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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 project = (v1, v2) => {
const scalar = (dotProduct(v1, v2) / dotProduct(v2, v2))
return [scalar * v2[0], scalar * 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 scalar 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 = project(delta, gesture.direction)
const perpendicularDir = perpendicular(gesture.direction)
const towardsPerpendicular = project(delta, perpendicularDir)
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
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