react 【七】各种hooks的使用/SPA的缺点

1、Hook

1.1 为什么会出现hook

1.2 useState

import React, { memo, useState } from "react";

const App = memo(() => {
  const [message, setMessage] = useState("Hello World");
  const [count, setCount] = useState(100);
  const [banners, setBanners] = useState([]);

  function changeMessage() {
    setMessage("你好啊, 李银河!");
  }

  return (
    <div>
      <h2>App: {message}</h2>
      <button onClick={changeMessage}>修改文本</button>
    </div>
  );
});

export default App;

1.3 useEffect

import React, { memo, useEffect } from 'react'
import { useState } from 'react'

const App = memo(() => {
  const [count, setCount] = useState(0)

  // 负责告知react, 在执行完当前组件渲染之后要执行的副作用代码
  useEffect(() => {
    // 1.监听事件
    // const unubscribe = store.subscribe(() => {
    // })
    // function foo() {
    // }
    // eventBus.on("why", foo)
    console.log("监听redux中数据变化, 监听eventBus中的why事件")

    // 返回值: 回调函数 => 组件被重新渲染或者组件卸载的时候执行
    return () => {
      console.log("取消监听redux中数据变化, 取消监听eventBus中的why事件")
    }
  })

  return (
    <div>
      <button onClick={e => setCount(count+1)}>+1({count})</button>
    </div>
  )
})

export default App

import React, { memo, useEffect } from 'react'
import { useState } from 'react'

const App = memo(() => {
  const [count, setCount] = useState(0)

  // 负责告知react, 在执行完当前组件渲染之后要执行的副作用代码
  useEffect(() => {
    // 1.修改document的title(1行)
    console.log("修改title")
  })

  // 一个函数式组件中, 可以存在多个useEffect
  useEffect(() => {
    // 2.对redux中数据变化监听(10行)
    console.log("监听redux中的数据")
    return () => {
      // 取消redux中数据的监听
    }
  })

  useEffect(() => {
    // 3.监听eventBus中的why事件(15行)
    console.log("监听eventBus的why事件")
    return () => {
      // 取消eventBus中的why事件监听
    }
  })

  return (
    <div>
      <button onClick={e => setCount(count+1)}>+1({count})</button>
    </div>
  )
})

export default App

import React, { memo, useEffect } from "react";
import { useState } from "react";

const App = memo(() => {
  const [count, setCount] = useState(0);
  const [message, setMessage] = useState("Hello World");

  // 只收到count的影响
  useEffect(() => {
    console.log("修改title:", count);
  }, [count]);

  // 不受任何影响 类似生命周期的挂载和卸载

  useEffect(() => {
    console.log("发送网络请求, 从服务器获取数据");

    return () => {
      console.log("会在组件被卸载时, 才会执行一次");
    };
  }, []);

  return (
    <div>
      <button onClick={(e) => setCount(count + 1)}>+1({count})</button>
      <button onClick={(e) => setMessage("你好啊")}>
        修改message({message})
      </button>
    </div>
  );
});

export default App;

1.4 useContext

import { createContext } from "react";

const UserContext = createContext();
const ThemeContext = createContext();

export { UserContext, ThemeContext };

import React, { memo, useContext } from 'react'
import { UserContext, ThemeContext } from "./context"

const App = memo(() => {
  // 使用Context
  const user = useContext(UserContext)
  const theme = useContext(ThemeContext)

  return (
    <div>
      <h2>User: {user.name}-{user.level}</h2>
      <h2 style={{color: theme.color, fontSize: theme.size}}>Theme</h2>
    </div>
  )
})

export default App

• 依然需要在index.js使用context

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
  <UserContext.Provider value={{name: "why", level: 99}}>
    <TokenContext.Provider value={'coderwhy'}>
      <App />
    </TokenContext.Provider>
  </UserContext.Provider>
);

1.5 useReducer

import React, { memo, useReducer } from 'react'
// import { useState } from 'react'

function reducer(state, action) {
  switch(action.type) {
    case "increment":
      return { ...state, counter: state.counter + 1 }
    case "decrement":
      return { ...state, counter: state.counter - 1 }
    case "add_number":
      return { ...state, counter: state.counter + action.num }
    case "sub_number":
      return { ...state, counter: state.counter - action.num }
    default:
      return state
  }
}

// useReducer+Context => redux

const App = memo(() => {
  // const [count, setCount] = useState(0)
  const [state, dispatch] = useReducer(reducer, { counter: 0, friends: [], user: {} })

  // const [counter, setCounter] = useState()
  // const [friends, setFriends] = useState()
  // const [user, setUser] = useState()

  return (
    <div>
      {/* 
当前计数: {count}
       setCount(count+1)}>+1
       setCount(count-1)}>-1
       setCount(count+5)}>+5
       setCount(count-5)}>-5
       setCount(count+100)}>+100 */}

      <h2>当前计数: {state.counter}</h2>
      <button onClick={e => dispatch({type: "increment"})}>+1</button>
      <button onClick={e => dispatch({type: "decrement"})}>-1</button>
      <button onClick={e => dispatch({type: "add_number", num: 5})}>+5</button>
      <button onClick={e => dispatch({type: "sub_number", num: 5})}>-5</button>
      <button onClick={e => dispatch({type: "add_number", num: 100})}>+100</button>
    </div>
  )
})

export default App

1.6 useCallback

import React, { memo, useState, useCallback, useRef } from "react";

// useCallback性能优化的点:
// 1.当需要将一个函数传递给子组件时, 最好使用useCallback进行优化, 将优化之后的函数, 传递给子组件

// props中的属性发生改变时, 组件本身就会被重新渲染
const HYHome = memo(function (props) {
  const { increment } = props;
  console.log("HYHome被渲染");
  return (
    <div>
      <button onClick={increment}>increment+1</button>

      {/* 100个子组件 */}
    </div>
  );
});

const App = memo(function () {
  const [count, setCount] = useState(0);
  const [message, setMessage] = useState("hello");

  console.log("App组件被重新渲染");

  // 闭包陷阱: useCallback
  // 闭包陷阱的结果可以参考如下案例 bar2()始终拿到的是why
  // 参数1 函数
  // 参数2 依赖值 只有依赖值发生变化 函数才会更新
  // const increment = useCallback(
  //   function foo() {
  //     console.log("increment");
  //     setCount(count + 1);
  //   },
  //   [count]
  // );

  // 进一步的优化: 当count发生改变时, 也使用同一个函数(了解)
  // 做法一: 将count依赖移除掉, 缺点: 闭包陷阱
  // 做法二: useRef, 在组件多次渲染时, 返回的是同一个值
  const countRef = useRef();
  countRef.current = count;
  const increment = useCallback(function foo() {
    console.log("increment");
    setCount(countRef.current + 1);
  }, []);

  // 普通的函数
  // 如果使用过方法的话 每次传入组件的props的数值都是新的会导致组件被重新渲染
  // const increment = () => {
  //   setCount(count + 1);
  // };

  return (
    <div>
      <h2>计数: {count}</h2>
      <button onClick={increment}>+1</button>

      <HYHome increment={increment} />

      <h2>message:{message}</h2>
      <button onClick={(e) => setMessage(Math.random())}>修改message</button>
    </div>
  );
});

// function foo(name) {
//   function bar() {
//     console.log(name)
//   }
//   return bar
// }

// const bar1 = foo("why")
// bar1() // why
// bar1() // why

// const bar2 = foo("kobe")
// bar2() // kobe

// bar1() // why

export default App;

1.7 useMemo

import React, { memo, useCallback } from "react";
import { useMemo, useState } from "react";

const HelloWorld = memo(function (props) {
  console.log("HelloWorld被渲染~");
  return <h2>Hello World</h2>;
});

function calcNumTotal(num) {
  // console.log("calcNumTotal的计算过程被调用~")
  let total = 0;
  for (let i = 1; i <= num; i++) {
    total += i;
  }
  return total;
}

const App = memo(() => {
  const [count, setCount] = useState(0);

  // const result = calcNumTotal(50)

  // 1.不依赖任何的值, 进行计算
  // 传入的是数值 只要数值没有发生变化 子组件就不会重新渲染
  const result = useMemo(() => {
    return calcNumTotal(50);
  }, []);

  // 2.依赖count
  // const result = useMemo(() => {
  //   return calcNumTotal(count*2)
  // }, [count])

  // 3.useMemo和useCallback的对比
  // useMemo是对值做优化 而useCallBack是对函数做优化
  function fn() {}
  // const increment = useCallback(fn, [])
  // const increment2 = useMemo(() => fn, [])

  // 4.使用useMemo对子组件渲染进行优化
  // const info = { name: "why", age: 18 }

  // 传入的是对象的话 APP组件重新渲染对象也会被重新创建 导致子组件也重新渲染
  const info = useMemo(() => ({ name: "why", age: 18 }), []);

  return (
    <div>
      <h2>计算结果: {result}</h2>
      <h2>计数器: {count}</h2>
      <button onClick={(e) => setCount(count + 1)}>+1</button>

      <HelloWorld result={result} info={info} />
    </div>
  );
});

export default App;

1.8 useRef

1.8.1 ref绑定dom

import React, { memo, useRef } from 'react'

const App = memo(() => {
  const titleRef = useRef()
  const inputRef = useRef()
  
  function showTitleDom() {
    console.log(titleRef.current)
    inputRef.current.focus()
  }

  return (
    <div>
      <h2 ref={titleRef}>Hello World</h2>
      <input type="text" ref={inputRef} />
      <button onClick={showTitleDom}>查看title的dom</button>
    </div>
  )
})

export default App

1.8.2 ref解决闭包缺陷

import React, { memo, useRef } from "react";
import { useCallback } from "react";
import { useState } from "react";

let obj = null;

const App = memo(() => {
  const [count, setCount] = useState(0);
  const nameRef = useRef();
  console.log(obj === nameRef);
  obj = nameRef;

  // 通过useRef解决闭包陷阱
  // 这里的ref使用是保持一致的 不会发生改变 所以将可以将count赋值给ref
  const countRef = useRef();
  countRef.current = count;

  // 这里闭包产生的原因是是 第一次就创建了一个函数 count 执行之后 = 1
  // 而在第二次执行的时候  依然是拿到count = 1的结果 并没有发生改变
  // 为了解决这个问题就使用ref保存的数值
  const increment = useCallback(() => {
    setCount(countRef.current + 1);
  }, []);

  return (
    <div>
      <h2>Hello World: {count}</h2>
      <button onClick={(e) => setCount(count + 1)}>+1</button>
      <button onClick={increment}>+1</button>
    </div>
  );
});

export default App;

1.9 useImperativeHandle

import React, { memo, useRef, forwardRef, useImperativeHandle } from "react";

const HelloWorld = memo(
  forwardRef((props, ref) => {
    const inputRef = useRef();

    // 子组件对父组件传入的ref进行处理
    // 不让父组件直接拿到input的所有方法 所以将自己想要抛出去的方法包裹起来
    useImperativeHandle(ref, () => {
      return {
        focus() {
          console.log("focus");
          inputRef.current.focus();
        },
        setValue(value) {
          inputRef.current.value = value;
        },
      };
    });

    return <input type="text" ref={inputRef} />;
  })
);

const App = memo(() => {
  const titleRef = useRef();
  const inputRef = useRef();

  function handleDOM() {
    // console.log(inputRef.current)

    inputRef.current.focus();

    // 不能直接通过.value的方法来修改 只能通过setValue的方法来修改
    // inputRef.current.value = ""
    inputRef.current.setValue("哈哈哈");
  }

  return (
    <div>
      <h2 ref={titleRef}>哈哈哈</h2>
      <HelloWorld ref={inputRef} />
      <button onClick={handleDOM}>DOM操作</button>
    </div>
  );
});

export default App;

1.10 useLayoutEffect

import React, { memo, useEffect, useLayoutEffect, useState } from "react";

const App = memo(() => {
  const [count, setCount] = useState(100);

  // 这时候已经显示到界面上 才执行修改count的数值会导致页面闪烁
  // useEffect(() => {
  //   console.log("useEffect");
  //   if (count === 0) {
  //     setCount(Math.random() + 99);
  //   }
  // });

  useLayoutEffect(() => {
    console.log("useLayoutEffect");
    if (count === 0) {
      setCount(Math.random() + 99);
    }
  });

  console.log("App render");

  return (
    <div>
      <h2>count: {count}</h2>
      <button onClick={(e) => setCount(0)}>设置为0</button>
    </div>
  );
});

export default App;

1.11 自定义Hook

1.11.1 什么是自定义Hook

import React, { memo, useEffect, useState } from 'react'

function useLogLife(cName) {
  useEffect(() => {
    console.log(cName + "组件被创建")
    return () => {
      console.log(cName + "组件被销毁")
    }
  }, [cName])
}

const Home = memo(() => {
  useLogLife("home")

  return <h1>Home Page</h1>
})

const About = memo(() => {
  useLogLife("about")

  return <h1>About Page</h1>
})

const App = memo(() => {
  const [isShow, setIsShow] = useState(true)

  useLogLife("app")

  return (
    <div>
      <h1>App Root Component</h1>
      <button onClick={e => setIsShow(!isShow)}>切换</button>
      { isShow && <Home/> }
      { isShow && <About/> }
    </div>
  )
})

export default App

1.11.2 Context的共享

import { createContext } from "react";

const UserContext = createContext();
const TokenContext = createContext();

export { UserContext, TokenContext };

const root = ReactDOM.createRoot(document.getElementById("root"));
root.render(
  <UserContext.Provider value={{ name: "why", level: 99 }}>
    <TokenContext.Provider value={"coderwhy"}>
      <App />
    </TokenContext.Provider>
  </UserContext.Provider>
);

Hook

import { useContext } from "react";
import { UserContext, TokenContext } from "../context";

function useUserToken() {
  const user = useContext(UserContext);
  const token = useContext(TokenContext);

  return [user, token];
}

export default useUserToken;

import React, { memo } from 'react'
import { useUserToken } from "./hooks"

// User/Token

const Home = memo(() => {
  const [user, token] = useUserToken()

  return <h1>Home Page: {user.name}-{token}</h1>
})

const About = memo(() => {
  const [user, token] = useUserToken()

  return <h1>About Page: {user.name}-{token}</h1>
})

const App = memo(() => {
  return (
    <div>
      <h1>App Root Component</h1>
      <Home/>
      <About/>
    </div>
  )
})

export default App

1.11.3 获取鼠标滚动位置

import React, { memo } from 'react'
import useScrollPosition from './hooks/useScrollPosition'
import "./style.css"

const Home = memo(() => {
  const [scrollX, scrollY] = useScrollPosition()

  return <h1>Home Page: {scrollX}-{scrollY}</h1>
})

const About = memo(() => {
  const [scrollX, scrollY] = useScrollPosition()

  return <h1>About Page: {scrollX}-{scrollY}</h1>
})

const App = memo(() => {
  return (
    <div className='app'>
      <h1>App Root Component</h1>
      <Home/>
      <About/>
    </div>
  )
})

export default App

import { useState, useEffect } from "react";

function useScrollPosition() {
  const [scrollX, setScrollX] = useState(0);
  const [scrollY, setScrollY] = useState(0);

  useEffect(() => {
    function handleScroll() {
      // console.log(window.scrollX, window.scrollY)
      // 监听到滚动就把值保存下来
      setScrollX(window.scrollX);
      setScrollY(window.scrollY);
    }

    window.addEventListener("scroll", handleScroll);
    return () => {
      window.removeEventListener("scroll", handleScroll);
    };
  }, []);

  return [scrollX, scrollY];
}

export default useScrollPosition;

1.11.4 storage

import { useEffect } from "react";
import { useState } from "react";

function useLocalStorage(key) {
  // 1.从localStorage中获取数据, 并且数据数据创建组件的state
  const [data, setData] = useState(() => {
    const item = localStorage.getItem(key);
    if (!item) return "";
    return JSON.parse(item);
  });

  // 2.监听data改变, 一旦发生改变就存储data最新值
  useEffect(() => {
    localStorage.setItem(key, JSON.stringify(data));
  }, [data]);

  // 3.将data/setData的操作返回给组件, 让组件可以使用和修改值
  return [data, setData];
}

export default useLocalStorage;

import React, { memo } from 'react'
import { useEffect } from 'react'
import { useState } from 'react'
import useLocalStorage from './hooks/useLocalStorage'

const App = memo(() => {
  // 通过key, 直接从localStorage中获取一个数据
  // const [token, setToken] = useState(localStorage.getItem("token"))
  // useEffect(() => {
  //   localStorage.setItem("token", token)
  // }, [token])
  const [token, setToken] = useLocalStorage("token")
  function setTokenHandle() {
    setToken("james")
  }

  // const [avatarUrl, setAvatarUrl] = useState(localStorage.getItem("avatarUrl"))
  // useEffect(() => {
  //   localStorage.setItem("avatarUrl", avatarUrl)
  // }, [avatarUrl])
  const [avatarUrl, setAvatarUrl] = useLocalStorage("avatarUrl")
  function setAvatarUrlHandle() {
    setAvatarUrl("http://www.james.com/cba.png")
  }

  return (
    <div className='app'>
      <h1>App Root Component: {token}</h1>
      <button onClick={setTokenHandle}>设置token</button>
      <h1>avatarURL: {avatarUrl}</h1>
      <button onClick={setAvatarUrlHandle}>设置新头像地址</button>
    </div>
  )
})

export default App

1.12 redux中的hook

import React, { memo } from 'react'
import { connect } from "react-redux"
import { addNumberAction, subNumberAction } from './store/modules/counter'

const App = memo((props) => {
  const { count, addNumber, subNumber } = props

  function addNumberHandle(num, isAdd = true) {
    if (isAdd) {
      addNumber(num)
    } else {
      subNumber(num)
    }
  }

  return (
    <div>
      <h2>当前计数: {count}</h2>
      <button onClick={e => addNumberHandle(1)}>+1</button>
      <button onClick={e => addNumberHandle(6)}>+6</button>
      <button onClick={e => addNumberHandle(6, false)}>-6</button>
    </div>
  )
})

const mapStateToProps = (state) => ({
  count: state.counter.count
})

const mapDispatchToProps = (dispatch) => ({
  addNumber(num) {
    dispatch(addNumberAction(num))
  },
  subNumber(num) {
    dispatch(subNumberAction(num))
  }
})

export default connect(mapStateToProps, mapDispatchToProps)(App)

import React, { memo } from "react";
import { useSelector, useDispatch, shallowEqual } from "react-redux";
import {
  addNumberAction,
  changeMessageAction,
  subNumberAction,
} from "./store/modules/counter";

// memo高阶组件包裹起来的组件有对应的特点: 只有props发生改变时, 才会重新渲染
// 而且useSelector监听的是state的变化 所以state变化的时候也会引起 重新渲染
// 加上shallowEqual 就只会在我们使用的参数发生变化的时候才重新渲染
const Home = memo((props) => {
  const { message } = useSelector(
    (state) => ({
      message: state.counter.message,
    }),
    shallowEqual
  );

  //  使用dispatch 可以直接使用reducer
  const dispatch = useDispatch();
  function changeMessageHandle() {
    dispatch(changeMessageAction("你好啊, 师姐!"));
  }

  console.log("Home render");

  return (
    <div>
      <h2>Home: {message}</h2>
      <button onClick={(e) => changeMessageHandle()}>修改message</button>
    </div>
  );
});

const App = memo((props) => {
  // 1.使用useSelector将redux中store的数据映射到组件内
  const { count } = useSelector(
    (state) => ({
      count: state.counter.count,
    }),
    shallowEqual
  );

  // 2.使用dispatch直接派发action
  const dispatch = useDispatch();
  function addNumberHandle(num, isAdd = true) {
    if (isAdd) {
      dispatch(addNumberAction(num));
    } else {
      dispatch(subNumberAction(num));
    }
  }

  console.log("App render");

  return (
    <div>
      <h2>当前计数: {count}</h2>
      <button onClick={(e) => addNumberHandle(1)}>+1</button>
      <button onClick={(e) => addNumberHandle(6)}>+6</button>
      <button onClick={(e) => addNumberHandle(6, false)}>-6</button>

      <Home />
    </div>
  );
});

export default App;

1.13 讲讲SPA和 Hydration

SPA页面

• 1、不利于SEO优化搜索引擎优化

• 2、首屏渲染速度慢

• 1、不利于SEO优化搜索引擎优化

• • 例如百度页面在进行关键词收录的时候 是通过请求index.html页面而SPA应用的index.html页面是很简单的，其他资源存在bundle中，另外还有一种就是获取meta配置 meta配置也是有限的，会导致获取到关于网站的资源比较少而导致网站排名靠后

• 2、首屏渲染速度慢

• • 网站的资源都是通过src = 'bundle.js'去获取 需要下载bundle.js文件 然后浏览器在执行bundle.js 再通过document.createElement讲dom渲染到页面上 如果请求的bundle.js文件过大 就会出现首屏渲染慢的问题

1.14 useId

import React, { memo, useId, useState } from 'react'

const App = memo(() => {
  const [count, setCount] = useState(0)

  const id = useId()
  console.log(id)

  return (
    <div>
      <button onClick={e => setCount(count+1)}>count+1:{count}</button>

      <label htmlFor={id}>
        用户名:<input id={id} type="text" />
      </label>
    </div>
  )
})

export default App

1.15 useTransition

faker 这个库可以模拟数据

import { faker } from '@faker-js/faker';

const namesArray = []

for (let i = 0; i < 10000; i++) {
  namesArray.push(faker.name.fullName())
}

export default namesArray

import React, { memo, useState, useTransition } from "react";
import namesArray from "./namesArray";

const App = memo(() => {
  const [showNames, setShowNames] = useState(namesArray);
  // pending startTransition 函数执行的状态
  const [pending, startTransition] = useTransition();

  function valueChangeHandle(event) {
    // 是在其他执行完成之后 在执行这里的函数
    startTransition(() => {
      const keyword = event.target.value;
      const filterShowNames = namesArray.filter((item) =>
        item.includes(keyword)
      );
      setShowNames(filterShowNames);
    });
  }

  return (
    <div>
      <input type="text" onInput={valueChangeHandle} />
      <h2>用户名列表: {pending && <span>data loading</span>} </h2>
      <ul>
        {showNames.map((item, index) => {
          return <li key={index}>{item}</li>;
        })}
      </ul>
    </div>
  );
});

export default App;

1.16 useDeferredValue

import React, { memo, useState, useDeferredValue } from "react";
import namesArray from "./namesArray";

const App = memo(() => {
  const [showNames, setShowNames] = useState(namesArray);

  // 跟useTransition的作用差不多 是对数据做处理而transition是对函数做处理
  const deferedShowNames = useDeferredValue(showNames);

  function valueChangeHandle(event) {
    const keyword = event.target.value;
    const filterShowNames = namesArray.filter((item) => item.includes(keyword));
    setShowNames(filterShowNames);
  }

  return (
    <div>
      <input type="text" onInput={valueChangeHandle} />
      <h2>用户名列表: </h2>
      <ul>
        {deferedShowNames.map((item, index) => {
          return <li key={index}>{item}</li>;
        })}
      </ul>
    </div>
  );
});

export default App;