public function watch(callable $cb, $cbData = null)
{
if ($this->req) {
$this->watchers[] = [$cb, $cbData];
if (!$this->parsing) {
$this->parsing = true;
\Amp\immediately(function () {
return $this->initIncremental();
});
}
} elseif (!$this->parsing) {
$this->watchers[] = [$cb, $cbData];
}
return $this;
}