javascript - 将折线与本机地图一起使用时出现空引用错误
问题描述
当我使用折线时,它给了我这个错误:
attempt to invoke interface method 'java.util.iterator java.util.list.iterator()' on a null object
起初,我以为我做错了什么,或者我误用了 google-map API,所以我尝试了一些硬编码的坐标(如下面的代码所示),但没有任何进展。如果我删除折线部分,代码将运行良好。我用谷歌搜索希望找到任何解决方案,但也没有任何成功。
- 我关于开发平台的一些信息:
- 世博会 2.19.1 版
- 纱线版本 1.16.0
- 节点版本 10.15.2
在安装了 android PI 的物理设备上进行测试。
import React from 'react';
import { StyleSheet, Text, View } from 'react-native';
import { Permissions, MapView } from 'expo'
// import Polyline from '@mapbox/polyline'
import { Polyline } from 'react-native-maps'
const locations = require('./locations.json')
export default class App extends React.Component {
state = {
latitude: null,
longitude: null,
locations: locations
}
componentDidMount() {
this.GetLocations()
}
GetLocations = async () => {
const { status } = await Permissions.getAsync(Permissions.LOCATION)
if (status !== 'granted') {
const response = await Permissions.askAsync(Permissions.LOCATION)
}
navigator.geolocation.getCurrentPosition(
({ coords: { latitude, longitude }}) => this.setState({ latitude, longitude}, this.mergeCoords),
(err) => console.log(`Error: ${err}`)
)
const { locations: [sampleLocation] } = this.state
this.setState({
desLatitude: sampleLocation.coords.latitude,
desLongitude: sampleLocation.coords.longitude,
}, this.mergeCoords)
}
mergeCoords = () => {
const { latitude, longitude, desLatitude, desLongitude } = this.state
const hasStartAndEnd = ( latitude !== null && desLatitude !== null )
// if the line have start and end
if (hasStartAndEnd) {
const concatStart = `${latitude},${longitude}`
const concatEnd = `${desLatitude},${desLongitude}`
this.getDirections(concatStart, concatEnd)
}
}
async getDirections(startLoc, desLoc) {
try {
// const res = await fetch(`https://maps.googleapis.com/maps/api/directions/json?key=MY_API_KEY&origin=${startLoc}&destination=${desLoc}`)
// const resJson = await res.json()
// const points = Polyline.decode(resJson.routes[0].overview_polyline.points)
// const coords = points.map( point => {
// return {
// latitude: point[0],
// longitude: point[1]
// }
// })
const coords = {
latitude: 31.262353,
longitude: 29.989506,
}
console.log("point, coords: ", coord)
this.setState({coords})
} catch(err) {
console.log('Error: ', err)
}
}
render() {
const { latitude, longitude, coords } = this.state
if (latitude) {
return (
<MapView
style={{ flex: 1 }}
showsUserLocation
initialRegion={{
latitude,
longitude,
latitudeDelta: 0.0922,
longitudeDelta: 0.0421,
}}
>
<MapView.Polyline
strokeWidth={2}
strokeColor="red"
coordinates={coords}
/>
</MapView>
)
}
return (
<View style={{ flex: 1, justifyContent: 'center', alignItems: 'center'}}>
<Text>We need your permissions !!</Text>
</View>
)
}
}
const styles = StyleSheet.create({
container: {
flex: 1,
backgroundColor: '#fff',
alignItems: 'center',
justifyContent: 'center',
},
});
解决方案
//use condional rendering
import React from 'react';
import { StyleSheet, Text, View } from 'react-native';
import { Permissions, MapView } from 'expo'
// import Polyline from '@mapbox/polyline'
import { Polyline } from 'react-native-maps'
const locations = require('./locations.json')
export default class App extends React.Component {
state = {
latitude: null,
longitude: null,
locations: locations,
direct : false
}
componentDidMount() {
this.GetLocations()
}
GetLocations = async () => {
const { status } = await Permissions.getAsync(Permissions.LOCATION)
if (status !== 'granted') {
const response = await Permissions.askAsync(Permissions.LOCATION)
}
navigator.geolocation.getCurrentPosition(
({ coords: { latitude, longitude }}) => this.setState({ latitude, longitude},
this.mergeCoords),
(err) => console.log(`Error: ${err}`)
)
const { locations: [sampleLocation] } = this.state
this.setState({
desLatitude: sampleLocation.coords.latitude,
desLongitude: sampleLocation.coords.longitude,
}, this.mergeCoords)
}
mergeCoords = () => {
const { latitude, longitude, desLatitude, desLongitude } = this.state
const hasStartAndEnd = ( latitude !== null && desLatitude !== null )
// if the line have start and end
if (hasStartAndEnd) {
const concatStart = `${latitude},${longitude}`
const concatEnd = `${desLatitude},${desLongitude}`
this.getDirections(concatStart, concatEnd)
}
}
async getDirections(startLoc, desLoc) {
try {
// const res = await fetch(`https://maps.googleapis.com/maps/api/directions/json?
key=MY_API_KEY&origin=${startLoc}&destination=${desLoc}`)
// const resJson = await res.json()
// const points = Polyline.decode(resJson.routes[0].overview_polyline.points)
// const coords = points.map( point => {
// return {
// latitude: point[0],
// longitude: point[1]
// }
// })
const coords = {
latitude: 31.262353,
longitude: 29.989506,
}
console.log("point, coords: ", coord)
this.setState({coords, direct:true})
} catch(err) {
console.log('Error: ', err)
}
}
render() {
const { latitude, longitude, coords } = this.state
if (latitude) {
return (
<MapView
style={{ flex: 1 }}
showsUserLocation
initialRegion={{
latitude,
longitude,
latitudeDelta: 0.0922,
longitudeDelta: 0.0421,
}}
>
{this.state.direct &&
<MapView.Polyline
strokeWidth={2}
strokeColor="red"
coordinates={coords}
/>}
</MapView>
)
}
return (
<View style={{ flex: 1, justifyContent: 'center', alignItems: 'center'}}>
<Text>We need your permissions !!</Text>
</View>
)
} }
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