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React Native Academy · 강의

핵심 기능: 오프라인을 지원하는 데이터 피드

React Query를 사용해 오프라인 지속성이 적용된 기본 데이터 피드를 구축하고, 당겨서 새로고침과 페이지네이션을 구현하며, 생성 및 삭제 변이를 추가하고, 목록 항목이 나타날 때 애니메이션을 적용합니다.

핵심 기능: 오프라인을 지원하는 데이터 피드은(는) CoddyKit의 무료 React Native Academy 강의입니다. 이것은 4개 중 3번째 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.

What Is Offline-First Design

Offline-first design means the app works correctly without an internet connection — users can view cached data, perform mutations that queue for sync, and experience no error messages just because the network is unavailable. Mobile users frequently move in and out of connectivity. A React Native app that breaks without a network feels fragile; one that keeps working feels polished and reliable.

Setting Up the QueryClient

React Query's QueryClient is the backbone of the data layer. Configure it with sensible defaults: a staleTime that avoids unnecessary refetches (e.g., 5 minutes for data that changes infrequently), and a gcTime (garbage collection time) that keeps unused data in the cache long enough to serve it from cache on reconnect. Pass this client to QueryClientProvider at the app root.

// src/services/queryClient.ts
import { QueryClient } from '@tanstack/react-query';

export const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 1000 * 60 * 5,  // 5 minutes — don't refetch too often
      gcTime: 1000 * 60 * 60,    // Keep in cache 1 hour after unmount
      retry: 2,
      retryDelay: (attempt) => Math.min(1000 * 2 ** attempt, 30000),
      refetchOnWindowFocus: false, // Not needed on mobile
      refetchOnReconnect: true,    // Sync when network returns
    },
  },
});

Persisting the Query Cache with AsyncStorage

To make data available when offline, persist the React Query cache to AsyncStorage. Use @tanstack/react-query-persist-client with an AsyncStorage persister. On app startup, the persisted cache is restored before the first network request — so users see their habits list immediately, even before the fetch completes. The cache is hydrated asynchronously, so show cached data optimistically and update when the fresh data arrives.

// App.tsx
import { PersistQueryClientProvider } from '@tanstack/react-query-persist-client';
import { createAsyncStoragePersister } from '@tanstack/query-async-storage-persister';
import AsyncStorage from '@react-native-async-storage/async-storage';
import { queryClient } from './src/services/queryClient';

const asyncStoragePersister = createAsyncStoragePersister({
  storage: AsyncStorage,
  key: 'REACT_QUERY_CACHE',
});

export default function App() {
  return (
    <PersistQueryClientProvider
      client={queryClient}
      persistOptions={{ persister: asyncStoragePersister }}
    >
      <AuthProvider>
        <RootNavigator />
      </AuthProvider>
    </PersistQueryClientProvider>
  );
}

Fetching the Habits List with useQuery

Build a custom hook useHabits that encapsulates the Supabase query inside useQuery. The query key ['habits', userId] scopes the cache to the current user, preventing data from leaking between accounts. React Query returns data, isLoading, and error — pass all three to the screen to show the correct UI state at each point.

// src/hooks/useHabits.ts
import { useQuery } from '@tanstack/react-query';
import { supabase } from '../services/supabase';
import { useAuth } from '../context/AuthContext';
import { Habit } from '../types';

async function fetchHabits(userId: string): Promise<Habit[]> {
  const { data, error } = await supabase
    .from('habits')
    .select('*')
    .eq('user_id', userId)
    .order('created_at', { ascending: true });

  if (error) throw error;
  return data;
}

export function useHabits() {
  const { session } = useAuth();
  const userId = session!.user.id;

  return useQuery({
    queryKey: ['habits', userId],
    queryFn: () => fetchHabits(userId),
    enabled: !!userId,
  });
}

Rendering the FlatList with Cached Data

Use the useHabits hook in the HabitList screen. Show a skeleton loader or spinner for the initial load, use the data array directly as the FlatList source, and handle the error state with a retry button. Because the cache is persisted, on subsequent app launches data is immediately available (no loading flash) while React Query fetches a fresh version in the background.

// src/screens/habits/HabitListScreen.tsx
import { FlatList, ActivityIndicator, View, Text } from 'react-native';
import { useHabits } from '../../hooks/useHabits';
import { HabitCard } from '../../components/HabitCard';

export function HabitListScreen() {
  const { data: habits, isLoading, error, refetch } = useHabits();

  if (isLoading && !habits) {
    return <ActivityIndicator style={{ flex: 1 }} />;
  }

  if (error) {
    return (
      <View>
        <Text>Failed to load habits.</Text>
        <Button title='Retry' onPress={() => refetch()} />
      </View>
    );
  }

  return (
    <FlatList
      data={habits}
      keyExtractor={(item) => item.id}
      renderItem={({ item }) => <HabitCard habit={item} />}
      onRefresh={refetch}
      refreshing={isLoading}
    />
  );
}

Creating Habits with useMutation

useMutation handles the insert operation. On onSuccess, call queryClient.invalidateQueries(['habits', userId]) to trigger a fresh fetch of the habits list. This ensures the newly created habit appears in the list immediately after creation without manually updating the cache. Set onError to show the user an error message if the insert fails.

// src/hooks/useCreateHabit.ts
import { useMutation, useQueryClient } from '@tanstack/react-query';
import { supabase } from '../services/supabase';
import { useAuth } from '../context/AuthContext';

export function useCreateHabit() {
  const queryClient = useQueryClient();
  const { session } = useAuth();
  const userId = session!.user.id;

  return useMutation({
    mutationFn: async ({ name, icon, color }) => {
      const { error } = await supabase.from('habits').insert({
        user_id: userId,
        name,
        icon,
        color,
      });
      if (error) throw error;
    },
    onSuccess: () => {
      queryClient.invalidateQueries({ queryKey: ['habits', userId] });
    },
    onError: (error) => {
      Alert.alert('Error', error.message);
    },
  });
}

Optimistic Updates for Check-In

For the daily check-in (marking a habit as done), use optimistic updates to update the UI instantly before the network request completes. When the user taps the check-in button, immediately update the cache with the assumed result. If the request fails, roll back the cache to the previous state. This makes the app feel instant even on slow networks.

// Optimistic update pattern with useMutation
const toggleCompletion = useMutation({
  mutationFn: (habitId) => insertCompletion(habitId, today),

  onMutate: async (habitId) => {
    // Cancel in-flight refetches
    await queryClient.cancelQueries({ queryKey: ['completions', userId] });

    // Save snapshot for rollback
    const previous = queryClient.getQueryData(['completions', userId]);

    // Optimistically add completion
    queryClient.setQueryData(['completions', userId], (old) => [
      ...old,
      { habit_id: habitId, completed_date: today }
    ]);

    return { previous }; // Pass snapshot to onError
  },

  onError: (_err, _habitId, context) => {
    // Roll back on failure
    queryClient.setQueryData(['completions', userId], context.previous);
  },

  onSettled: () => {
    queryClient.invalidateQueries({ queryKey: ['completions', userId] });
  },
});

Network Status and Offline Banner

Show users a subtle offline banner when the device has no internet connection so they understand why their changes haven't synced. Use @react-native-community/netinfo to listen for connectivity changes. React Query automatically pauses mutations when offline and retries them when connectivity returns — the banner simply makes this behavior visible to the user.

import NetInfo from '@react-native-community/netinfo';
import { useEffect, useState } from 'react';

export function OfflineBanner() {
  const [isOffline, setIsOffline] = useState(false);

  useEffect(() => {
    const unsubscribe = NetInfo.addEventListener((state) => {
      setIsOffline(!state.isConnected);
    });
    return unsubscribe;
  }, []);

  if (!isOffline) return null;

  return (
    <View style={{
      backgroundColor: '#FF3B30',
      padding: 8,
      alignItems: 'center'
    }}>
      <Text style={{ color: 'white', fontWeight: 'bold' }}>
        No internet connection — changes will sync when online
      </Text>
    </View>
  );
}

Pagination with FlatList and useInfiniteQuery

For feeds with many items, implement pagination using useInfiniteQuery. It fetches pages of data and appends them to a running list. Pass fetchNextPage to FlatList's onEndReached handler so the next page loads automatically as the user scrolls. Use getNextPageParam to derive the next page cursor from the last page's response.

import { useInfiniteQuery } from '@tanstack/react-query';

export function useHabitsPaginated() {
  return useInfiniteQuery({
    queryKey: ['habits', 'paginated'],
    queryFn: ({ pageParam = 0 }) =>
      supabase.from('habits').select('*')
        .range(pageParam, pageParam + 19),  // 20 per page
    getNextPageParam: (lastPage, allPages) => {
      const nextOffset = allPages.length * 20;
      return lastPage.data?.length === 20 ? nextOffset : undefined;
    },
    initialPageParam: 0,
  });
}

// In screen:
const { data, fetchNextPage, hasNextPage } = useHabitsPaginated();
const habits = data?.pages.flatMap(p => p.data) ?? [];

<FlatList
  data={habits}
  onEndReached={() => hasNextPage && fetchNextPage()}
  onEndReachedThreshold={0.3}
/>

Animating List Item Entry

Make the feed feel dynamic by animating items as they appear. A simple fade-in + slide-up on mount using the Animated API adds polish without heavy libraries. Create a reusable AnimatedListItem wrapper that each item passes through. The animation runs once on mount — use useRef to prevent re-triggering on re-renders.

import { useRef, useEffect } from 'react';
import { Animated } from 'react-native';

export function AnimatedListItem({ children, index }) {
  const opacity = useRef(new Animated.Value(0)).current;
  const translateY = useRef(new Animated.Value(20)).current;

  useEffect(() => {
    Animated.parallel([
      Animated.timing(opacity, {
        toValue: 1,
        duration: 300,
        delay: index * 50,  // Stagger by index
        useNativeDriver: true,
      }),
      Animated.timing(translateY, {
        toValue: 0,
        duration: 300,
        delay: index * 50,
        useNativeDriver: true,
      }),
    ]).start();
  }, []);

  return (
    <Animated.View style={{ opacity, transform: [{ translateY }] }}>
      {children}
    </Animated.View>
  );
}

Streak Calculation Logic

The streak is the number of consecutive days a habit was completed up to and including today. Calculate it in JavaScript from the completions array. Sort completions by date descending, start from today and walk backwards, counting consecutive days. This logic is pure JavaScript — no network call needed if the completions are already in the React Query cache.

// src/utils/streaks.ts
export function calculateStreak(completedDates: string[]): number {
  if (completedDates.length === 0) return 0;

  const sorted = [...completedDates].sort().reverse();
  const today = new Date().toISOString().split('T')[0];

  let streak = 0;
  let current = new Date(today);

  for (const dateStr of sorted) {
    const expected = current.toISOString().split('T')[0];
    if (dateStr === expected) {
      streak++;
      current.setDate(current.getDate() - 1);
    } else if (dateStr < expected) {
      break; // Gap found
    }
  }

  return streak;
}

// Usage:
const streak = calculateStreak(
  completions
    .filter(c => c.habit_id === habit.id)
    .map(c => c.completed_date)
);

Quick Check

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

Lesson Recap

In this lesson you learned: how to configure React Query with persistence to serve cached data while offline, how to build custom hooks with useQuery and useMutation for habits CRUD, and how to implement optimistic updates for immediate UI feedback. You also saw how to add pagination, entry animations, and streak calculation from cached data. Next up we polish, test, and ship to both stores.

자주 묻는 질문

“핵심 기능: 오프라인을 지원하는 데이터 피드” 강의는 무료인가요?

네 — “핵심 기능: 오프라인을 지원하는 데이터 피드” 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.

“핵심 기능: 오프라인을 지원하는 데이터 피드”에서 뭘 배우나요?

React Query를 사용해 오프라인 지속성이 적용된 기본 데이터 피드를 구축하고, 당겨서 새로고침과 페이지네이션을 구현하며, 생성 및 삭제 변이를 추가하고, 목록 항목이 나타날 때 애니메이션을 적용합니다. 브라우저에서 직접 실행하는 실습 코드로 React Native Academy을(를) 배우며, 24/7 AI 튜터가 강의를 진행하면서 질문에 답변해줍니다.

React Native Academy을(를) 시작하는 데 경험이 필요한가요?

사전 경험은 필요하지 않습니다. CoddyKit의 React Native Academy은(는) 초급자부터 고급 학습자까지를 위해 구성되어 있으므로, 여기서 시작하거나 처음부터 시작할 수 있으며 자신의 속도대로 진행할 수 있습니다. 이것은 4개 중 3번째 강의입니다.

“핵심 기능: 오프라인을 지원하는 데이터 피드” 강의는 얼마나 걸리나요?

대부분의 CoddyKit 강의는 약 5~10분이 소요됩니다. 각 강의는 간결하고 인터랙티브하여 꾸준한 진행이 가능하며, 웹과 앱에서 중단한 부분부터 바로 시작할 수 있습니다.

이 React Native Academy 강의에서 코드를 작성하고 실행할 수 있나요?

네. 모든 React Native Academy 강의에는 내장 코드 에디터가 포함되어 있으므로, 브라우저에서 바로 실제 코드를 작성하고 실행한 후 즉시 AI 피드백을 받을 수 있습니다 — 로컬 설정이 필요 없습니다.

이 강의의 모든 강의

  1. 아키텍처 및 기술 스택 계획하기
  2. 인증 흐름과 보호된 경로
  3. 핵심 기능: 오프라인을 지원하는 데이터 피드
  4. 다듬기, 테스트 및 양대 스토어 출시
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