constructor() {
    super();
    this.unsubscribe = null;
    this.ref = firebase.firestore().collection("termins");
  }
  state = {
    snapshots: [],
    loading: true
  };
componentDidMount() {
    this.unsubscribe = this.ref.onSnapshot(this.onCollectionUpdate)
}

componentWillUnmount() {
    this.unsubscribe();
}
onCollectionUpdate = (querySnapshot) => {
  const todos = [];
  querySnapshot.forEach((doc) => {
    const { time, salonname } = doc.data();

    todos.push({
      key: doc.id,
      doc, // DocumentSnapshot
      time,
      salonname,
    });
  });

  this.setState({
    todos,
    loading: false,
 });
}