如何在 react-native 中从 Location.getCurrentPositionAsync() 的结果中获

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
}

我正在使用世博会应用程序在我的智能手机(Galaxy J5 10)中运行这个项目。所有位置权限都授予该应用程序，但仍然无法正常工作。我尝试了很多文档，但它不起作用。我该如何纠正这一点。

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 null 时，您不会为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，则会引发错误。

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

当this.state.errorMessage和this.state.location都为 null 时，您的text只是一个字符串，因此它会导致MapView出错，因为您正在尝试访问字符串上的对象属性。

你应该做这样的事情：

设置加载状态的初始值
在_getLocationAsync中设置加载状态
仅在授予权限时检查位置
重构渲染，以便它处理组件的加载(它应该显示 3 种不同的输出之一，未加载，加载但有错误，已加载但有位置)

这是重构

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'
}
});

看起来你已经用JSON.stringify(location)将对象转换为字符串。

不能使用 . 运算符访问字符串的属性。

相反，请尝试将其保留为对象并将值传递给映射组件。

您正在将位置转换为 JSON 字符串。纬度坐标应该是浮点值。试试这个：

state = {
region: {
longitude: -122.0840052, //Replace with any initial value here
latitude: 37.4220181,
longitudeDelta: 0.04,
latitudeDelta: 0.09
}
};
let { coords } = await Location.getCurrentPositionAsync({});
this.setState({ region: {
longitude: coords.longitude,
latitude: coords.latitude,
longitudeDelta: 0.04,
latitudeDelta: 0.09
} 
});
<MapView region={this.state.region} />

希望这有帮助！

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