pluck specification

This is the engineering specification for the MotionObservable operator: pluck.

This operator is reserved for platforms that support keyed type introspection or dictionary-based streams.

Overview

pluck extracts a value from a dictionary using a given path. Paths are dot-delimited keys, e.g. translate or translate.x.

Example usage

stream.pluck('x')

upstream        path  |  downstream
{x: 50, y: 20}  x     |  50
{x: 10, y:  0}  x     |  10
{x:  5, y: 60}  x     |   5
{x: 60, y: 35}  x     |  60
{x: 53, y: 32}  x     |  53
{x: 21, y: 57}  x     |  21
{x: 57, y: 74}  x     |  57

MVP

Expose a pluck operator API

Accept a single String argument path.

pluck<U>(path: string): MotionObservable<U>

Emit the new value

Split the path by . and iterate through the incoming dictionary with the resulting array of key values. Emit the final value or nothing if no keyed value exists.

const pathSegments = path.split('.');

return function plucker(value) {
  let result = value;

  for (let pathSegment of pathSegments) {
    result = result[pathSegment];
  }

  return result;
};