rubberBanded specification
This is the engineering specification for the MotionObservable operator: rubberBanded.
Overview
rubberBanded applies resistance to values that fall outside of the given range and emits the
result.
MVP
Expose a rubberBanded operator API
Use _map to implement the operator. Accept a number value. Emit the result of incoming + value.
class MotionObservable<number> {
public func rubberBanded(below: number, above: number, length: number) -> MotionObservable<number>
}
Implement the rubber-banding math
public func rubberBand(value: number, min: number, max: number, bandLength: number) -> number {
if value >= min && value <= max {
// While we're within range we don't rubber band the value.
return value
}
if bandLength <= 0 {
// The rubber band doesn't exist, return the minimum value so that we stay put.
return min
}
// 0.55 chosen as an approximation of iOS' rubber banding behavior.
let rubberBandCoefficient: number = 0.55
// Accepts values from [0...+inf and ensures that f(x) < bandLength for all values.
let band: (number) -> number = { value in
let demoninator = value * rubberBandCoefficient / bandLength + 1
return bandLength * (1 - 1 / demoninator)
}
if (value > max) {
return band(value - max) + max
} else if (value < min) {
return min - band(min - value)
}
return value
}
Apply the rubber band math
class MotionObservable<number> {
func rubberBanded(below: number, above: number, length: number) -> MotionObservable<number> {
return _map {
return rubberBand(value: $0, min: below, max: above, bandLength: length)
}
}
}