export function Watcher(db, selector, opts) {
	const subscribers = new Set();
	let changes = null;

	return function subscribe(fn) {
		subscribers.add(fn);

		if (subscribers.size === 1 && !changes) {
			changes = db
				.changes(
					Object.assign(
						{
							since: 'now',
							live: true,
							selector
						},
						opts
					)
				)
				.on('change', change => {
					const { id, deleted, doc } = change;
					subscribers.forEach(s => s(id, !!deleted, doc));
				})
				.on('error', err => {
					subscribers.clear();
					throw err;
				});
		}
		return () => {
			subscribers.delete(fn);
			if (subscribers.size === 0 && changes) {
				changes.cancel();
				changes = null;
			}
		};
	};
}