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我正在尝试创建位置跟踪器的简单示例，但我遇到了以下情况。我的基本目标是通过按开始/结束按钮来切换位置手表。我通过实现自定义反应钩子来分离关注点，然后在 App 组件中使用它：

使用WatchLocation.js

import {useEffect, useRef, useState} from "react"
import {PermissionsAndroid} from "react-native"
import Geolocation from "react-native-geolocation-service"

const watchCurrentLocation = async (successCallback, errorCallback) => {
  if (!(await PermissionsAndroid.check(PermissionsAndroid.PERMISSIONS.ACCESS_FINE_LOCATION))) {
    errorCallback("Permissions for location are not granted!")
  }
  return Geolocation.watchPosition(successCallback, errorCallback, {
    timeout: 3000,
    maximumAge: 500,
    enableHighAccuracy: true,
    distanceFilter: 0,
    useSignificantChanges: false,
  })
}

const stopWatchingLocation = (watchId) => {
  Geolocation.clearWatch(watchId)
  // Geolocation.stopObserving()
}

export default useWatchLocation = () => {
  const [location, setLocation] = useState()
  const [lastError, setLastError] = useState()
  const [locationToggle, setLocationToggle] = useState(false)
  const watchId = useRef(null)

  const startLocationWatch = () => {
    watchId.current = watchCurrentLocation(
      (position) => {
        setLocation(position)
      },
      (error) => {
        setLastError(error)
      }
    )
  }

  const cancelLocationWatch = () => {
    stopWatchingLocation(watchId.current)
    setLocation(null)
    setLastError(null)
  }

  const setLocationWatch = (flag) => {
    setLocationToggle(flag)
  }

  // execution after render when locationToggle is changed
  useEffect(() => {
    if (locationToggle) {
      startLocationWatch()
    } else cancelLocationWatch()
    return cancelLocationWatch()
  }, [locationToggle])

  // mount / unmount
  useEffect(() => {
    cancelLocationWatch()
  }, [])

  return { location, lastError, setLocationWatch }
}

应用程序.js

import React from "react"
import {Button, Text, View} from "react-native"

import useWatchLocation from "./hooks/useWatchLocation"

export default App = () => {
  const { location, lastError, setLocationWatch } = useWatchLocation()
  return (
    <View style={{ margin: 20 }}>
      <View style={{ margin: 20, alignItems: "center" }}>
        <Text>{location && `Time: ${new Date(location.timestamp).toLocaleTimeString()}`}</Text>
        <Text>{location && `Latitude: ${location.coords.latitude}`}</Text>
        <Text>{location && `Longitude: ${location.coords.longitude}`}</Text>
        <Text>{lastError && `Error: ${lastError}`}</Text>
      </View>
      <View style={{ marginTop: 20, width: "100%", flexDirection: "row", justifyContent: "space-evenly" }}>
        <Button onPress={() => {setLocationWatch(true)}} title="START" />
        <Button onPress={() => {setLocationWatch(false)}} title="STOP" />
      </View>
    </View>
  )
}

我搜索了多个在线示例，上面的代码应该可以工作。但问题是当按下停止按钮时，即使我调用Geolocation.clearWatch(watchId) ，位置仍然会不断更新。

我包装了 Geolocation 调用以处理位置许可和其他可能的调试内容。似乎使用useWatchLocation中的useRef挂钩保存的watchId值无效。我的猜测是基于尝试在 Geolocation.clearWatch(watchId) 之后立即调用Geolocation.stopObserving( )。订阅停止，但我收到警告：

使用现有订阅调用 stopObserving。

所以我假设原始订阅没有被清除。

我错过了什么/做错了什么？

编辑：我想出了解决方案。但是由于 isMounted 模式通常被认为是反模式：有没有人有更好的解决方案？

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1 回答 1

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好的，问题解决了isMounted 模式。isMounted.current 设置为locationToggle生效true并在内部设置cancelLocationWatch为false：

const isMounted = useRef(null)

...
    
useEffect(() => {
        if (locationToggle) {
          isMounted.current = true              // <--
          startLocationWatch()
        } else cancelLocationWatch()
        return () => cancelLocationWatch()
      }, [locationToggle])
    
...    

const cancelLocationWatch = () => {
        stopWatchingLocation(watchId.current)
        setLocation(null)
        setLastError(null)
        isMounted.current = false               // <--
      }

并检查挂载/卸载效果、成功和错误回调：

const startLocationWatch = () => {
    watchId.current = watchCurrentLocation(
      (position) => {
        if (isMounted.current) {                // <--
          setLocation(position)
        }
      },
      (error) => {
        if (isMounted.current) {                // <--
          setLastError(error)
        }
      }
    )
  }
于 2020-10-04T20:41:31.480 回答