Port of Android platform gesture detection code to C++

ui/events/gesture_detection/gesture_provider.cc

+// Copyright 2014 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 "ui/events/gesture_detection/gesture_provider.h"
+
+#include <cmath>
+
+#include "base/auto_reset.h"
+#include "base/debug/trace_event.h"
+#include "ui/events/gesture_detection/gesture_event_params.h"
+#include "ui/events/gesture_detection/motion_event.h"
+
+namespace ui {
+namespace {
+
+// Double-tap drag zoom sensitivity (speed).
+const float kDoubleTapDragZoomSpeed = 0.005f;
+
+const char* GetMotionEventActionName(MotionEvent::Action action) {
+  switch(action) {
+    case MotionEvent::ACTION_POINTER_DOWN: return "ACTION_POINTER_DOWN";
+    case MotionEvent::ACTION_POINTER_UP:   return "ACTION_POINTER_UP";
+    case MotionEvent::ACTION_DOWN:         return "ACTION_DOWN";
+    case MotionEvent::ACTION_UP:           return "ACTION_UP";
+    case MotionEvent::ACTION_CANCEL:       return "ACTION_CANCEL";
+    case MotionEvent::ACTION_MOVE:         return "ACTION_MOVE";
+  }
+  return "";
+}
+
+GestureEventParams CreateGesture(GestureEventType type,
+                                 base::TimeTicks time,
+                                 float x,
+                                 float y,
+                                 const GestureEventParams::Data& extra_data) {
+  return GestureEventParams(type, time, x, y, extra_data);
+}
+
+GestureEventParams CreateGesture(GestureEventType type,
+                                 base::TimeTicks time,
+                                 float x,
+                                 float y) {
+  return CreateGesture(type, time, x, y, GestureEventParams::Data());
+}
+
+GestureEventParams CreateGesture(GestureEventType type,
+                                 const MotionEvent& event,
+                                 const GestureEventParams::Data& extra_data) {
+  return CreateGesture(
+      type, event.GetEventTime(), event.GetX(), event.GetY(), extra_data);
+}
+
+GestureEventParams CreateGesture(GestureEventType type,
+                                 const MotionEvent& event) {
+  return CreateGesture(type, event, GestureEventParams::Data());
+}
+
+float Round(float f) {
+  return (f > 0.f) ? std::floor(f + 0.5f) : std::ceil(f - 0.5f);
+}
+
+}  // namespace
+
+// GestureProvider:::Config
+
+GestureProvider::Config::Config() : disable_click_delay(false) {}
+
+GestureProvider::Config::~Config() {}
+
+// GestureProvider::ScaleGestureListener
+
+class GestureProvider::ScaleGestureListenerImpl
+    : public ScaleGestureDetector::ScaleGestureListener {
+ public:
+  ScaleGestureListenerImpl(const ScaleGestureDetector::Config& config,
+                           GestureProvider* provider)
+      : scale_gesture_detector_(config, this),
+        provider_(provider),
+        ignore_detector_events_(false),
+        pinch_event_sent_(false) {}
+
+  bool OnTouchEvent(const MotionEvent& event) {
+    // TODO: Need to deal with multi-touch transition.
+    const bool in_scale_gesture = IsScaleGestureDetectionInProgress();
+    bool handled = scale_gesture_detector_.OnTouchEvent(event);
+    if (!in_scale_gesture &&
+        (event.GetAction() == MotionEvent::ACTION_UP ||
+         event.GetAction() == MotionEvent::ACTION_CANCEL)) {
+      return false;
+    }
+    return handled;
+  }
+
+  // ScaleGestureDetector::ScaleGestureListener implementation.
+  virtual bool OnScaleBegin(const ScaleGestureDetector& detector) OVERRIDE {
+    if (ignore_detector_events_)
+      return false;
+    pinch_event_sent_ = false;
+    return true;
+  }
+
+  virtual void OnScaleEnd(const ScaleGestureDetector& detector) OVERRIDE {
+    if (!pinch_event_sent_)
+      return;
+    provider_->Send(
+        CreateGesture(GESTURE_PINCH_END, detector.GetEventTime(), 0, 0));
+    pinch_event_sent_ = false;
+  }
+
+  virtual bool OnScale(const ScaleGestureDetector& detector) OVERRIDE {
+    if (ignore_detector_events_)
+      return false;
+    if (!pinch_event_sent_) {
+      pinch_event_sent_ = true;
+      provider_->Send(CreateGesture(GESTURE_PINCH_BEGIN,
+                                    detector.GetEventTime(),
+                                    detector.GetFocusX(),
+                                    detector.GetFocusY()));
+    }
+    GestureEventParams::Data pinch_data;
+    pinch_data.pinch_update.scale = detector.GetScaleFactor();
+    provider_->Send(CreateGesture(GESTURE_PINCH_UPDATE,
+                                  detector.GetEventTime(),
+                                  detector.GetFocusX(),
+                                  detector.GetFocusY(),
+                                  pinch_data));
+    return true;
+  }
+
+  bool IsScaleGestureDetectionInProgress() const {
+    return !ignore_detector_events_ && scale_gesture_detector_.IsInProgress();
+  }
+
+  void set_ignore_detector_events(bool value) {
+    // Note that returning false from OnScaleBegin / OnScale makes the
+    // gesture detector not to emit further scaling notifications
+    // related to this gesture. Thus, if detector events are enabled in
+    // the middle of the gesture, we don't need to do anything.
+    ignore_detector_events_ = value;
+  }
+
+ private:
+  ScaleGestureDetector scale_gesture_detector_;
+
+  GestureProvider* const provider_;
+
+  // Completely silence scaling events. Used in WebView when zoom support
+  // is turned off.
+  bool ignore_detector_events_;
+
+  // Whether any pinch zoom event has been sent to native.
+  bool pinch_event_sent_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScaleGestureListenerImpl);
+};
+
+// GestureProvider::GestureListener
+
+class GestureProvider::GestureListenerImpl
+    : public GestureDetector::GestureListener,
+      public GestureDetector::DoubleTapListener {
+ public:
+  GestureListenerImpl(
+      const GestureDetector::Config& gesture_detector_config,
+      const SnapScrollController::Config& snap_scroll_controller_config,
+      bool disable_click_delay,
+      GestureProvider* provider)
+      : gesture_detector_(gesture_detector_config, this, this),
+        snap_scroll_controller_(snap_scroll_controller_config),
+        provider_(provider),
+        px_to_dp_(1.0f / snap_scroll_controller_config.device_scale_factor),
+        disable_click_delay_(disable_click_delay),
+        scaled_touch_slop_(gesture_detector_config.scaled_touch_slop),
+        scaled_touch_slop_square_(scaled_touch_slop_ * scaled_touch_slop_),
+        double_tap_timeout_(gesture_detector_config.double_tap_timeout),
+        ignore_single_tap_(false),
+        seen_first_scroll_event_(false),
+        double_tap_mode_(DOUBLE_TAP_MODE_NONE),
+        double_tap_y_(0),
+        support_double_tap_(true),
+        double_tap_drag_zoom_anchor_x_(0),
+        double_tap_drag_zoom_anchor_y_(0),
+        last_raw_x_(0),
+        last_raw_y_(0),
+        accumulated_scroll_error_x_(0),
+        accumulated_scroll_error_y_(0) {
+    UpdateDoubleTapListener();
+  }
+
+  virtual ~GestureListenerImpl() {}
+
+  bool OnTouchEvent(const MotionEvent& e,
+                    bool is_scale_gesture_detection_in_progress) {
+    snap_scroll_controller_.SetSnapScrollingMode(
+        e, is_scale_gesture_detection_in_progress);
+
+    if (is_scale_gesture_detection_in_progress)
+      SetIgnoreSingleTap(true);
+
+    if (e.GetAction() == MotionEvent::ACTION_POINTER_DOWN ||
+        e.GetAction() == MotionEvent::ACTION_CANCEL) {
+      EndDoubleTapDragIfNecessary(e);
+    } else if (e.GetAction() == MotionEvent::ACTION_DOWN) {
+      gesture_detector_.set_is_longpress_enabled(true);
+    }
+
+    return gesture_detector_.OnTouchEvent(e);
+  }
+
+  // GestureDetector::GestureListener implementation.
+  virtual bool OnDown(const MotionEvent& e) OVERRIDE {
+    current_down_time_ = e.GetEventTime();
+    ignore_single_tap_ = false;
+    seen_first_scroll_event_ = false;
+    last_raw_x_ = e.GetRawX();
+    last_raw_y_ = e.GetRawY();
+    accumulated_scroll_error_x_ = 0;
+    accumulated_scroll_error_y_ = 0;
+
+    GestureEventParams::Data tap_down_data;
+    tap_down_data.tap.width = tap_down_data.tap.height = e.GetTouchMajor();
+    provider_->Send(CreateGesture(GESTURE_TAP_DOWN, e, tap_down_data));
+
+    // Return true to indicate that we want to handle touch.
+    return true;
+  }
+
+  virtual bool OnScroll(const MotionEvent& e1,
+                        const MotionEvent& e2,
+                        float raw_distance_x,
+                        float raw_distance_y) OVERRIDE {
+    float distance_x = raw_distance_x;
+    float distance_y = raw_distance_y;
+    if (!seen_first_scroll_event_) {
+      // Remove the touch slop region from the first scroll event to avoid a
+      // jump.
+      seen_first_scroll_event_ = true;
+      double distance =
+          std::sqrt(distance_x * distance_x + distance_y * distance_y);
+      double epsilon = 1e-3;
+      if (distance > epsilon) {
+        double ratio = std::max(0., distance - scaled_touch_slop_) / distance;
+        distance_x *= ratio;
+        distance_y *= ratio;
+      }
+    }
+    snap_scroll_controller_.UpdateSnapScrollMode(distance_x, distance_y);
+    if (snap_scroll_controller_.IsSnappingScrolls()) {
+      if (snap_scroll_controller_.IsSnapHorizontal()) {
+        distance_y = 0;
+      } else {
+        distance_x = 0;
+      }
+    }
+
+    last_raw_x_ = e2.GetRawX();
+    last_raw_y_ = e2.GetRawY();
+    if (!provider_->IsScrollInProgress()) {
+      // Note that scroll start hints are in distance traveled, where
+      // scroll deltas are in the opposite direction.
+      GestureEventParams::Data scroll_data;
+      scroll_data.scroll_begin.delta_x_hint = -raw_distance_x;
+      scroll_data.scroll_begin.delta_y_hint = -raw_distance_y;
+      provider_->Send(CreateGesture(GESTURE_SCROLL_BEGIN,
+                                    e2.GetEventTime(),
+                                    e1.GetX(),
+                                    e1.GetY(),
+                                    scroll_data));
+    }
+
+    // distance_x and distance_y is the scrolling offset since last OnScroll.
+    // Because we are passing integers to Blink, this could introduce
+    // rounding errors. The rounding errors will accumulate overtime.
+    // To solve this, we should be adding back the rounding errors each time
+    // when we calculate the new offset.
+    // TODO(jdduke): Determine if we can simpy use floating point deltas, as
+    // WebGestureEvent also takes floating point deltas for GestureScrollUpdate.
+    int dx = (int)(distance_x + accumulated_scroll_error_x_);
+    int dy = (int)(distance_y + accumulated_scroll_error_y_);
+    accumulated_scroll_error_x_ += (distance_x - dx);
+    accumulated_scroll_error_y_ += (distance_y - dy);
+
+    if (dx || dy) {
+      GestureEventParams::Data scroll_data;
+      scroll_data.scroll_update.delta_x = -dx;
+      scroll_data.scroll_update.delta_y = -dy;
+      provider_->Send(CreateGesture(GESTURE_SCROLL_UPDATE, e2, scroll_data));
+    }
+
+    return true;
+  }
+
+  virtual bool OnFling(const MotionEvent& e1,
+                       const MotionEvent& e2,
+                       float velocity_x,
+                       float velocity_y) OVERRIDE {
+    if (snap_scroll_controller_.IsSnappingScrolls()) {
+      if (snap_scroll_controller_.IsSnapHorizontal()) {
+        velocity_y = 0;
+      } else {
+        velocity_x = 0;
+      }
+    }
+
+    provider_->Fling(
+        e2.GetEventTime(), e1.GetX(), e1.GetY(), velocity_x, velocity_y);
+    return true;
+  }
+
+  virtual void OnShowPress(const MotionEvent& e) OVERRIDE {
+    GestureEventParams::Data show_press_data;
+    // TODO(jdduke): Expose minor axis length and rotation in |MotionEvent|.
+    show_press_data.show_press.width = e.GetTouchMajor();
+    show_press_data.show_press.height = show_press_data.show_press.width;
+    provider_->Send(CreateGesture(GESTURE_SHOW_PRESS, e, show_press_data));
+  }
+
+  virtual bool OnSingleTapUp(const MotionEvent& e) OVERRIDE {
+    if (IsPointOutsideCurrentSlopRegion(e.GetRawX(), e.GetRawY())) {
+      provider_->SendTapCancelIfNecessary(e);
+      ignore_single_tap_ = true;
+      return true;
+    }
+    // This is a hack to address the issue where user hovers
+    // over a link for longer than double_tap_timeout_, then
+    // OnSingleTapConfirmed() is not triggered. But we still
+    // want to trigger the tap event at UP. So we override
+    // OnSingleTapUp() in this case. This assumes singleTapUp
+    // gets always called before singleTapConfirmed.
+    if (!ignore_single_tap_) {
+      if (e.GetEventTime() - current_down_time_ > double_tap_timeout_) {
+        return OnSingleTapConfirmed(e);
+      } else if (IsDoubleTapDisabled() || disable_click_delay_) {
+        // If double-tap has been disabled, there is no need to wait
+        // for the double-tap timeout.
+        return OnSingleTapConfirmed(e);
+      } else {
+        // Notify Blink about this tapUp event anyway,
+        // when none of the above conditions applied.
+        provider_->Send(CreateGesture(GESTURE_SINGLE_TAP_UNCONFIRMED, e));
+      }
+    }
+
+    return provider_->SendLongTapIfNecessary(e);
+  }
+
+  // GestureDetector::DoubleTapListener implementation.
+  virtual bool OnSingleTapConfirmed(const MotionEvent& e) OVERRIDE {
+    // Long taps in the edges of the screen have their events delayed by
+    // ContentViewHolder for tab swipe operations. As a consequence of the delay
+    // this method might be called after receiving the up event.
+    // These corner cases should be ignored.
+    if (ignore_single_tap_)
+      return true;
+
+    ignore_single_tap_ = true;
+
+    GestureEventParams::Data tap_data;
+    tap_data.tap.tap_count = 1;
+    tap_data.tap.width = tap_data.tap.height = e.GetTouchMajor();
+    provider_->Send(CreateGesture(GESTURE_SINGLE_TAP_CONFIRMED, e, tap_data));
+    return true;
+  }
+
+  virtual bool OnDoubleTap(const MotionEvent& e) OVERRIDE { return false; }
+
+  virtual bool OnDoubleTapEvent(const MotionEvent& e) OVERRIDE {
+    switch (e.GetAction()) {
+      case MotionEvent::ACTION_DOWN:
+        // Note that this will be called before the corresponding |onDown()|
+        // of the same ACTION_DOWN event.  Thus, the preceding TAP_DOWN
+        // should be cancelled prior to sending a new one (in |onDown()|).
+        double_tap_drag_zoom_anchor_x_ = e.GetX();
+        double_tap_drag_zoom_anchor_y_ = e.GetY();
+        double_tap_mode_ = DOUBLE_TAP_MODE_DRAG_DETECTION_IN_PROGRESS;
+        // If a long-press fires during a double-tap, the GestureDetector
+        // will stop feeding MotionEvents to |onDoubleTapEvent()|,
+        // preventing double-tap drag zoom. Long press detection will be
+        // re-enabled on the next ACTION_DOWN.
+        gesture_detector_.set_is_longpress_enabled(false);
+        break;
+      case MotionEvent::ACTION_MOVE:
+        if (double_tap_mode_ == DOUBLE_TAP_MODE_DRAG_DETECTION_IN_PROGRESS) {
+          float distance_x = double_tap_drag_zoom_anchor_x_ - e.GetX();
+          float distance_y = double_tap_drag_zoom_anchor_y_ - e.GetY();
+
+          // Begin double-tap drag zoom mode if the move distance is
+          // further than the threshold.
+          if (IsDistanceGreaterThanTouchSlop(distance_x, distance_y)) {
+            GestureEventParams::Data scroll_data;
+            scroll_data.scroll_begin.delta_x_hint = -distance_x;
+            scroll_data.scroll_begin.delta_y_hint = -distance_y;
+            provider_->Send(
+                CreateGesture(GESTURE_SCROLL_BEGIN, e, scroll_data));
+            provider_->Send(
+                CreateGesture(GESTURE_PINCH_BEGIN,
+                              e.GetEventTime(),
+                              Round(double_tap_drag_zoom_anchor_x_),
+                              Round(double_tap_drag_zoom_anchor_y_)));
+            double_tap_mode_ = DOUBLE_TAP_MODE_DRAG_ZOOM;
+          }
+        } else if (double_tap_mode_ == DOUBLE_TAP_MODE_DRAG_ZOOM) {
+          provider_->Send(CreateGesture(GESTURE_SCROLL_UPDATE, e));
+
+          float dy = double_tap_y_ - e.GetY();
+          GestureEventParams::Data pinch_data;
+          pinch_data.pinch_update.scale =
+              std::pow(dy > 0 ? 1.0f - kDoubleTapDragZoomSpeed
+                              : 1.0f + kDoubleTapDragZoomSpeed,
+                       std::abs(dy * px_to_dp_));
+          provider_->Send(CreateGesture(GESTURE_PINCH_UPDATE,
+                                        e.GetEventTime(),
+                                        Round(double_tap_drag_zoom_anchor_x_),
+                                        Round(double_tap_drag_zoom_anchor_y_),
+                                        pinch_data));
+        }
+        break;
+      case MotionEvent::ACTION_UP:
+        if (double_tap_mode_ != DOUBLE_TAP_MODE_DRAG_ZOOM) {
+          // Normal double-tap gesture.
+          provider_->Send(CreateGesture(GESTURE_DOUBLE_TAP, e));
+        }
+        EndDoubleTapDragIfNecessary(e);
+        break;
+      case MotionEvent::ACTION_CANCEL:
+        EndDoubleTapDragIfNecessary(e);
+        break;
+      default:
+        NOTREACHED() << "Invalid double-tap event.";
+        break;
+    }
+    double_tap_y_ = e.GetY();
+    return true;
+  }
+
+  virtual bool OnLongPress(const MotionEvent& e) OVERRIDE {
+    DCHECK(!IsDoubleTapInProgress());
+    SetIgnoreSingleTap(true);
+
+    GestureEventParams::Data long_press_data;
+    long_press_data.long_press.width = e.GetTouchMajor();
+    long_press_data.long_press.height = long_press_data.long_press.width;
+    provider_->Send(CreateGesture(GESTURE_LONG_PRESS, e, long_press_data));
+
+    // Returning true puts the GestureDetector in "longpress" mode, disabling
+    // further scrolling.  This is undesirable, as it is quite common for a
+    // longpress gesture to fire on content that won't trigger a context menu.
+    return false;
+  }
+
+  void UpdateDoubleTapSupportForPlatform(bool support_double_tap) {
+    DCHECK(!IsDoubleTapInProgress());
+    DoubleTapMode double_tap_mode =
+        support_double_tap ? DOUBLE_TAP_MODE_NONE : DOUBLE_TAP_MODE_DISABLED;
+    if (double_tap_mode_ == double_tap_mode)
+      return;
+    double_tap_mode_ = double_tap_mode;
+    UpdateDoubleTapListener();
+  }
+
+  void UpdateDoubleTapSupportForPage(bool support_double_tap) {
+    if (support_double_tap_ == support_double_tap)
+      return;
+    support_double_tap_ = support_double_tap;
+    UpdateDoubleTapListener();
+  }
+
+  bool IsDoubleTapDisabled() const {
+    return double_tap_mode_ == DOUBLE_TAP_MODE_DISABLED || !support_double_tap_;
+  }
+
+  bool IsClickDelayDisabled() const { return disable_click_delay_; }
+
+  bool IsDoubleTapInProgress() const {
+    return double_tap_mode_ != DOUBLE_TAP_MODE_DISABLED &&
+           double_tap_mode_ != DOUBLE_TAP_MODE_NONE;
+  }
+
+ private:
+  enum DoubleTapMode {
+    DOUBLE_TAP_MODE_NONE,
+    DOUBLE_TAP_MODE_DRAG_DETECTION_IN_PROGRESS,
+    DOUBLE_TAP_MODE_DRAG_ZOOM,
+    DOUBLE_TAP_MODE_DISABLED
+  };
+
+  bool IsPointOutsideCurrentSlopRegion(float x, float y) const {
+    return IsDistanceGreaterThanTouchSlop(last_raw_x_ - x, last_raw_y_ - y);
+  }
+
+  bool IsDistanceGreaterThanTouchSlop(float distance_x,
+                                      float distance_y) const {
+    return distance_x * distance_x + distance_y * distance_y >
+           scaled_touch_slop_square_;
+  }
+
+  void SetIgnoreSingleTap(bool value) { ignore_single_tap_ = value; }
+
+  void EndDoubleTapDragIfNecessary(const MotionEvent& event) {
+    if (!IsDoubleTapInProgress())
+      return;
+    if (double_tap_mode_ == DOUBLE_TAP_MODE_DRAG_ZOOM) {
+      provider_->Send(CreateGesture(GESTURE_PINCH_END, event));
+      provider_->Send(CreateGesture(GESTURE_SCROLL_END, event));
+    }
+    double_tap_mode_ = DOUBLE_TAP_MODE_NONE;
+    UpdateDoubleTapListener();
+  }
+
+  void UpdateDoubleTapListener() {
+    if (IsDoubleTapDisabled()) {
+      // Defer nulling the DoubleTapListener until the double-tap gesture is
+      // complete.
+      if (IsDoubleTapInProgress())
+        return;
+      gesture_detector_.set_doubletap_listener(NULL);
+    } else {
+      gesture_detector_.set_doubletap_listener(this);
+    }
+  }
+
+  GestureDetector gesture_detector_;
+  SnapScrollController snap_scroll_controller_;
+
+  GestureProvider* const provider_;
+
+  const float px_to_dp_;
+
+  // Whether the click delay should always be disabled by sending clicks for
+  // double-tap gestures.
+  const bool disable_click_delay_;
+
+  const int scaled_touch_slop_;
+
+  // Cache of square of the scaled touch slop so we don't have to calculate it
+  // on every touch.
+  const int scaled_touch_slop_square_;
+
+  const base::TimeDelta double_tap_timeout_;
+
+  base::TimeTicks current_down_time_;
+
+  // TODO(klobag): This is to avoid a bug in GestureDetector. With multi-touch,
+  // always_in_tap_region_ is not reset. So when the last finger is up,
+  // OnSingleTapUp() will be mistakenly fired.
+  bool ignore_single_tap_;
+
+  // Used to remove the touch slop from the initial scroll event in a scroll
+  // gesture.
+  bool seen_first_scroll_event_;
+
+  // Indicate current double-tap mode state.
+  int double_tap_mode_;
+
+  // On double-tap this will store the y coordinates of the touch.
+  float double_tap_y_;
+
+  // The page's viewport and scale sometimes allow us to disable double-tap
+  // gesture detection,
+  // according to the logic in ContentViewCore.onRenderCoordinatesUpdated().
+  bool support_double_tap_;
+
+  // x, y coordinates for an Anchor on double-tap drag zoom.
+  float double_tap_drag_zoom_anchor_x_;
+  float double_tap_drag_zoom_anchor_y_;
+
+  // Used to track the last rawX/Y coordinates for moves.  This gives absolute
+  // scroll distance.
+  // Useful for full screen tracking.
+  float last_raw_x_;
+  float last_raw_y_;
+
+  // Used to track the accumulated scroll error over time. This is used to
+  // remove the
+  // rounding error we introduced by passing integers to webkit.
+  float accumulated_scroll_error_x_;
+  float accumulated_scroll_error_y_;
+
+  DISALLOW_COPY_AND_ASSIGN(GestureListenerImpl);
+};
+
+// GestureProvider
+
+GestureProvider::GestureProvider(const Config& config,
+                                 GestureProviderClient* client)
+    : client_(client),
+      needs_show_press_event_(false),
+      needs_tap_ending_event_(false),
+      touch_scroll_in_progress_(false),
+      pinch_in_progress_(false) {
+  DCHECK(client);
+  InitGestureDetectors(config);
+}
+
+GestureProvider::~GestureProvider() {}
+
+bool GestureProvider::OnTouchEvent(const MotionEvent& event) {
+  TRACE_EVENT1("input", "GestureProvider::OnTouchEvent",
+               "action", GetMotionEventActionName(event.GetAction()));
+  if (!CanHandle(event))
+    return false;
+
+  const bool was_touch_scrolling_ = touch_scroll_in_progress_;
+  const bool in_scale_gesture =
+      scale_gesture_listener_->IsScaleGestureDetectionInProgress();
+
+  if (event.GetAction() == MotionEvent::ACTION_DOWN) {
+    current_down_event_ = event.Clone();
+    touch_scroll_in_progress_ = false;
+    needs_show_press_event_ = true;
+    current_longpress_time_ = base::TimeTicks();
+    SendTapCancelIfNecessary(event);
+  }
+
+  bool handled = gesture_listener_->OnTouchEvent(event, in_scale_gesture);
+  handled |= scale_gesture_listener_->OnTouchEvent(event);
+
+  if (event.GetAction() == MotionEvent::ACTION_UP ||
+      event.GetAction() == MotionEvent::ACTION_CANCEL) {
+    // "Last finger raised" could be an end to movement, but it should
+    // only terminate scrolling if the event did not cause a fling.
+    if (was_touch_scrolling_ && !handled)
+      EndTouchScrollIfNecessary(event.GetEventTime(), true);
+
+    // We shouldn't necessarily cancel a tap on ACTION_UP, as the double-tap
+    // timeout may yet trigger a SINGLE_TAP_CONFIRMED.
+    if (event.GetAction() == MotionEvent::ACTION_CANCEL)
+      SendTapCancelIfNecessary(event);
+
+    current_down_event_.reset();
+  }
+
+  return true;
+}
+
+void GestureProvider::ResetGestureDetectors() {
+  if (!current_down_event_)
+    return;
+  scoped_ptr<MotionEvent> cancel_event = current_down_event_->Cancel();
+  gesture_listener_->OnTouchEvent(*cancel_event, false);
+  scale_gesture_listener_->OnTouchEvent(*cancel_event);
+}
+
+void GestureProvider::CancelActiveTouchSequence() {
+  if (!current_down_event_)
+    return;
+  OnTouchEvent(*current_down_event_->Cancel());
+  current_down_event_.reset();
+}
+
+void GestureProvider::UpdateMultiTouchSupport(bool support_multi_touch_zoom) {
+  scale_gesture_listener_->set_ignore_detector_events(
+      !support_multi_touch_zoom);
+}
+
+void GestureProvider::UpdateDoubleTapSupportForPlatform(
+    bool support_double_tap) {
+  gesture_listener_->UpdateDoubleTapSupportForPlatform(support_double_tap);
+}
+
+void GestureProvider::UpdateDoubleTapSupportForPage(bool support_double_tap) {
+  gesture_listener_->UpdateDoubleTapSupportForPage(support_double_tap);
+}
+
+bool GestureProvider::IsScrollInProgress() const {
+  // TODO(wangxianzhu): Also return true when fling is active once the UI knows
+  // exactly when the fling ends.
+  return touch_scroll_in_progress_;
+}
+
+bool GestureProvider::IsPinchInProgress() const { return pinch_in_progress_; }
+
+bool GestureProvider::IsDoubleTapInProgress() const {
+  return gesture_listener_->IsDoubleTapInProgress();
+}
+
+void GestureProvider::InitGestureDetectors(const Config& config) {
+  TRACE_EVENT0("input", "GestureProvider::InitGestureDetectors");
+  gesture_listener_.reset(
+      new GestureListenerImpl(config.gesture_detector_config,
+                              config.snap_scroll_controller_config,
+                              config.disable_click_delay,
+                              this));
+
+  scale_gesture_listener_.reset(
+      new ScaleGestureListenerImpl(config.scale_gesture_detector_config, this));
+}
+
+bool GestureProvider::CanHandle(const MotionEvent& event) const {
+  return event.GetAction() == MotionEvent::ACTION_DOWN || current_down_event_;
+}
+
+void GestureProvider::Fling(base::TimeTicks time,
+                            float x,
+                            float y,
+                            float velocity_x,
+                            float velocity_y) {
+  if (!velocity_x && !velocity_y) {
+    EndTouchScrollIfNecessary(time, true);
+    return;
+  }
+
+  if (!touch_scroll_in_progress_) {
+    // The native side needs a GESTURE_SCROLL_BEGIN before GESTURE_FLING_START
+    // to send the fling to the correct target. Send if it has not sent.
+    // The distance traveled in one second is a reasonable scroll start hint.
+    GestureEventParams::Data scroll_data;
+    scroll_data.scroll_begin.delta_x_hint = velocity_x;
+    scroll_data.scroll_begin.delta_y_hint = velocity_y;
+    Send(CreateGesture(GESTURE_SCROLL_BEGIN, time, x, y, scroll_data));
+  }
+  EndTouchScrollIfNecessary(time, false);
+
+  GestureEventParams::Data fling_data;
+  fling_data.fling_start.velocity_x = velocity_x;
+  fling_data.fling_start.velocity_y = velocity_y;
+  Send(CreateGesture(GESTURE_FLING_START, time, x, y, fling_data));
+}
+
+void GestureProvider::Send(const GestureEventParams& gesture) {
+  DCHECK(!gesture.time.is_null());
+  // The only valid events that should be sent without an active touch sequence
+  // are SHOW_PRESS and TAP_CONFIRMED, potentially triggered by the double-tap
+  // delay timing out.
+  DCHECK(current_down_event_ || gesture.type == GESTURE_SINGLE_TAP_CONFIRMED ||
+         gesture.type == GESTURE_SHOW_PRESS);
+
+  switch (gesture.type) {
+    case GESTURE_TAP_DOWN:
+      needs_tap_ending_event_ = true;
+      break;
+    case GESTURE_SINGLE_TAP_UNCONFIRMED:
+      needs_show_press_event_ = false;
+      break;
+    case GESTURE_SINGLE_TAP_CONFIRMED:
+      if (needs_show_press_event_)
+        Send(CreateGesture(
+            GESTURE_SHOW_PRESS, gesture.time, gesture.x, gesture.y));
+      needs_tap_ending_event_ = false;
+      break;
+    case GESTURE_DOUBLE_TAP:
+      needs_tap_ending_event_ = false;
+      break;
+    case GESTURE_TAP_CANCEL:
+      if (!needs_tap_ending_event_)
+        return;
+      needs_tap_ending_event_ = false;
+      break;
+    case GESTURE_SHOW_PRESS:
+      needs_show_press_event_ = false;
+      break;
+    case GESTURE_LONG_TAP:
+      needs_tap_ending_event_ = false;
+      current_longpress_time_ = base::TimeTicks();
+      break;
+    case GESTURE_LONG_PRESS:
+      DCHECK(!scale_gesture_listener_->IsScaleGestureDetectionInProgress());
+      current_longpress_time_ = gesture.time;
+      break;
+    case GESTURE_PINCH_UPDATE:
+      pinch_in_progress_ = true;
+      break;
+    case GESTURE_PINCH_END:
+      pinch_in_progress_ = false;
+      break;
+    case GESTURE_SCROLL_BEGIN:
+      touch_scroll_in_progress_ = true;
+      SendTapCancelIfNecessary(*current_down_event_);
+      break;
+    case GESTURE_SCROLL_END:
+      touch_scroll_in_progress_ = false;
+      break;
+    default:
+      break;
+  };
+
+  client_->OnGestureEvent(gesture);
+}
+
+void GestureProvider::SendTapCancelIfNecessary(const MotionEvent& event) {
+  if (!needs_tap_ending_event_)
+    return;
+  current_longpress_time_ = base::TimeTicks();
+  Send(CreateGesture(GESTURE_TAP_CANCEL, event));
+}
+
+bool GestureProvider::SendLongTapIfNecessary(const MotionEvent& event) {
+  if (event.GetAction() == MotionEvent::ACTION_UP &&
+      !current_longpress_time_.is_null() &&
+      !scale_gesture_listener_->IsScaleGestureDetectionInProgress()) {
+    SendTapCancelIfNecessary(event);
+    GestureEventParams::Data long_tap_data;
+    long_tap_data.long_press.width = event.GetTouchMajor();
+    long_tap_data.long_press.height = long_tap_data.long_press.width;
+    Send(CreateGesture(GESTURE_LONG_TAP, event, long_tap_data));
+    return true;
+  }
+  return false;
+}
+
+void GestureProvider::EndTouchScrollIfNecessary(base::TimeTicks time,
+                                                bool send_scroll_end_event) {
+  if (!touch_scroll_in_progress_)
+    return;
+  touch_scroll_in_progress_ = false;
+  if (send_scroll_end_event)
+    Send(CreateGesture(GESTURE_SCROLL_END, time, 0, 0));
+}
+
+}  //  namespace ui