javascript - 如何从本地的 Location.getCurrentPositionAsync() 结果中获取经纬度

Question

这是我获取设备当前位置的代码。

import React, { Component } from 'react';
import { Platform, Text, View, StyleSheet, FlatList } from 'react-native';
import { Constants, Location, Permissions, MapView } from 'expo';

export default class Home extends Component {

    state = {
        location: null,
        errorMessage: null,
    };

    componentWillMount() {
        if (Platform.OS === 'android' && !Constants.isDevice) {
            this.setState({
                errorMessage: 'Oops, this will not work on Sketch in an Android emulator. Try it on your device!',
            });
        } else {
            this._getLocationAsync();
        }
    }

    _getLocationAsync = async () => {
        let { status } = await Permissions.askAsync(Permissions.LOCATION);
        if (status !== 'granted') {
            this.setState({
                errorMessage: 'Permission to access location was denied',
            });
        }

        let location = await Location.getCurrentPositionAsync({ enableHighAccuracy: true });
        this.setState({ location });
    };

    render() {
        let text = 'Waiting..';
        if (this.state.errorMessage) {
            text = this.state.errorMessage;
        } else if (this.state.location) {
            text = JSON.stringify(this.state.location);
        }
        console.log(text)
        return (
            <MapView
                style={{ flex: 1 }}
                region={{
                    latitude: text.coords.latitude,
                    longitude: text.coords.longitude,
                    latitudeDelta: 0.1,
                    longitudeDelta: 0.1,
                }}
            />
        );
    }
}

const styles = StyleSheet.create({
    container: {
        flex: 1,
        alignItems: 'center',
        justifyContent: 'center',
        paddingTop: Constants.statusBarHeight,
        backgroundColor: '#ecf0f1',
    },
    paragraph: {
        margin: 24,
        fontSize: 18,
        textAlign: 'center',
    },
});

我想做的是将纬度和对数传递给 MapView。但这不起作用。

的输出console.log(text) 是

{
  "timestamp":1551594077000,
  "mocked":false,
  "coords":{
    "heading":0,
    "longitude":80.4380389,
    "speed":0,
    "altitude":-78,
    "latitude":6.0140343,
    "accuracy":21.238000869750977

  }

我正在使用 expo 应用程序在我的智能手机 (galaxy j5 10) 中运行这个项目。所有位置权限都已授予该应用程序，但仍无法正常工作。我尝试了很多文档，但没有用。我该如何纠正这个。

Answer 1

您的错误是由您的渲染方法引起的:

render() {
    let text = 'Waiting..';
    if (this.state.errorMessage) {
        text = this.state.errorMessage;
    } else if (this.state.location) {
        text = JSON.stringify(this.state.location);
    }
    console.log(text)
    return (
        <MapView
            style={{ flex: 1 }}
            region={{
                latitude: text.coords.latitude,
                longitude: text.coords.longitude,
                latitudeDelta: 0.1,
                longitudeDelta: 0.1,
            }}
        />
    );
}

当 this.state.errorMessage 为空时，您没有为 this.state.location 设置值，因此您的 MapView 会尝试使用您设置为 text 的值，这不起作用，因为 this.state.location 为 null，如果您尝试访问其上的值，将会抛出错误。

当您获得位置时，您可以使用 JSON.stringify 将位置对象转换为字符串，但这会阻止您访问该对象的属性。

当 this.state.errorMessage 和 this.state.location 都为空时，您的 text 只是一个字符串，因此它会导致MapView 出错，因为您正试图访问字符串上的对象属性。

你应该这样做:

1. 设置加载状态的初始值
2. 在 _getLocationAsync 中设置加载状态
3. 仅在获得许可后才检查位置
4. 重构渲染，使其处理组件的加载(它应该显示 3 种不同输出之一，未加载、加载但有错误、已加载但有位置)

这里是重构

export default class Home extends Component {
  state = {
    location: null,
    errorMessage: null,
    loaded: false
  };
  // componentWillMount has been deprecated, use componentDidMount instead
  componentDidMount () {
    if (Platform.OS === 'android' && !Constants.isDevice) {
      this.setState({
        errorMessage: 'Oops, this will not work on Sketch in an Android emulator. Try it on your device!',
        loaded:true
      });
    } else {
      this._getLocationAsync();
    }
  }

  _getLocationAsync = async () => {
    let { status } = await Permissions.askAsync(Permissions.LOCATION);
    if (status !== 'granted') {
      this.setState({
        errorMessage: 'Permission to access location was denied',
        loaded: true
      });
    } else {
      // only check the location if it has been granted
      // you also may want to wrap this in a try/catch as async functions can throw
      let location = await Location.getCurrentPositionAsync({ enableHighAccuracy: true });
      this.setState({ location, loaded: true, errorMessage: null });
    }
  };

  render () {
    // check to see if we have loaded
    if (this.state.loaded) {
      // if we have an error message show it
      if (this.state.errorMessage) {
        return (
          <View style={styles.container}>
            <Text>{JSON.stringify(this.state.errorMessage)}</Text>
          </View>
        );
      } else if (this.state.location) {
        // if we have a location show it
        return (
          <MapView
            style={{ flex: 1 }}
            region={{
              latitude: this.state.location.coords.latitude,
              longitude: this.state.location.coords.longitude,
              latitudeDelta: 0.1,
              longitudeDelta: 0.1
            }}
          />
        );
      }
    } else {
      // if we haven't loaded show a waiting placeholder
      return (
        <View style={styles.container}>
          <Text>Waiting...</Text>
        </View>
      );
    }
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    alignItems: 'center',
    justifyContent: 'center',
    paddingTop: Constants.statusBarHeight,
    backgroundColor: '#ecf0f1'
  },
  paragraph: {
    margin: 24,
    fontSize: 18,
    textAlign: 'center'
  }
});