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使用 useState 管理组件状态

介绍 useState 钩子，将计数器或切换值存储在状态中，并通过调用状态设置函数触发重新渲染。

使用 useState 管理组件状态 是 CoddyKit 上的免费 React Native Academy 课时。这是第 2 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 React Native Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 React Native Academy 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

What Is State?

State is data that can change over time and that a component owns and manages internally. Unlike props (which come from the parent and are read-only), state is created and updated by the component itself. When state changes, React automatically re-renders the component to reflect the new values in the UI. Common state examples include: whether a modal is open, the text the user has typed, the currently selected tab, or items fetched from an API. State is what makes React components interactive.

import { View, Text, TouchableOpacity } from 'react-native';
import { useState } from 'react';

export default function Counter() {
  // Declare state: count starts at 0
  const [count, setCount] = useState(0);

  return (
    <View style={{ padding: 24, alignItems: 'center' }}>
      <Text style={{ fontSize: 48, fontWeight: 'bold' }}>{count}</Text>
      <TouchableOpacity
        onPress={() => setCount(count + 1)}
        style={{ marginTop: 16, padding: 12, backgroundColor: '#4f86f7', borderRadius: 8 }}
      >
        <Text style={{ color: '#fff', fontSize: 18 }}>Increment</Text>
      </TouchableOpacity>
    </View>
  );
}

The useState Hook Signature

useState is a React Hook that you call at the top level of your component function. It takes one argument — the initial state value — and returns an array with two elements: the current state value and a setter function. You destructure these using array destructuring. The naming convention is [value, setValue]. The setter function is what you call to update state — never mutate the state variable directly. Each state variable is independent; call useState multiple times for multiple pieces of state.

import { useState } from 'react';

function MyComponent() {
  // Syntax: const [value, setter] = useState(initialValue);
  const [count, setCount] = useState(0);          // number
  const [name, setName] = useState('');            // string
  const [isVisible, setIsVisible] = useState(false); // boolean
  const [items, setItems] = useState([]);          // array
  const [user, setUser] = useState(null);          // object or null

  // Update state by calling the setter:
  // setCount(5);          ← sets to 5
  // setName('Alice');     ← sets to 'Alice'
  // setIsVisible(true);   ← sets to true
}

State Updates Trigger Re-Renders

When you call a state setter, React schedules a re-render of the component. During the next render, useState returns the new value. React is smart about this — it batches multiple state updates that happen in the same event handler and does a single re-render. Also important: the state value inside the current render is immutable. Calling setCount(count + 1) does not immediately change count in the current function execution — the new value appears in the next render cycle.

import { useState } from 'react';
import { View, Text, TouchableOpacity } from 'react-native';

export default function Example() {
  const [count, setCount] = useState(0);

  function handlePress() {
    // These do NOT stack: count is still 0 in this render
    setCount(count + 1); // schedules count = 1
    setCount(count + 1); // also schedules count = 1, not 2!

    // To correctly base on previous value, use functional update:
    // setCount(prev => prev + 1); // prev = 0, result = 1
    // setCount(prev => prev + 1); // prev = 1, result = 2
  }

  return (
    <TouchableOpacity onPress={handlePress}>
      <Text>{count}</Text>
    </TouchableOpacity>
  );
}

Functional Updates with Previous State

When your new state depends on the previous state, always use the functional update form: setState(prev => prev + 1). React guarantees that prev is the most recent state value, even if multiple updates are batched. This is critical when incrementing counters, toggling booleans, or adding items to arrays in rapid succession (e.g., fast button taps). The object literal form (setState(state + 1)) can produce bugs when multiple updates happen before the component re-renders.

import { useState } from 'react';
import { View, Text, TouchableOpacity } from 'react-native';

export default function SafeCounter() {
  const [count, setCount] = useState(0);

  function increment() {
    // Safe: uses previous state, handles batching correctly
    setCount(prev => prev + 1);
  }

  function decrement() {
    setCount(prev => Math.max(0, prev - 1)); // min 0
  }

  function reset() {
    setCount(0); // direct value ok when not based on previous
  }

  return (
    <View style={{ alignItems: 'center', gap: 12, padding: 24 }}>
      <Text style={{ fontSize: 48 }}>{count}</Text>
      <TouchableOpacity onPress={increment}><Text>+</Text></TouchableOpacity>
      <TouchableOpacity onPress={decrement}><Text>-</Text></TouchableOpacity>
      <TouchableOpacity onPress={reset}><Text>Reset</Text></TouchableOpacity>
    </View>
  );
}

Toggling Boolean State

A very common pattern is toggling a boolean state between true and false. Use the functional update form with the logical NOT operator: setIsOpen(prev => !prev). This is cleaner than setIsOpen(!isOpen) because it correctly handles rapid presses. Common use cases include showing/hiding a modal, expanding/collapsing an accordion section, switching between play and pause, or toggling a favorite star icon. Booleans are perhaps the most frequently used type of component state.

import { useState } from 'react';
import { View, Text, TouchableOpacity, Modal, StyleSheet } from 'react-native';

export default function ToggleModal() {
  const [isVisible, setIsVisible] = useState(false);

  return (
    <View style={styles.container}>
      <TouchableOpacity
        style={styles.btn}
        onPress={() => setIsVisible(prev => !prev)}
      >
        <Text style={styles.btnText}>Toggle Modal</Text>
      </TouchableOpacity>

      <Modal visible={isVisible} transparent animationType='fade'>
        <View style={styles.overlay}>
          <Text style={styles.modalText}>Hello from the modal!</Text>
          <TouchableOpacity onPress={() => setIsVisible(false)}>
            <Text style={{ color: '#fff', marginTop: 16 }}>Close</Text>
          </TouchableOpacity>
        </View>
      </Modal>
    </View>
  );
}

const styles = StyleSheet.create({
  container: { flex: 1, justifyContent: 'center', alignItems: 'center' },
  btn: { padding: 12, backgroundColor: '#4f86f7', borderRadius: 8 },
  btnText: { color: '#fff', fontWeight: 'bold' },
  overlay: { flex: 1, backgroundColor: 'rgba(0,0,0,0.7)', justifyContent: 'center', alignItems: 'center' },
  modalText: { color: '#fff', fontSize: 20, fontWeight: 'bold' },
});

State with Strings: Controlled Text Input

Binding a TextInput to state creates a controlled input. The value prop sets the displayed text (from state), and the onChangeText callback updates the state whenever the user types. This makes the state the single source of truth for the input's value — you can read, validate, or transform the text at any time by reading the state variable. Without binding to state (uncontrolled), you would need a ref to read the input value, which is less idiomatic in React.

import { useState } from 'react';
import { View, Text, TextInput, StyleSheet } from 'react-native';

export default function SearchBox() {
  const [query, setQuery] = useState('');

  return (
    <View style={styles.container}>
      <TextInput
        value={query}
        onChangeText={setQuery} // same as: (text) => setQuery(text)
        placeholder='Search...'
        style={styles.input}
      />
      <Text style={styles.preview}>
        You typed: <Text style={{ fontWeight: 'bold' }}>{query}</Text>
      </Text>
      <Text style={styles.count}>{query.length} characters</Text>
    </View>
  );
}

const styles = StyleSheet.create({
  container: { padding: 16, gap: 8 },
  input: { borderWidth: 1, borderColor: '#ccc', borderRadius: 8, padding: 12, fontSize: 16 },
  preview: { fontSize: 14, color: '#555' },
  count: { fontSize: 12, color: '#999' },
});

State with Arrays: Adding and Removing Items

When state is an array, never mutate it directly with push or splice. Instead, create a new array with the change and pass it to the setter. To add an item: setItems(prev => [...prev, newItem]). To remove an item: setItems(prev => prev.filter(item => item.id !== id)). To update an item: setItems(prev => prev.map(item => item.id === id ? {...item, done: true} : item)). These immutable patterns let React detect changes and schedule efficient re-renders.

import { useState } from 'react';
import { View, Text, TouchableOpacity, TextInput, FlatList } from 'react-native';

export default function TodoList() {
  const [todos, setTodos] = useState([]);
  const [input, setInput] = useState('');

  function addTodo() {
    if (!input.trim()) return;
    setTodos(prev => [...prev, { id: Date.now().toString(), text: input.trim(), done: false }]);
    setInput('');
  }

  function toggleDone(id) {
    setTodos(prev => prev.map(t => t.id === id ? { ...t, done: !t.done } : t));
  }

  return (
    <View style={{ flex: 1, padding: 16 }}>
      <TextInput value={input} onChangeText={setInput} placeholder='Add a todo' />
      <TouchableOpacity onPress={addTodo}><Text>Add</Text></TouchableOpacity>
      <FlatList
        data={todos}
        keyExtractor={t => t.id}
        renderItem={({ item }) => (
          <TouchableOpacity onPress={() => toggleDone(item.id)}>
            <Text style={{ textDecorationLine: item.done ? 'line-through' : 'none' }}>
              {item.text}
            </Text>
          </TouchableOpacity>
        )}
      />
    </View>
  );
}

State with Objects

For related pieces of state, group them into a single object state variable. When updating object state, spread the previous state to keep unchanged fields: setState(prev => ({ ...prev, name: 'Alice' })). This only overwrites the name field while keeping all other fields intact. Alternatively, each field can be its own useState — neither approach is universally better. Use an object when the fields always change together (like a form with name, email, and password); use separate states when they change independently.

import { useState } from 'react';
import { View, TextInput, Text, StyleSheet } from 'react-native';

export default function ProfileForm() {
  const [form, setForm] = useState({
    name: '',
    email: '',
    bio: '',
  });

  function updateField(field, value) {
    setForm(prev => ({ ...prev, [field]: value }));
  }

  return (
    <View style={{ padding: 16, gap: 12 }}>
      <TextInput
        value={form.name}
        onChangeText={v => updateField('name', v)}
        placeholder='Name'
        style={styles.input}
      />
      <TextInput
        value={form.email}
        onChangeText={v => updateField('email', v)}
        placeholder='Email'
        style={styles.input}
      />
      <Text style={{ color: '#666' }}>Preview: {form.name} — {form.email}</Text>
    </View>
  );
}

const styles = StyleSheet.create({
  input: { borderWidth: 1, borderColor: '#ccc', borderRadius: 8, padding: 12 },
});

Lazy Initial State

If computing the initial state is expensive (reading from AsyncStorage, parsing a large dataset), pass a function to useState instead of a value: useState(() => computeExpensiveValue()). React calls this initializer function only once on the component's first render, not on every subsequent render. This is called lazy initialization. Passing an expensive computation as a plain value (useState(expensiveCompute())) would run it on every render, causing unnecessary work even though the initial state is only used once.

import { useState } from 'react';

// Without lazy init: computeExpensiveList() runs on EVERY render
const [list, setList] = useState(computeExpensiveList()); // BAD

// With lazy init: function is called ONCE on mount
const [list, setList] = useState(() => computeExpensiveList()); // GOOD

// Real example: JSON.parse is cheap, but still good practice:
const [settings, setSettings] = useState(() => {
  const stored = SomeStorage.get('settings');
  return stored ? JSON.parse(stored) : { theme: 'light', lang: 'en' };
});

Derived Values vs. Redundant State

Don't store values in state if they can be derived from existing state or props. For example, if you have a list of todos in state, don't also store a completedCount in state — compute it as todos.filter(t => t.done).length during render. Storing derived values in state creates synchronization bugs: you forget to update one when you update the other. Only put things in state that you can't derive, can't compute during render, or that are asynchronously set (like API response data). This rule keeps state minimal and bug-free.

import { useState } from 'react';
import { Text, View } from 'react-native';

export default function TodoStats() {
  const [todos, setTodos] = useState([
    { id: '1', text: 'Buy groceries', done: true },
    { id: '2', text: 'Write code', done: false },
    { id: '3', text: 'Go for a walk', done: true },
  ]);

  // DERIVED: computed from existing state, not stored in state
  const total = todos.length;
  const completed = todos.filter(t => t.done).length;
  const remaining = total - completed;

  return (
    <View style={{ padding: 16 }}>
      <Text>Total: {total}</Text>
      <Text>Completed: {completed}</Text>
      <Text>Remaining: {remaining}</Text>
    </View>
  );
}

useState vs useRef: When Not to Use State

Not every value that changes in a component needs to be in state. If a value changes but does not need to trigger a re-render, store it in a useRef instead. Common examples: a timer ID returned from setTimeout, a flag tracking whether a network request is in flight, or the previous value of a prop. Putting these in state causes unnecessary re-renders. A useRef persists its value across renders (like state) but updating it does not schedule a re-render. The golden rule: if the UI doesn't need to change when the value changes, use useRef.

import { useState, useRef, useEffect } from 'react';
import { Text, TouchableOpacity, View } from 'react-native';

export default function Timer() {
  const [elapsed, setElapsed] = useState(0);
  const intervalRef = useRef(null); // timer ID in ref — no re-render needed

  function start() {
    if (intervalRef.current) return; // already running
    intervalRef.current = setInterval(() => {
      setElapsed(prev => prev + 1); // state — needs re-render to show
    }, 1000);
  }

  function stop() {
    clearInterval(intervalRef.current);
    intervalRef.current = null;
  }

  useEffect(() => () => clearInterval(intervalRef.current), []); // cleanup

  return (
    <View style={{ alignItems: 'center', padding: 24, gap: 16 }}>
      <Text style={{ fontSize: 48 }}>{elapsed}s</Text>
      <TouchableOpacity onPress={start}><Text>Start</Text></TouchableOpacity>
      <TouchableOpacity onPress={stop}><Text>Stop</Text></TouchableOpacity>
    </View>
  );
}

Quick Check

Test your understanding of React Native Mobile Development concepts from this lesson.

Lesson Recap

In this lesson you learned: useState declares state variables and provides a setter that triggers re-renders, functional updates safely derive new state from the previous value, and derived values should be computed during render instead of stored as redundant state. Next up we explore lifting state up to share data between sibling components.

常见问题解答

「使用 useState 管理组件状态」课时是免费的吗？

是的 — 「使用 useState 管理组件状态」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 React Native Academy 课程的其余内容，请升级到 CoddyKit PRO。 React Native Academy 课程共包含 4 节课。

「使用 useState 管理组件状态」这节课中我会学到什么？

介绍 useState 钩子，将计数器或切换值存储在状态中，并通过调用状态设置函数触发重新渲染。你通过在浏览器中直接运行的动手代码来练习 React Native Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 React Native Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 React Native Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 2 节课，共 4 节。

「使用 useState 管理组件状态」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

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能。每节 React Native Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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