live-set/map.js.flow

Class representing a changing and transformable collection of items.

/* @flow */

import LiveSet from '.';

export default function map<T,U>(liveSet: LiveSet<T>, cb: (value: T) => U): LiveSet<U> {
  return new LiveSet({
    scheduler: liveSet.getScheduler(),
    read() {
      const s = new Set();
      liveSet.values().forEach(value => {
        s.add(cb(value));
      });
      return s;
    },
    listen(setValues, controller) {
      const m: Map<T,U> = new Map();

      const sub = liveSet.subscribe({
        start() {
          const s = new Set();
          liveSet.values().forEach(value => {
            const newValue = cb(value);
            m.set(value, newValue);
            s.add(newValue);
          });
          setValues(s);
        },
        next(changes) {
          changes.forEach(change => {
            if (change.type === 'add') {
              const newValue = cb(change.value);
              m.set(change.value, newValue);
              controller.add(newValue);
            } else if (change.type === 'remove') {
              const newValue = m.get(change.value);
              if (!newValue) throw new Error('removed item not in liveset');
              m.delete(change.value);
              controller.remove(newValue);
            }
          });
        },
        error(err) {
          controller.error(err);
        },
        complete() {
          controller.end();
        }
      });

      return sub;
    }
  });
}