somewhat functioning gesture service

1 files changed, 71 insertions, 0 deletions

diff --git a/src/services/gesture_service/gesture_service.js b/src/services/gesture_service/gesture_service.js
index e69de29b..2e39003c 100644
--- a/src/services/gesture_service/gesture_service.js
+++ b/src/services/gesture_service/gesture_service.js
@@ -0,0 +1,71 @@
+
+const DIRECTION_LEFT = [-1, 0]
+const DIRECTION_RIGHT = [1, 0]
+const DIRECTION_UP = [0, -1]
+const DIRECTION_DOWN = [0, 1]
+
+const deltaCoord = (oldCoord, newCoord) => [newCoord[0] - oldCoord[0], newCoord[1] - oldCoord[1]]
+
+const touchEventCoord = e => ([e.touches[0].screenX, e.touches[0].screenY])
+
+const vectorLength = v => Math.sqrt(v[0] * v[0] + v[1] * v[1])
+
+const perpendicular = v => [v[1], v[0]]
+
+const dotProduct = (v1, v2) => v1[0] * v2[0] + v1[1] * v2[1]
+
+const vectorFlatten = (v1, v2) => [v1[0] * v2[0], v1[1] * v2[1]]
+
+// direction: either use the constants above or an arbitrary 2d vector.
+// threshold: how many Px to move from touch origin before checking if the
+//    callback should be called.
+// divergentTolerance: a scalr for much of divergent direction we tolerate when
+//    above threshold. for example, with 1.0 we only call the callback if
+//    divergent component of delta is < 1.0 * direction component of delta.
+const swipeGesture = (direction, onSwipe, threshold = 30, perpendicularTolerance = 1.0) => {
+  return {
+    direction,
+    onSwipe,
+    threshold,
+    perpendicularTolerance,
+    _startPos: [0, 0],
+    _swiping: false
+  }
+}
+
+const beginSwipe = (event, gesture) => {
+  gesture._startPos = touchEventCoord(event)
+  gesture._swiping = true
+}
+
+const updateSwipe = (event, gesture) => {
+  if (!gesture._swiping) return
+  // movement too small
+  const delta = deltaCoord(gesture._startPos, touchEventCoord(event))
+  if (vectorLength(delta) < gesture.threshold) return
+  // movement is opposite from direction
+  if (dotProduct(delta, gesture.direction) < 0) return
+  // movement perpendicular to direction is too much
+  const towardsDir = vectorFlatten(gesture.direction, delta)
+  const perpendicularDir = perpendicular(gesture.direction)
+  const towardsPerpendicular = vectorFlatten(perpendicularDir, delta)
+  if (
+    vectorLength(towardsDir) <
+    gesture.perpendicularTolerance * vectorLength(towardsPerpendicular)
+  ) return
+
+  gesture.onSwipe()
+  gesture._swiping = false
+}
+
+const GestureService = {
+  DIRECTION_LEFT,
+  DIRECTION_RIGHT,
+  DIRECTION_UP,
+  DIRECTION_DOWN,
+  swipeGesture,
+  beginSwipe,
+  updateSwipe
+}
+
+export default GestureService