React Native Map - Getting markers from the server

问题

I am trying to dynamically get nearby markers from the server and print on a map without success. When I update map location I am getting below warnings. The expected behaviour is getting markers printed on the map when I drag map but instead it doesn't show any dynamic fetched markers but show warnings.

20:27:15: [Unhandled promise rejection: TypeError: undefined is not an object (evaluating 'object[key]')] - node_modules/immutability-helper/index.js:81:44 in - node_modules/immutability-helper/index.js:73:29 in update - ... 16 more stack frames from framework internals

App.js (Main file)

import React, { Component } from 'react';
import { Text, View, StyleSheet, Switch, Alert, AppRegistry } from 'react-native';
import MapView from 'react-native-maps';
import Fetchdata from './Fetchdata.js';

const styles = StyleSheet.create({
  container: {
    ...StyleSheet.absoluteFillObject,
    height: 400,
    width: 400,
    justifyContent: 'flex-end',
    alignItems: 'center',
  },
  map: {
    ...StyleSheet.absoluteFillObject,
  },
});

export default class myScreen extends Component {
  render() {
    return (
      <View style ={styles.container}>
        <Fetchdata />
      </View>
    );
  }
} 

Fetchdata.js (the file getting server data)

import React, { Component } from 'react'
import { Text, View, StyleSheet, Switch, Alert, AppRegistry} from 'react-native'
import MapView, {Marker, ProviderPropType} from 'react-native-maps';
import update from 'immutability-helper';

const styles = StyleSheet.create({
  container: {
    ...StyleSheet.absoluteFillObject,
    height: 400,
    width: 400,
    justifyContent: 'flex-end',
    alignItems: 'center',
  },
  map: {
    ...StyleSheet.absoluteFillObject,
  },
});
export default class Fetchdata extends Component {
  constructor (props) {
    super(props);
  };

  state = {
    latitude: 40.3565,
    longitude: 27.9774,
    markers: [
      {
        key: 1,
        latlng: {
          latitude: 40.3565,
          longitude: 27.9774
        }
      }
    ]
  };
  componentDidMount = () => {
    navigator.geolocation.getCurrentPosition(
        (position) => {
          this.setState({
            latitude: position.coords.latitude,
            longitude: position.coords.longitude,
            error: null,
      });
    },
      (error) => this.setState({ error: error.message }),
      { enableHighAccuracy: true, timeout: 20000, maximumAge: 1000 },
    );
   }
   onRegionChange (region) {
       fetch('https://isg.info.tr/query_maps.php' + '?latitude=' + region.latitude + '&longitude=' + region.longitude , {method: 'GET'})
        .then((response) => response.json())
        .then((responseJson) => {
          var newlatlng;
          for (i = 0 ; i< responseJson.length; i++) {
            newlatlng = update(this.state, {markers: { 
                                                        key: { $set: i }, 
                                                        latlng: { latitude: { $set: responseJson.latitude },
                                                        longitude: { $set:  responseJson.longitude  } 
                                                      } 
                                            }        
                                          });
            this.setState({markers});
          }
        })
   };
   render() {
      return (
        <View style={styles.container}>
          <MapView
                style={styles.map}
                initialRegion={{
                latitude: this.state.latitude,
                longitude: this.state.longitude,
                latitudeDelta: 0.015,
                longitudeDelta: 0.015,
            }}
            onRegionChange={this.onRegionChange.bind(this)}
            >
            {this.state.markers.map((marker, index) => (
              <MapView.Marker key={index} coordinate={marker.latlng} title={'marker.title'} />
            ))}
          </MapView>
      </View>
      );
   }
}
Fetchdata.propTypes = {
  provider: ProviderPropType,
};

Jsondata get from the server:

[{"latlng":{"latitude":"40.3565","longitude":"27.9774"}},{"latlng":{"latitude":"40.3471","longitude":"27.9598"}},{"latlng":{"latitude":"40","longitude":"27.9708"}}]

回答1:

I am not sure about the update function but here you are assigning undefined values:

latlng: { latitude: { $set: responseJson.latitude }, longitude: { $set: responseJson.longitude }

responseJson does not have latitude and longitude as children. They have LatLng

来源：https://stackoverflow.com/questions/49905341/react-native-map-getting-markers-from-the-server