反作用钩USESTEAT阵列

Question

这里面有什么问题吗？

    import React, { useState } from "react"; 
    import ReactDOM from "react-dom";

    const StateSelector = () => {   
    const initialValue = [
    { id: 0,value: " --- Select a State ---" }];

      const allowedState = [
        { id: 1, value: "Alabama" },
        { id: 2, value: "Georgia" },
        { id: 3, value: "Tennessee" }
        ];

      const [stateOptions, setStateValues] = useState(initialValue);  
      // initialValue.push(...allowedState);

      console.log(initialValue.length);

      setStateValues(allowedState); // Not sure why cannot I reset the state in here for an array.

         return (<div>
          <label>Select a State:</label>
          <select>
            {stateOptions.map((localState, index) => (
              <option key={localState.id}>{localState.value}</option>
            ))}
          </select>
        </div>   ); };

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

Answer 1

您不应该在呈现函数中设置状态（或执行任何带有副作用的其他操作）。使用钩子时，可以为此使用useEffect。

以下版本有效：

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

const StateSelector = () => {
  const initialValue = [
    { id: 0, value: " --- Select a State ---" }];

  const allowedState = [
    { id: 1, value: "Alabama" },
    { id: 2, value: "Georgia" },
    { id: 3, value: "Tennessee" }
  ];

  const [stateOptions, setStateValues] = useState(initialValue);
  // initialValue.push(...allowedState);

  console.log(initialValue.length);
  // ****** BEGINNING OF CHANGE ******
  useEffect(() => {
    // Should not ever set state during rendering, so do this in useEffect instead.
    setStateValues(allowedState);
  }, []);
  // ****** END OF CHANGE ******

  return (<div>
    <label>Select a State:</label>
    <select>
      {stateOptions.map((localState, index) => (
        <option key={localState.id}>{localState.value}</option>
      ))}
    </select>
  </div>);
};

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

它在一个代码沙箱里。