实用的react hook

接触react-hook已经很久了，用了之后，发现真的很强大，记录一下最近使用的想法

useState

首先接触到的就是useState
state只是一个状态，但是之前class就需要写的很大，相比之下，写成hook就简化了很多

class Age extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      age: 20
    };
  }

  handleOnChange = e => {
    const { name, value } = e.target;
    this.setState({ [name]: value });
  };

  render() {
    return (
      <>
        <div>age: {this.state.age}</div>
        <input name="age" onChange={handleOnChange} />
      </>
    );
  }
}

用了hook之后，就变得很简洁了

const Age = () => {
  const [age, setAge] = React.useState(20);

  const handleOnChange = e => {
    const { value } = e.target;
    setAge(value);
  };

  return (
    <>
      <div>age: {age}</div>
      <input name="age" onChange={handleOnChange} />
    </>
  );
};

useEffect

对于useEffect我最初以为是用来替代生命周期，比如componentDidMount，但是事实证明他真的很强大，[name]括号里面的是依赖，依赖一变，就会重新执行，多个依赖就更棒了，如果是用之前的写法，就要在 componentWillReceiveProps里面写很多if来比较nextProps和this.props
而且这个最强大就是可以抽出来，当一个反复利用reuseable的hook

传入不同的localStorageKey，就可以存不同的数据到localStorage，这样就方便重复实用了，而且每次value变了，就会自动保存到localStorage

const useLocalStorage = localStorageKey => {
  const [value, setValue] = React.useState(
    localStorage && localStorage.getItem(localStorageKey)
  );

  React.useEffect(() => {
    localStorage && localStorage.setItem(localStorageKey, value);
  }, [value, localStorageKey]);

  return [value, setValue];
};

useRef

有些时候，会发现，其实数据并没有变化，但是还是触发了setState或者是别的callback，这样会导致，调用的函数太多，从而影响了性能，所以比较两个对象是不是相等，只有当不等，才更新，这样可以减小一点re-render。
这时候，就是useRef登场的时候了，搭配实用lodash.isEqual, 可以比较大的object

const usePrevious = value => {
  const ref = useRef();

  useEffect(() => {
    ref.current = value;
  }, [value]);

  return ref.current;
};

#使用
const myPreviousState = usePrevious(conditions);
useEffect(() => {
 if (myPreviousState && !_.isEqual(myPreviousState, conditions)) {
        onChange(conditions);
 }
}, [myPreviousState, conditions, onChange ])

useContext

context呢，我之前一直觉得这个东西很高大上，很复杂，一直不敢用，但是后来发现，他就是对应上下文，比如传递props，一般都是使用parent传给child，但是如果，A -> B -> C -> D，但其实B和C都没用到，只有D用到了，那么D要来修改A，就需要再往上传递，导致B和C一直在反反复复被重新渲染，这时候，当当当当，context就可以发挥很大的作用，在A里面包一下Provider，然后到D直接用createContext和useContext拿出来就可以用了，这样代码也不会忘了忘记中间的B，C的pass props，看起来也清楚，何乐而不为呢

const NameContext = createContext();

const A = () => (
   <NameContext.Provider value={{ username, setUserName }}>
     <B />
   </NameContext.Provider>
);

const Names = () => {
  const { username, setUserName } = useContext(NameContext);

  const { names, onChange } = useNames({ username, setUserName });

  return (
    <div>
      <label>firstName</label>
      <input
        name="firstName"
        value={names && names.firstName}
        onChange={onChange}
      />
      <label>lastName</label>
      <input
        name="lastName"
        value={names && names.lastName}
        onChange={onChange}
      />
    </div>
  );
};

useReducer

最后是用到redux, 之前对于redux的理解一直如果当一个对象，要被很多不同的地方共享和修改，那么放到redux是最合适不过。但是知道我遇到了，一个嵌套很深的一个object，要更新里面的某一个字段真的要写好多好多层，然后我翻到了hook对于useReducer的介绍

Another option is useReducer, which is more suited for managing state objects that contain multiple sub-values.

useReducer is usually preferable to useState when you have complex state logic that involves multiple sub-values or when the next state depends on the previous one. useReducer also lets you optimize performance for components that trigger deep updates because you can pass dispatch down instead of callbacks

现在的reducer是相对于useState而言的，他更加适合复杂的，有很多sub-values的对象的更新

import React, { useEffect, useContext, createContext, useReducer } from "react";
const NameContext = createContext();

const init = initial => {
  return { name: initial };
};

const reducer = (state, action) => {
  switch (action.type) {
    case "firstName":
      return {
        ...state,
        name: {
          firstName: action.payload
        }
      };
    case "lastName":
      return {
        ...state,
        name: {
          lastName: action.payload
        }
      };
    default:
      return state;
  }
};

const useNames = ({ username, setUserName }) => {
  // const [names, setNames] = React.useState(username);
  const [state, dispatch] = useReducer(reducer, username, init);
  const names = state.name;

  const onChange = e => {
    const { name, value } = e.target;
    dispatch({ type: name, payload: value });
  };

  useEffect(() => {
    setUserName(names);
  }, [names, setUserName]);

  return {
    names,
    onChange
  };
};

const Names = () => {
  const { username, setUserName } = useContext(NameContext);
  const { names, onChange } = useNames({ username, setUserName });
  return (
    <div>
      <label>firstName</label>
      <input
        name="firstName"
        value={names && names.firstName}
        onChange={onChange}
      />
      <label>lastName</label>
      <input
        name="lastName"
        value={names && names.lastName}
        onChange={onChange}
      />
    </div>
  );
};

const App = () => {
  const [username, setUserName] = React.useState({
    firstName: "Rachel",
    lastName: "Green"
  });

  return (
    <NameContext.Provider value={{ username, setUserName }}>
      <Names />
    </NameContext.Provider>
  );
};

export default App;

完成了一个简简单单的name 表单组

image.png