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react的statehook useState Hook详细使用

article 2025/1/13 4:06:11

1. 基本用法

1.1 声明状态

import React, { useState } from 'react';

function Example() {
  // 声明一个叫 count 的 state，初始值为 0
  const [count, setCount] = useState(0);
  
  return (
    <div>
      <p>当前计数: {count}</p>
      <button onClick={() => setCount(count + 1)}>
        增加
      </button>
    </div>
  );
}

1.2 多个状态声明

function UserForm() {
  const [name, setName] = useState('');
  const [age, setAge] = useState(0);
  const [email, setEmail] = useState('');

  return (
    <form>
      <input
        value={name}
        onChange={e => setName(e.target.value)}
        placeholder="姓名"
      />
      <input
        type="number"
        value={age}
        onChange={e => setAge(Number(e.target.value))}
        placeholder="年龄"
      />
      <input
        value={email}
        onChange={e => setEmail(e.target.value)}
        placeholder="邮箱"
      />
    </form>
  );
}

2. 状态更新

2.1 基础更新

function Counter() {
  const [count, setCount] = useState(0);

  // 直接设置新值
  const increment = () => setCount(count + 1);
  
  // 使用函数式更新
  const decrement = () => setCount(prevCount => prevCount - 1);

  return (
    <div>
      <button onClick={decrement}>-</button>
      <span>{count}</span>
      <button onClick={increment}>+</button>
    </div>
  );
}

2.2 对象状态更新

function UserProfile() {
  const [user, setUser] = useState({
    name: '',
    age: 0,
    email: ''
  });

  // 更新对象的某个属性
  const updateName = (name) => {
    setUser(prevUser => ({
      ...prevUser,
      name
    }));
  };

  // 批量更新多个属性
  const updateProfile = (updates) => {
    setUser(prevUser => ({
      ...prevUser,
      ...updates
    }));
  };

  return (
    <div>
      <input
        value={user.name}
        onChange={e => updateName(e.target.value)}
      />
      <button onClick={() => updateProfile({ age: 25, email: 'test@example.com' })}>
        更新信息
      </button>
    </div>
  );
}

2.3 数组状态更新

function TodoList() {
  const [todos, setTodos] = useState([]);

  // 添加项
  const addTodo = (text) => {
    setTodos(prevTodos => [
      ...prevTodos,
      { id: Date.now(), text, completed: false }
    ]);
  };

  // 删除项
  const removeTodo = (id) => {
    setTodos(prevTodos => prevTodos.filter(todo => todo.id !== id));
  };

  // 更新项
  const toggleTodo = (id) => {
    setTodos(prevTodos => prevTodos.map(todo =>
      todo.id === id
        ? { ...todo, completed: !todo.completed }
        : todo
    ));
  };

  return (
    <div>
      <button onClick={() => addTodo('新任务')}>添加任务</button>
      <ul>
        {todos.map(todo => (
          <li key={todo.id}>
            <input
              type="checkbox"
              checked={todo.completed}
              onChange={() => toggleTodo(todo.id)}
            />
            <span>{todo.text}</span>
            <button onClick={() => removeTodo(todo.id)}>删除</button>
          </li>
        ))}
      </ul>
    </div>
  );
}

3. 高级用法

3.1 惰性初始化

function ExpensiveComponent() {
  // 惰性初始化：初始值通过函数计算
  const [state, setState] = useState(() => {
    const initialValue = expensiveComputation();
    return initialValue;
  });

  return <div>{state}</div>;
}

3.2 状态重置

function ResetableCounter() {
  const [count, setCount] = useState(0);

  // 重置状态到初始值
  const reset = () => setCount(0);

  return (
    <div>
      Count: {count}
      <button onClick={() => setCount(count + 1)}>增加</button>
      <button onClick={reset}>重置</button>
    </div>
  );
}

3.3 派生状态

function DerivedStateExample() {
  const [list, setList] = useState([1, 2, 3, 4, 5]);
  
  // 派生状态：基于现有状态计算
  const evenNumbers = list.filter(n => n % 2 === 0);
  const total = list.reduce((sum, n) => sum + n, 0);

  return (
    <div>
      <p>所有数字: {list.join(', ')}</p>
      <p>偶数: {evenNumbers.join(', ')}</p>
      <p>总和: {total}</p>
    </div>
  );
}

4. 常见使用场景

4.1 表单处理

function Form() {
  const [formData, setFormData] = useState({
    username: '',
    password: '',
    email: ''
  });

  const handleChange = (e) => {
    const { name, value } = e.target;
    setFormData(prev => ({
      ...prev,
      [name]: value
    }));
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    console.log('提交表单:', formData);
  };

  return (
    <form onSubmit={handleSubmit}>
      <input
        name="username"
        value={formData.username}
        onChange={handleChange}
      />
      <input
        name="password"
        type="password"
        value={formData.password}
        onChange={handleChange}
      />
      <input
        name="email"
        type="email"
        value={formData.email}
        onChange={handleChange}
      />
      <button type="submit">提交</button>
    </form>
  );
}

4.2 异步数据加载

function DataFetcher() {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(false);
  const [error, setError] = useState(null);

  const fetchData = async () => {
    setLoading(true);
    try {
      const response = await fetch('https://api.example.com/data');
      const result = await response.json();
      setData(result);
      setError(null);
    } catch (err) {
      setError(err.message);
      setData(null);
    } finally {
      setLoading(false);
    }
  };

  return (
    <div>
      <button onClick={fetchData}>加载数据</button>
      {loading && <p>加载中...</p>}
      {error && <p>错误: {error}</p>}
      {data && <pre>{JSON.stringify(data, null, 2)}</pre>}
    </div>
  );
}

5. 性能优化

5.1 避免不必要的更新

function OptimizedComponent() {
  const [count, setCount] = useState(0);
  
  // 使用回调函数避免重复创建函数
  const increment = useCallback(() => {
    setCount(c => c + 1);
  }, []);

  return <button onClick={increment}>Count: {count}</button>;
}

5.2 状态分割

// 不好的做法：将不相关的状态放在一起
function BadExample() {
  const [state, setState] = useState({
    count: 0,
    user: null,
    theme: 'light'
  });
}

// 好的做法：将状态分开管理
function GoodExample() {
  const [count, setCount] = useState(0);
  const [user, setUser] = useState(null);
  const [theme, setTheme] = useState('light');
}