:
* Original events are fed into ProcessEvent().
* Outgoing events (potentially resampled for resampling) are added to a queue
* and can be consumed by calling ConsumeOutgoingEvents(). Touch events which
* are not touch move events are forwarded instantly and not resampled. Only
* touch move events are resampled. Whenever a touch move event is received, it
* gets delayed until NotifyFrame() is called, at which point it is resampled
* into a resampled version for the given frame timestamp, and added to the
* outgoing queue. If no touch move event is received between two consecutive
* frames, this is treated as a stop in the touch motion. If the last outgoing
* event was an resampled touch move event, we return back to the non-resampled
* state by emitting a copy of the last original touch move event, which has
* unmodified position data. Touch events which are not touch move events also
* force a return to the non-resampled state before they are moved to the
* outgoing queue.
class TouchResampler final {
// Feed a touch event into the interpolater. Returns an ID that can be used to
// match outgoing events to this incoming event, to track data associated with
// this event.
uint64_t ProcessEvent(MultiTouchInput&& aInput);
// Emit events, potentially resampled, for this timestamp. The events are put
// into the outgoing queue. May not emit any events if there's no update.
void NotifyFrame(const TimeStamp& aTimeStamp);
// Returns true between the start and the end of a touch gesture. During this
// time, the caller should keep itself registered with the system frame
// callback mechanism, so that NotifyFrame() can be called on every frame.
// (Otherwise, if we only registered the callback after receiving a touch move
// event, the frame callback might be delayed by a full frame.)
bool InTouchingState() const { return mCurrentTouches.HasTouch(); }
struct OutgoingEvent {
// The event, potentially modified from the original for resampling.
MultiTouchInput mEvent;
// Some(eventId) if this event is a modified version of an original event,
// Nothing() if this is an extra event.
Maybe<uint64_t> mEventId;
// Returns the outgoing events that were produced since the last call.
// No event IDs will be skipped. Returns at least one outgoing event for each
// incoming event (possibly after a delay), and potential extra events with
// no originating event ID.
// Outgoing events should be consumed after every call to ProcessEvent() and
// after every call to NotifyFrame().
std::queue<OutgoingEvent> ConsumeOutgoingEvents() {
return std::move(mOutgoingEvents);
// Add the event to the outgoing queue.
void EmitEvent(MultiTouchInput&& aInput, uint64_t aEventId) {
mLastEmittedEventTime = aInput.mTimeStamp;
mOutgoingEvents.push(OutgoingEvent{std::move(aInput), Some(aEventId)});
// Emit an event that does not correspond to an incoming event.
void EmitExtraEvent(MultiTouchInput&& aInput) {
mLastEmittedEventTime = aInput.mTimeStamp;
mOutgoingEvents.push(OutgoingEvent{std::move(aInput), Nothing()});
// Move any touch move events that we deferred for resampling to the outgoing
// queue unmodified, leaving mDeferredTouchMoveEvents empty.
void FlushDeferredTouchMoveEventsUnresampled();
// Must only be called if mInResampledState is true and
// mDeferredTouchMoveEvents is empty. Emits mOriginalOfResampledTouchMove,
// with a potentially adjusted timestamp for correct ordering.
void ReturnToNonResampledState();
// Takes historical touch data from mRemainingTouchData and prepends it to the
// data in aInput.
void PrependLeftoverHistoricalData(MultiTouchInput* aInput);
struct DataPoint {
TimeStamp mTimeStamp;
ScreenIntPoint mPosition;
struct TouchInfo {
void Update(const SingleTouchData& aTouch, const TimeStamp& aEventTime);
ScreenIntPoint ResampleAtTime(const ScreenIntPoint& aLastObservedPosition,
const TimeStamp& aTimeStamp);
int32_t mIdentifier = 0;
Maybe<DataPoint> mBaseDataPoint;
Maybe<DataPoint> mLatestDataPoint;
struct CurrentTouches {
void UpdateFromEvent(const MultiTouchInput& aInput);
bool HasTouch() const { return !mTouches.IsEmpty(); }
TimeStamp LatestDataPointTime() { return mLatestDataPointTime; }
ScreenIntPoint ResampleTouchPositionAtTime(
int32_t aIdentifier, const ScreenIntPoint& aLastObservedPosition,
const TimeStamp& aTimeStamp);
void ClearDataPoints() {
for (auto& touch : mTouches) {
touch.mBaseDataPoint = Nothing();
touch.mLatestDataPoint = Nothing();
nsTArray<TouchInfo>::iterator TouchByIdentifier(int32_t aIdentifier);
nsTArray<TouchInfo> mTouches;
TimeStamp mLatestDataPointTime;
// The current touch positions with historical data points. This data only
// contains original non-resampled positions from the incoming touch events.
CurrentTouches mCurrentTouches;
// Incoming touch move events are stored here until NotifyFrame is called.
std::queue<std::pair<MultiTouchInput, uint64_t>> mDeferredTouchMoveEvents;
// Stores any touch samples that were not included in the last emitted touch
// move event because they were in the future compared to the emitted event's
// timestamp. These data points should be prepended to the historical data of
// the next emitted touch move evnt.
// Can only be non-empty if mInResampledState is true.
std::unordered_map<int32_t, nsTArray<SingleTouchData::HistoricalTouchData>>
// If we're in an resampled state, because the last outgoing event was a
// resampled touch move event, then this contains a copy of the unresampled,
// original touch move event.
// Some() iff mInResampledState is true.
Maybe<MultiTouchInput> mOriginalOfResampledTouchMove;
// The stream of outgoing events that can be consumed by our caller.
std::queue<OutgoingEvent> mOutgoingEvents;
// The timestamp of the event that was emitted most recently, or the null
// timestamp if no event has been emitted yet.
TimeStamp mLastEmittedEventTime;
uint64_t mNextEventId = 0;
// True if the last outgoing event was a touch move event with an resampled
// position. We only want to stay in this state as long as a continuous stream
// of touch move events is coming in.
bool mInResampledState = false;
} // namespace widget
} // namespace mozilla
#endif // mozilla_widget_TouchResampler_h