Slop region check for multi-finger scroll

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 <stddef.h>
 
 #include <cmath>
 
@@ skipping 100 matching lines @@
       current_down_time_ = event.GetEventTime();
       current_longpress_time_ = base::TimeTicks();
       ignore_single_tap_ = false;
       scroll_event_sent_ = false;
       pinch_event_sent_ = false;
       show_press_event_sent_ = false;
       gesture_detector_.set_longpress_enabled(true);
       tap_down_point_ = gfx::PointF(event.GetX(), event.GetY());
       max_diameter_before_show_press_ = event.GetTouchMajor();
     }
-
     gesture_detector_.OnTouchEvent(event);
     scale_gesture_detector_.OnTouchEvent(event);
 
     if (action == MotionEvent::ACTION_UP ||
         action == MotionEvent::ACTION_CANCEL) {
       // Note: This call will have no effect if a fling was just generated, as
       // |Fling()| will have already signalled an end to touch-scrolling.
       if (scroll_event_sent_)
         Send(CreateGesture(ET_GESTURE_SCROLL_END, event));
       current_down_time_ = base::TimeTicks();
@@ skipping 149 matching lines @@
     // Return true to indicate that we want to handle touch.
     return true;
   }
 
   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 (!scroll_event_sent_ && e2.GetPointerCount() == 1) {
-      // Remove the touch slop region from the first scroll event to
-      // avoid a jump. Touch slop isn't used for multi-finger
-      // gestures, so in those cases we don't subtract the slop.
+    if (!scroll_event_sent_ && e2.GetPointerCount() < 3) {
+      // Remove the touch slop region from the first scroll event to avoid a
+      // jump. Touch slop isn't used for scroll gestures with greater than 2
+      // pointers down, in those cases we don't subtract the slop.
       float distance =
           std::sqrt(distance_x * distance_x + distance_y * distance_y);
       float epsilon = 1e-3f;
       if (distance > epsilon) {
-        float ratio =
-            std::max(0.f,
-                     distance - config_.gesture_detector_config.touch_slop) /
-            distance;
+        float subtraction_value = config_.gesture_detector_config.touch_slop /
+                                  e2.GetPointerCount();
+        float ratio = std::max(0.f, distance - subtraction_value ) / distance;

[tdresser, 2016/06/10 19:34:57]

Is this logic correct? If the two pointers move in opposite directions and
exceed their slop regions, and then both scroll 1px down, what will happen?

[sahel, 2016/06/23 22:34:44]

Done.

         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
@@ skipping 525 matching lines @@
   // null'ing of the listener until the sequence has ended.
   if (current_down_event_)
     return;
 
   const bool double_tap_enabled =
       double_tap_support_for_page_ && double_tap_support_for_platform_;
   gesture_listener_->SetDoubleTapEnabled(double_tap_enabled);
 }
 
 }  //  namespace ui