Port of Android platform gesture detection code to C++

ui/events/gesture_detection/snap_scroll_controller.cc

Index: ui/events/gesture_detection/snap_scroll_controller.cc
diff --git a/ui/events/gesture_detection/snap_scroll_controller.cc b/ui/events/gesture_detection/snap_scroll_controller.cc
new file mode 100644
index 0000000000000000000000000000000000000000..e9f318c14b29aed7d3bdf0d1c276aec9477627cc
--- /dev/null
+++ b/ui/events/gesture_detection/snap_scroll_controller.cc
@@ -0,0 +1,115 @@
+// 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/snap_scroll_controller.h"
+
+#include <cmath>
+
+#include "ui/events/gesture_detection/motion_event.h"
+
+namespace ui {
+namespace {
+const int kSnapBound = 16;
+}  // namespace
+
+SnapScrollController::Config::Config()
+    : screen_width_pixels(1), screen_height_pixels(1), device_scale_factor(1) {}
+
+SnapScrollController::Config::~Config() {}
+
+SnapScrollController::SnapScrollController(const Config& config)
+    : channel_distance_(CalculateChannelDistance(config)),
+      snap_scroll_mode_(SNAP_NONE),
+      first_touch_x_(-1),
+      first_touch_y_(-1),
+      distance_x_(0),
+      distance_y_(0) {}
+
+SnapScrollController::~SnapScrollController() {}
+
+void SnapScrollController::UpdateSnapScrollMode(float distance_x,
+                                                float distance_y) {
+  if (snap_scroll_mode_ == SNAP_HORIZ || snap_scroll_mode_ == SNAP_VERT) {
+    distance_x_ += std::abs(distance_x);
+    distance_y_ += std::abs(distance_y);
+    if (snap_scroll_mode_ == SNAP_HORIZ) {
+      if (distance_y_ > channel_distance_) {
+        snap_scroll_mode_ = SNAP_NONE;
+      } else if (distance_x_ > channel_distance_) {
+        distance_x_ = 0;
+        distance_y_ = 0;
+      }
+    } else {
+      if (distance_x_ > channel_distance_) {
+        snap_scroll_mode_ = SNAP_NONE;
+      } else if (distance_y_ > channel_distance_) {
+        distance_x_ = 0;
+        distance_y_ = 0;
+      }
+    }
+  }
+}
+
+void SnapScrollController::SetSnapScrollingMode(
+    const MotionEvent& event,
+    bool is_scale_gesture_detection_in_progress) {
+  switch (event.GetAction()) {
+    case MotionEvent::ACTION_DOWN:
+      snap_scroll_mode_ = SNAP_NONE;
+      first_touch_x_ = event.GetX();
+      first_touch_y_ = event.GetY();
+      break;
+    // Set scrolling mode to SNAP_X if scroll towards x-axis exceeds kSnapBound
+    // and movement towards y-axis is trivial.
+    // Set scrolling mode to SNAP_Y if scroll towards y-axis exceeds kSnapBound
+    // and movement towards x-axis is trivial.
+    // Scrolling mode will remain in SNAP_NONE for other conditions.
+    case MotionEvent::ACTION_MOVE:
+      if (!is_scale_gesture_detection_in_progress &&
+          snap_scroll_mode_ == SNAP_NONE) {
+        int x_diff = static_cast<int>(std::abs(event.GetX() - first_touch_x_));
+        int y_diff = static_cast<int>(std::abs(event.GetY() - first_touch_y_));
+        if (x_diff > kSnapBound && y_diff < kSnapBound) {
+          snap_scroll_mode_ = SNAP_HORIZ;
+        } else if (x_diff < kSnapBound && y_diff > kSnapBound) {
+          snap_scroll_mode_ = SNAP_VERT;
+        }
+      }
+      break;
+    case MotionEvent::ACTION_UP:
+    case MotionEvent::ACTION_CANCEL:
+      first_touch_x_ = -1;
+      first_touch_y_ = -1;
+      distance_x_ = 0;
+      distance_y_ = 0;
+      break;
+    default:
+      break;
+  }
+}
+
+// static
+float SnapScrollController::CalculateChannelDistance(const Config& config) {
+  float channel_distance = 16.f;
+
+  const float screen_size = std::abs(
+      hypot((float)config.screen_width_pixels / config.device_scale_factor,
+            (float)config.screen_height_pixels / config.device_scale_factor));
+  if (screen_size < 3.f) {
+    channel_distance = 16.f;
+  } else if (screen_size < 5.f) {
+    channel_distance = 22.f;
+  } else if (screen_size < 7.f) {
+    channel_distance = 28.f;
+  } else {
+    channel_distance = 34.f;
+  }
+  channel_distance = channel_distance * config.device_scale_factor;
+  if (channel_distance < 16.f)
+    channel_distance = 16.f;
+
+  return channel_distance;
+}
+
+}  // namespace ui