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React Native Academy · Lektion

Kernfunktion: Daten-Feed mit Offline-Unterstützung

Erstellen Sie den Hauptdaten-Feed mit React Query und Offline-Persistenz, implementieren Sie Pull-to-Refresh und Pagination, fügen Sie Mutationen zum Erstellen und Löschen hinzu und animieren Sie das Einblenden von Listenelementen.

Kernfunktion: Daten-Feed mit Offline-Unterstützung ist eine kostenlose React Native Academy-Lektion auf CoddyKit. Dies ist Lektion 3 von 4. Du kannst die komplette Lektion unten kostenlos lesen – dann übst du sie direkt im Browser mit einem integrierten Code-Editor und einem KI-Tutor rund um die Uhr. Sie ist Teil des React Native Academy-Lernpfads, und dein Fortschritt wird über Web und CoddyKit-App synchronisiert. Der React Native Academy-Kurs umfasst insgesamt 4 Lektionen.

Teile dieser Lektion wurden noch nicht übersetzt und werden auf Englisch angezeigt.

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.

Häufig gestellte Fragen

Ist die Lektion „Kernfunktion: Daten-Feed mit Offline-Unterstützung“ kostenlos?

Ja — der vollständige Text von „Kernfunktion: Daten-Feed mit Offline-Unterstützung“ ist hier im Web kostenlos zu lesen. Um sie interaktiv zu üben (integrierter Code-Editor und 24/7 KI-Tutor) und den Rest des React Native Academy-Kurses freizuschalten, upgrade auf CoddyKit PRO. Der React Native Academy-Kurs umfasst insgesamt 4 Lektionen.

Was lerne ich in „Kernfunktion: Daten-Feed mit Offline-Unterstützung“?

Erstellen Sie den Hauptdaten-Feed mit React Query und Offline-Persistenz, implementieren Sie Pull-to-Refresh und Pagination, fügen Sie Mutationen zum Erstellen und Löschen hinzu und animieren Sie das… Du übst React Native Academy mit praktischem Code, den du direkt im Browser ausführst, und ein 24/7 KI-Tutor beantwortet deine Fragen während du die Lektion bearbeitest.

Brauche ich Erfahrung, um React Native Academy zu starten?

Keine Vorkenntnisse erforderlich. React Native Academy auf CoddyKit ist für Anfänger bis fortgeschrittene Lernende strukturiert, sodass du hier starten oder von Anfang an beginnen und in deinem eigenen Tempo voranschreiten kannst. Dies ist Lektion 3 von 4.

Wie lange dauert die Lektion „Kernfunktion: Daten-Feed mit Offline-Unterstützung“?

Die meisten CoddyKit-Lektionen dauern etwa 5–10 Minuten. Jede ist kompakt und interaktiv, sodass du stetig Fortschritte machst und genau dort weitermachst, wo du aufgehört hast – im Web und in der App.

Kann ich in dieser React Native Academy-Lektion Code schreiben und ausführen?

Ja. Jede React Native Academy-Lektion enthält einen integrierten Code-Editor, sodass du echten Code direkt in deinem Browser schreibst und ausführst und sofort KI-Feedback erhältst — ohne lokale Einrichtung erforderlich.

Alle Lektionen in diesem Kurs

  1. Architektur und Tech-Stack planen
  2. Authentifizierungsablauf und geschützte Routen
  3. Kernfunktion: Daten-Feed mit Offline-Unterstützung
  4. Feinschliff, Tests und Veröffentlichung in beiden Stores
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