dragstream WIP

function pointersStream = function(target) {
  return Rx.Observable.from(
    [
      'pointerdown',
      'pointermove',
      'pointerup',
    ].map(
      type => Rx.Observable.fromEvent(target, type)
    )
  ).mergeAll().filter(
    event => event.pressure || event.type === 'pointerup'
  ).groupBy(
    event => event.pointerId
  ).map(
    pointerEventStream => pointerEventStream.map(
      event => (
        {
          id: event.pointerId,
          x: event.pageX,
          y: event.pageY,
          pressure: event.pressure,
        }
      )
    ).takeUntil(pointer => !pointer.pressure)
  ).zipLatest(
    currentPointers => // [pointer1, pointer2]
  );
}

// expected result
function zipLatest(predicate) {
  return Rx.Observable.create(
    observer => {
      const source = this;
      const latestMap = new Map();

      function dispatch() {
        observer.next(
          latestMap.values()
        );
      }

      source.subscribe(
        {
          next(innerStream) {
            innerStream.subscribe(
              {
                next(value) {
                  latestMap.set(innerStream, value);
                  dispatch();
                },

                complete() {
                  latestMap.delete(innerStream);
                  dispatch();
                }
              }
            )
          }
        }
      );
    }
  );
}

As a pen: http://codepen.io/appsforartists/pen/ALLNgO?editors=0011