GestureRecognizer specification

This is the engineering specification for the GestureRecognizer abstract type.

Overview

A gesture recognizer listens to input events and translates them into meaningful interaction events.

MVP

Abstract type

GestureRecognizer is a protocol, if your language has that concept.

Pseudo-code example:

protocol GestureRecognizer {
}

Element API

A gesture recognizer should expose an API for reading the element to which the recognizer is associated.

protocol GestureRecognizer {
  let element

Element association

How a gesture recognizer is associated with an element is a platform implementation detail.

Event API

A gesture recognizer should expose an API for adding objects that will receive state change events.

These observers should be invoked each time the gesture recognizer’s state value is written to.

protocol GestureRecognizer {
  func addStateChangeObserver(observer)

Enabled API

A gesture recognizer should expose an API for changing its enabled state.

A disabled gesture recognizer will not emit any state change events.

protocol GestureRecognizer {
  var enabled: Bool

States

A gesture recognizer can be in any one of the following states:

enum GestureRecognizerState {
  case Possible
  case Began
  case Changed
  case Ended
  case Cancelled
  case Failed

  case Recognized = Ended
}

State API

A gesture recognizer has a read-only state API.

protocol GestureRecognizer {
  let state: GestureRecognizerState

Location API

A gesture recognizer should expose an API for reading the centroid touch location in a given element’s coordinate system.

The method is responsible for converting the touch location to the relevant coordinate system.

protocol GestureRecognizer {
  func locationInElement(element: Element) -> Point