无法访问 React 中的上下文值

Question

I'm trying to pass a value from one component within a context consumer to another component as a prop it says it's undefined .

<ReadingSessionContext.Consumer>
  {(context) => {
    console.dir(context.state.session.duration) // Value is printed here
    return (
      ...
        <Timer duration={context.state.session.duration} />
      ...
    )
  }}
</ReadingSessionContext.Consumer>

And the Timer component

class Timer extends React.Component {
  state = {
    "minutes": this.props.duration.split(":")[1].parseInt(), // Returns error here
    "seconds": this.props.duration.split(":")[2].parseInt()  // Returns error here
  }

  componentDidMount() {
    console.dir(this.props) // Value is undefined here
    this.myInterval = setInterval(() => {
      const { seconds, minutes } = this.state;

      if (seconds > 0) {
        this.setState(({ seconds }) => ({
          seconds: seconds - 1
        }))
      }

      if (seconds === 0) {
        if (minutes === 0) {
          clearInterval(this.myInterval)
        } else {
          this.setState(({ minutes }) => ({
            minutes: minutes - 1,
            seconds: 59
          }))
        }
      }
    }, 1000)
  }

  render() {
    const { minutes, seconds } = this.state;

    return (
      <Typography component="h1" variant="h5">
        Time Remaining: { minutes }:{ seconds < 10 ? `0${ seconds }` : seconds }
      </Typography>
    )
  }
}

I've also tried to refactor this so that the Timer component consumes the context, rather than passing it as a prop:

function Timer() {
  const context = useContext(ReadingSessionContext);
  const [minutes, setMinutes] = useState(3);
  const [seconds, setSeconds] = useState(0);

  useEffect(() => {
    let duration = context.state.session.duration; // Value is accessable here
    console.log(duration); // This logs to the console correctly, and it is a string
    setMinutes(duration.split(":")[1].parseInt()); // Throws error here
    setSeconds(duration.split(":")[2].parseInt()); // Throws error here

    let timerInterval = setInterval(() => {
      if (seconds > 0) {
        setSeconds(seconds - 1);
      }

      if (seconds === 0) {
        if (minutes === 0) {
          clearInterval(this.timerInterval)
        } else {
          setMinutes(minutes - 1);
          setSeconds(59)
        }
      }
    }, 1000);
  });
}

In each case, the error that gets thrown is - Uncaught TypeError: Cannot read property 'split' of undefined

However, in every instance where I inspect the value before calling .split() it tells me that the value exists, and is in fact correct, but all of a sudden stops existing as soon as I try to perform some action on the string

Provider.jsx

import React from "react";
import axios from "axios";
import ReadingSessionContext from "./Context";

export default class ReadingSessionProvider extends React.Component {

  /**
   * Set the initial state of the `ReadingSessionProvider`
   * @param {*} props 
   */
  state = {
    "translationUrl": process.env.REACT_APP_BACKEND_URL + "translate/",
    "readingSessionUrl": process.env.REACT_APP_BACKEND_URL + "reading-sessions/",
    "session": {},
    "book": {},
    "translations": [],
    "serverPage": 1,
    "clientPage": 0,
    "limit": 10,
    "totalResults": 0,
    "sessionId": 0,
    "headers": {
      "Content-type": "application/json",
      "Authorization": "Token " +  localStorage.getItem("token"),
    }
  }

  /**
   * After the component mounts, call the `getReadingSession` method
   * and update the state with response
   */
  async componentDidMount() {
    let data = await this.getReadingSession();
    this.setState({"session": data.data});
    this.setState({"book": data.data.library_item.book});
    await this.getTranslations()
  }

  /**
   * Call the API and get data for this specific reading session
   */
  async getReadingSession() {
    let result = await axios.get(
      this.state.readingSessionUrl + window.location.href.split('/')[5] + "/",
      {headers: this.state.headers}
    );
    return result;
  }

  makeUrl = sessionId => {
    return `${this.state.translationUrl}?page=${this.state.serverPage}&limit=${this.state.limit}&sessionId=${this.state.session.id}`;
  }

  /**
   * Make the API call to the server to retrieve a list of the translations
   * for the currently logged in user.
   */
  getTranslations = async () => {
    try {
      let url = `${this.state.translationUrl}?page=${this.state.serverPage}&limit=${this.state.limit}&sessionId=${this.state.session.id}`
      let response = await axios.get(url, {headers: this.state.headers});
      await this.setState({"translations": response.data.results});
      await this.setState({"totalResults": response.data.count});
    } catch (error) {
      console.log(error);
    }
  }

  /**
   * Submit the text that the user has input and get the updated 
   * list of translations from the API
   */
  submitText = async (e, text) => {
    console.log("hello?")
    let data = {
      "text_to_be_translated": text,
      "session": this.state.session.id
    };

    try {
      await axios.post(this.state.translationUrl, data, {headers: this.state.headers});
      let paginationUrl = `${this.state.translationUrl}?page=${this.state.serverPage}&limit=${this.state.limit}&sessionId=${this.state.session.id}`;
      this.getTranslations(paginationUrl);
    } catch (error) {
      console.dir(error);
    }
  }

  setSessionId = sessionId => {
    this.setState({"sessionId": sessionId});
    console.log("called")
  }

  handleChangePage = async (event, newPage) => {
    this.setState({"serverPage": newPage + 1})
    this.setState({"clientPage": newPage})
    let url = await `${this.state.translationUrl}translate/?page=${newPage + 1}&limit=${this.state.limit}&sessionId=${this.state.session.id}`
    console.log(url)
    await this.getTranslations(url);
  }

  render() {
    return (
      <ReadingSessionContext.Provider value={{
        state: this.state,
        getTranslations: this.getTranslations,
        submitText: this.submitText,
        handleChangePage: this.handleChangePage,
        setSessionId: this.setSessionId,
        makeUrl: this.makeUrl

      }}>
        {this.props.children}
      </ReadingSessionContext.Provider>
    )
  }
}

Context.jsx

import React from "react";

const ReadingSessionContext = React.createContext();

export default ReadingSessionContext;

Answer 1

Okay. You're really close. You still need to wrap your root component or hierachy with ReadingSessionProvider. So this is how it works generally.

1. You create the context with const ReadingSessionContext = React.createContext();
2. You use it to make a context provider. Which you have done when you created the the ReadingSessionProvider class.
3. You use the ReadingSessionProvider wrap the app section you want to access the consumer. So say

<ReadingSessionProvider>
  <App />
</ReadingSessionProvider>

4. Now the children of App component can use the values set in the ReadingSessionProvider. Either with useContext(ReadingSessionContext) or with ReadingSessionContext.Consumer

See this video from Wes Bos for more information: https://youtu.be/XLJN4JfniH4