React Native Academy · 강의

복잡한 상태 로직에 useReducer 사용하기

여러 useState 호출을 하나의 useReducer로 대체하고, 동작 유형을 사용하는 리듀서 함수를 작성하며, UI 이벤트에서 동작을 디스패치합니다.

복잡한 상태 로직에 useReducer 사용하기은(는) CoddyKit의 무료 React Native Academy 강의입니다. 이것은 4개 중 2번째 강의입니다. 아래에서 전체 강의를 무료로 읽을 수 있으며, 내장 코드 에디터와 24/7 AI 튜터와 함께 브라우저에서 직접 실습할 수 있습니다. 이 강의는 React Native Academy 학습 경로의 일부이며, 진행 상황이 웹과 CoddyKit 앱에 동기화됩니다. React Native Academy 강의에는 총 4개의 강의가 포함되어 있습니다.

이 강의의 일부는 아직 번역되지 않았으며 영어로 표시됩니다.

Why useReducer?

When a component has several useState calls whose updates are interrelated, the logic can become hard to follow. useReducer centralizes all state transitions in a single reducer function, making complex state logic easier to understand, test, and debug — the same pattern made popular by Redux.

The Reducer Function

A reducer is a pure function that takes the current state and an action object, and returns the next state. It must be pure — no side effects, no API calls, no mutations of the incoming state object. Return a new object for the new state. The action.type string identifies what transition to perform.

function counterReducer(state, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    case 'reset':
      return { count: 0 };
    default:
      return state;
  }
}

Calling useReducer

useReducer takes the reducer function and an initial state, and returns [state, dispatch]. The state is the current state value. The dispatch function sends an action to the reducer to trigger a state transition and schedule a re-render.

import { useReducer } from 'react';

const initialState = { count: 0 };

function Counter() {
  const [state, dispatch] = useReducer(counterReducer, initialState);

  return (
    <View>
      <Text>Count: {state.count}</Text>
      <Button title='+' onPress={() => dispatch({ type: 'increment' })} />
      <Button title='-' onPress={() => dispatch({ type: 'decrement' })} />
      <Button title='Reset' onPress={() => dispatch({ type: 'reset' })} />
    </View>
  );
}

Passing Payload with Actions

Actions can carry additional data in a payload property. The reducer reads action.payload to know the specifics of the transition. For example, an 'addItem' action can carry the new item's data in its payload, which the reducer appends to the state's items array.

function listReducer(state, action) {
  switch (action.type) {
    case 'addItem':
      return { items: [...state.items, action.payload] };
    case 'removeItem':
      return { items: state.items.filter((i) => i.id !== action.payload.id) };
    default:
      return state;
  }
}

// Dispatch with payload
dispatch({ type: 'addItem', payload: { id: '1', text: 'Buy milk' } });

Managing a Shopping Cart

A shopping cart is a great real-world use case for useReducer because it involves multiple related state transitions: adding items, removing items, updating quantities, and clearing the cart. Each action type maps to a clear, testable transition in the reducer function.

const initialCartState = { items: [], total: 0 };

function cartReducer(state, action) {
  switch (action.type) {
    case 'ADD':
      return {
        items: [...state.items, action.payload],
        total: state.total + action.payload.price,
      };
    case 'CLEAR':
      return initialCartState;
    default:
      return state;
  }
}

Initializing State Lazily

Pass a third argument to useReducer — an init function — to compute the initial state lazily. React calls init(initialArg) instead of using initialArg directly. This is useful when the initial state is derived from a prop or requires an expensive computation.

function init(initialCount) {
  return { count: initialCount };
}

function Counter({ startCount }) {
  const [state, dispatch] = useReducer(
    counterReducer,
    startCount, // passed to init
    init         // called once on mount
  );
}

useReducer vs useState

Use useState for simple, independent values (a boolean, a string, a number). Reach for useReducer when state transitions depend on multiple values at once, when the next state depends on the current state in complex ways, or when you want to co-locate all state logic in one testable function.

// useState — fine for simple independent values
const [name, setName] = useState('');
const [age, setAge] = useState(0);

// useReducer — better when these fields relate to each other
const [userForm, dispatch] = useReducer(userFormReducer, {
  name: '',
  age: 0,
  isValid: false,
});

Form State with useReducer

A complex registration form with many fields and validation logic benefits from useReducer. Define action types like 'CHANGE_FIELD', 'VALIDATE', and 'SUBMIT'. The reducer handles all field updates and validation in one place instead of scattered across multiple useState setters and useEffect calls.

function formReducer(state, action) {
  switch (action.type) {
    case 'CHANGE_FIELD':
      return {
        ...state,
        values: { ...state.values, [action.field]: action.value },
        errors: { ...state.errors, [action.field]: null },
      };
    case 'SET_ERROR':
      return { ...state, errors: { ...state.errors, ...action.errors } };
    default:
      return state;
  }
}

Testing a Reducer in Isolation

Because reducers are pure functions with no side effects, they are extremely easy to unit test. Call the reducer directly with a state and an action, and assert on the returned state. You do not need to mount any component or mock any API to test the logic.

// Pure unit test — no React needed
test('increment action increases count by 1', () => {
  const state = { count: 5 };
  const next = counterReducer(state, { type: 'increment' });
  expect(next.count).toBe(6);
});

test('reset returns count to 0', () => {
  const state = { count: 100 };
  const next = counterReducer(state, { type: 'reset' });
  expect(next.count).toBe(0);
});

Combining useReducer with useContext

A powerful pattern is pairing useReducer with useContext to build a mini Redux-like global store. Export the dispatch function via context so any descendant component can dispatch actions without prop drilling. This avoids installing a third-party library for moderate complexity apps.

const StoreContext = createContext(null);
const DispatchContext = createContext(null);

export function StoreProvider({ children }) {
  const [state, dispatch] = useReducer(appReducer, initialState);
  return (
    <StoreContext.Provider value={state}>
      <DispatchContext.Provider value={dispatch}>
        {children}
      </DispatchContext.Provider>
    </StoreContext.Provider>
  );
}

Dispatching from Multiple Components

With the dispatch function available via context, any nested component can trigger state transitions without receiving props. Import the useContext(DispatchContext) hook, call dispatch with the appropriate action type, and the reducer handles the update centrally. This scales well as the app grows.

function AddToCartButton({ product }) {
  const dispatch = useContext(DispatchContext);

  return (
    <Button
      title='Add to Cart'
      onPress={() => dispatch({ type: 'ADD', payload: product })}
    />
  );
}

Quick Check

Test your understanding of useReducer for complex state logic from this lesson.

Lesson Recap

In this lesson you learned: useReducer centralizes state transitions in a pure reducer function, dispatch actions with a type and optional payload to trigger transitions, and pair useReducer with useContext for a scalable global state pattern without Redux. Next up we explore useCallback and useMemo for performance optimization.

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네 — “복잡한 상태 로직에 useReducer 사용하기” 전체 내용을 이 웹사이트에서 무료로 읽을 수 있습니다. 인터랙티브하게 실습하려면(내장 코드 에디터와 24/7 AI 튜터), CoddyKit PRO로 업그레이드하면 React Native Academy 강의 전체를 잠금 해제할 수 있습니다. React Native Academy 강의에는 총 4개의 강의가 포함되어 있습니다.

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여러 useState 호출을 하나의 useReducer로 대체하고, 동작 유형을 사용하는 리듀서 함수를 작성하며, UI 이벤트에서 동작을 디스패치합니다. 브라우저에서 직접 실행하는 실습 코드로 React Native Academy을(를) 배우며, 24/7 AI 튜터가 강의를 진행하면서 질문에 답변해줍니다.

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