zoukankan      html  css  js  c++  java
  • [React] Safely setState on a Mounted React Component through the useEffect Hook

    In the class version of this component, we had a method called safeSetState which would check whether the component was still mounted before trying to call setState. This is because our graphql client library is unable to cancel in-flight requests. Let's make that same kind of thing work by tracking the mounted state of our component using the useRef and useEffect hooks.

    We want a "lock", which should run once when component inited, after component unmounted, it should be reseted.

    We can use 'useRef' to build a container to hold our lock:

      const mountedRef = useRef(false);

    Then we can use useEffect:

      useEffect(() => {
        mountedRef.current = true
        return () => (mountedRef.current = false)
      }, [])

    The reason to use '[]' as second arguement, is because we don't want useEffect be triggered second times, we only want to run once, therefore, we use empty array, it won't trigger scecond time.

    Then we can create a safe setSetate function:

      const [state, setState] = useReducer(
        (state, newState) => ({...state, ...newState}),
        {
          loaded: false,
          fetching: false,
          data: null,
          error: null,
        },
      )
    
     const setSafeState = (...args) => mountedRef.current && setState(...args);

    ----

    Full code:

    import {useContext, useReducer, useEffect, useRef} from 'react'
    import PropTypes from 'prop-types'
    import isEqual from 'lodash/isEqual'
    import * as GitHub from '../../../github-client'
    
    function useSetState(initialState) {
      return useReducer(
        (state, newState) => ({...state, ...newState}),
        initialState,
      )
    }
    
    function useSafeSetState(initialState) {
      const [state, setState] = useSetState(initialState)
    
      const mountedRef = useRef(false)
      useEffect(() => {
        mountedRef.current = true
        return () => (mountedRef.current = false)
      }, [])
      const safeSetState = (...args) => mountedRef.current && setState(...args)
    
      return [state, safeSetState]
    }
    
    function Query({query, variables, normalize = data => data, children}) {
      const client = useContext(GitHub.Context)
      const [state, setState] = useSafeSetState({
        loaded: false,
        fetching: false,
        data: null,
        error: null,
      })
    
      useEffect(() => {
        if (isEqual(previousInputs.current, [query, variables])) {
          return
        }
        setState({fetching: true})
        client
          .request(query, variables)
          .then(res =>
            setState({
              data: normalize(res),
              error: null,
              loaded: true,
              fetching: false,
            }),
          )
          .catch(error =>
            setState({
              error,
              data: null,
              loaded: false,
              fetching: false,
            }),
          )
      })
    
      const previousInputs = useRef()
      useEffect(() => {
        previousInputs.current = [query, variables]
      })
    
      return children(state)
    }
    
    Query.propTypes = {
      query: PropTypes.string.isRequired,
      variables: PropTypes.object,
      children: PropTypes.func.isRequired,
      normalize: PropTypes.func,
    }
    
    export default Query
  • 相关阅读:
    Android对话框自定义标题
    JSONObject和JSONArray的关系
    Java的List排序
    重写onStart()函数
    SSH框架执行自己定义的SQL语句
    nested exception is org.hibernate.QueryException: could not resolve property
    java.lang.NullPointerException org.apache.struts2.impl.StrutsActionProxy.getErrorMessage(StrutsActionProxy.java:69)
    Java项目打包部署war文件
    一道腾讯面试题
    SSH服务器与Android通信(3)--Android客户端发送数据
  • 原文地址:https://www.cnblogs.com/Answer1215/p/10410150.html
Copyright © 2011-2022 走看看