componentDidMount() {
    this.getCurrentLocation(); // Get users current location
}

componentDidUpdate(prevProps, prevState) {
    // If prevState changes
    if (prevState.newLatitude !== this.state.newLatitude) {
        // calculate the distance between initial and new coordinates
        this.getDistance();
    }
}

componentWillUnmount() {
    // Remember to clear all listeners
    navigator.geolocation.clearWatch(this.watchUserUserPosition());
}

getLocation() {
    navigator.geolocation.getCurrentPosition((position) => {
        this.setState({
            latitude: position.coords.latitude,
            longitude: position.coords.longitude
        }, () => {
            this.sendToFirebase();
            this.watchUserPosition(); //continue listening for location changes
        });
    },
    (error) => {
        //Handle error
    },
    { enableHighAccuracy: false, timeout: 200000, maximumAge: 1000 },
    );
}

watchUserPosition() {
    navigator.geolocation.watchPosition(
      (position) => {
        this.setState({
          newLatitude: position.coords.latitude,
          newLongitude: position.coords.longitude,
          error: null,
        });
      },
      (error) => this.setState({ error: error.message }),
      { enableHighAccuracy: true, timeout: 20000, maximumAge: 1000, distanceFilter: 10 },
    );
  }

getDistance() {
    let initial = {latitude: this.state.latitude, longitude: this.state.longitude};
    let newCoord = {latitude: this.state.newLatitude, longitude: this.state.newLongitude};
    const distance = geolib.getDistance(initial, newCoord);
    if (distance >== 100) {
        Alert.alert("Success!", "You have reached 100meters!");
        this.sendToFirebase(); // whatever you are saving to firebase
    }
}