React上下文useReducer没有正确更新

Question

我在让reducer与React上下文一起工作时遇到麻烦。在中buttonbar.js，有两个按钮应该用来更新状态。通过过滤当前数据来更新状态。单击按钮，我没有收到任何错误，但是它也没有执行任何操作。我认为问题出在减速器上。

context.js

import React, { useState, useEffect } from "react";
import * as moment from "moment";
import axios from "axios";

export const Context = React.createContext();

const url = "https://projects.fivethirtyeight.com/polls/polls.json";

export const filterReducer = (state, action) => {
  switch (action.type) {
    case "SHOW_ALL":
      return state.polls;
    case "SHOW_APPROVAL":
      return state.polls.filter(e => e.type === "trump-approval");
    default:
      return state.polls;
  }
};

export function Provider({ children }) {
  let intialState = {
    polls: [],
    dispatch: action => this.setState(state => filterReducer(state, action))
  };

  const [state, setState, dispatch] = useState(intialState);

  useEffect(() => {
    var dateRange = moment()
      .subtract(7, "days")
      .calendar();

    axios
      .get(url)
      .then(res => {
        setState({
          polls: res.data
            .filter(e => Date.parse(e.endDate) >= Date.parse(dateRange))
            .reverse()
        });
      }, [])
      .catch(error => console.log(error));
  }, []);

  return (
    <Context.Provider value={[state, setState, dispatch]}>
      {children}
    </Context.Provider>
  );
}

// export const Consumer = Context.Consumer;

buttonbar.js

import React, { useContext, useState, useEffect, useReducer } from "react";
import { Context, filterReducer } from "../context";

const ButtonBar = () => {
  const [state, setState] = useContext(Context);
  const [filter, dispatch] = useReducer(filterReducer, state);

  const showAll = () => {
    dispatch({ type: "SHOW_ALL" });
    console.log("showAll clicked");
  };
  const showApproval = () => {
    dispatch({ type: "SHOW_APPROVAL" });
    console.log("showApproval clicked");
  };

  return (
    <div class="mb-2">
      <button class="btn btn-primary btn-sm" name="all" onClick={showAll}>
        All
      </button>{" "}
      <button
        class="btn btn-primary btn-sm"
        name="trump approval"
        onClick={showApproval}
      >
        Trump Approval
      </button>
    </div>
  );
};

export default ButtonBar;

Answer 1

有几件事，你没有做对。

首先，您将 initialState 与调度方法一起使用，而您试图通过useState使用不正确的第三个参数来获取此调度值

其次，由于您使用的是 reducer 模式，因此最好使用 if useReducerhook

第三，千万不要在reducer中过滤数据，否则下次要显示所有数据时，完整的数据将丢失，只保留过滤后的数据。相反，你必须有它的选择器。

相关代码：

import React, {
  useEffect,
  useContext,
  useReducer,
  useMemo,
  useState
} from "react";
import ReactDOM from "react-dom";

import "./styles.css";
import moment from "moment";
import axios from "axios";

export const Context = React.createContext();

const url = "https://projects.fivethirtyeight.com/polls/polls.json";

export const filterReducer = (state, action) => {
  switch (action.type) {
    case "ADD_POLLS":
      console.log(action.payload);
      return action.payload;
    default:
      return state.polls;
  }
};

export function Provider({ children }) {
  const [state, dispatch] = useReducer(filterReducer);

  useEffect(() => {
    var dateRange = moment()
      .subtract(7, "days")
      .calendar();

    axios
      .get(url)
      .then(res => {
        dispatch({
          type: "ADD_POLLS",
          payload: res.data
            .filter(e => Date.parse(e.endDate) >= Date.parse(dateRange))
            .reverse()
        });
      }, [])
      .catch(error => console.log(error));
  }, []);

  return (
    <Context.Provider value={[state, dispatch]}>{children}</Context.Provider>
  );
}
const ButtonBar = () => {
  const [polls] = useContext(Context);
  const [state, setState] = useState(polls);
  useEffect(() => {
    setState(polls);
  }, [polls]);
  const filterResult = useMemo(() => {
    return filter => {
      switch (filter) {
        case "SHOW_ALL":
          setState(polls);
          break;
        case "SHOW_APPROVAL":
          setState(polls.filter(e => e.type === "trump-approval"));
          break;
        default:
          return;
      }
    };
  }, [polls]);

  return (
    <div class="mb-2">
      <button
        class="btn btn-primary btn-sm"
        name="all"
        onClick={() => filterResult("SHOW_ALL")}
      >
        All
      </button>{" "}
      <button
        class="btn btn-primary btn-sm"
        name="trump approval"
        onClick={() => filterResult("SHOW_APPROVAL")}
      >
        Trump Approval
      </button>
      <div>{(state || []).length}</div>
      <pre>{JSON.stringify(state, null, 4)}</pre>
    </div>
  );
};

const rootElement = document.getElementById("root");
ReactDOM.render(
  <Provider>
    <ButtonBar />
  </Provider>,
  rootElement
);

工作演示

Answer 2

您错误地使用了 useReducer Hook，仅仅因为您在组件中使用了 useReducer Hook，并不意味着您正在更新全局上下文状态。

所以在你的buttonbar.js中

  const [filter, dispatch] = useReducer(filterReducer, state);

  const showAll = () => {
    dispatch({ type: "SHOW_ALL" });
    console.log("showAll clicked");
  };
  const showApproval = () => {
    dispatch({ type: "SHOW_APPROVAL" });
    console.log("showApproval clicked");
  };

您正在使用减速器正确更新状态，但它只会更新本地组件状态，而不是全局上下文状态。

如果你来自 redux，这似乎是违反直觉的。

在上下文中，状态在父组件中包含和更改，因此只需将上述代码移动到父组件，然后通过上下文访问它即可。

export function Provider({ children }) {
  let intialState = {
    polls: [],
    dispatch: action => this.setState(state => filterReducer(state, action))
  };

  // 2 args not 3
  const [state, setState] = useState(intialState);

  const [filter, dispatch] = useReducer(filterReducer, state);

  const showAll = () => {
    dispatch({ type: "SHOW_ALL" });
    console.log("showAll clicked");
  };
  const showApproval = () => {
    dispatch({ type: "SHOW_APPROVAL" });
    console.log("showApproval clicked");
  };

将状态和函数传递给 value prop

   <Context.Provider value={{
                          showAllProp: () => showAll(),
                          showApprovalProp: () => showApproval(),
                          filterProp: filter }}>
      {children}
    </Context.Provider>

然后，您可以使用 value 属性在子组件中访问这些值和函数。

   const context = useContext(Context);  

  <button class="btn btn-primary btn-sm" name="all" onClick={context.showAllProp}>
    All
  </button>{" "}
  <button
    class="btn btn-primary btn-sm"
    name="trump approval"
    onClick={context.showApprovalProp}
  >

这本质上就是将上下文与组件连接起来的方式。