ScaleGestureRecognizer specification

This is the engineering specification for the ScaleGestureRecognizer object.

Overview

A scale gesture recognizer listens to input events and generates scale events relative to an initial scale.

This gesture recognizer requires two input independent event streams in order to calculate the scale.

MVP

Is a GestureRecognizer

ScaleGestureRecognizer conforms to the GestureRecognizer protocol.

Pseudo-code example:

class ScaleGestureRecognizer: GestureRecognizer {
}

Scale API

Expose an API for reading the current scale of the gesture recognizer in relation to the associated element.

For two-dimensional displays the scale should be expressed in terms of a linear scale applied to both the x and y components.

class ScaleGestureRecognizer {
  func scale() -> Number

Velocity API

Expose an API for reading the current scale velocity of the input events in relation to the associated element.

TODO: Spec out an implementation for calculating velocity.

class ScaleGestureRecognizer {
  func velocity() -> Number

Recognition threshold API

Expose an API for setting the scale threshhold that must be passed before this gesture recognizer begins emitting Changed gesture events.

The default value is platform-dependent.

class ScaleGestureRecognizer {
  var recognitionThreshold: Vector

Event algorithm

When two input streams are registered with the gesture recognizer then a vector should be calculated from the first stream’s position to the second’s. This is the reference vector.

Calculate a new vector from the first event stream’s position to the second on each new event. The generated scale gesture event value is the relative length of this new vector to the reference vector.