Everything in Mobile Dev, page 6
Handling async API calls in Redux
Middleware intercepts dispatched actions, Thunk dispatches functions for simple flows, Saga uses generators for complex orchestration.
Redux principles: store, actions, reducers
One store holds all state; actions are plain objects describing what happened; pure reducers compute the next immutable state from current state and action; dispatch drives the cycle.
Context re-renders when unrelated value changes
Every consumer re-renders when the Provider value reference changes, regardless of which field it uses; fix by splitting into separate contexts or memoizing and selecting.
Local state versus global state management
Keep state local with useState/useReducer when it is used by one component or its subtree; reach for Redux or Zustand when many distant components share and mutate the same state.
Prop drilling and the Context API
Prop drilling is threading props through many intermediate components that do not use them; Context provides a value to a subtree so consumers read it directly.
SectionList versus FlatList for grouped data
FlatList takes a flat data array; SectionList takes a sections array of objects each with a data array and optional title, and supports renderSectionHeader and sticky headers.
FlatList virtualization and windowing trade-offs
FlatList renders only items within a window of viewports around the visible area, unmounting others; larger windowSize means fewer blanks but more memory, smaller means less memory but more blank flashes.
Preventing unnecessary FlatList item re-renders
Extract the row into a React.memo component, pass stable memoized callbacks and primitive props, and keep renderItem referentially stable so only changed items re-render.
Optimizing FlatList with variable item heights
Implement getItemLayout to return each item's length and cumulative offset so FlatList skips measurement and scrollToIndex works precisely; precompute heights per item type.
Implementing infinite scroll with FlatList
Use onEndReached with onEndReachedThreshold to fetch the next page, append results, and show a footer loader; guard with a loading flag to prevent duplicate fetches.
Re-rendering FlatList on external state change
FlatList is a PureComponent and only checks data and extraData by reference, so pass the external value via extraData and ensure renderItem reads current state.
Diagnosing slow FlatList scroll and blank cells
Memoize renderItem and items, supply getItemLayout for fixed heights, tune windowSize, maxToRenderPerBatch, initialNumToRender, and removeClippedSubviews; profile first.
FlatList data, renderItem, and keyExtractor
Data is the array, renderItem maps each item to a component, keyExtractor returns a stable unique key letting React reconcile and reuse rows.
ScrollView versus FlatList for long lists
ScrollView mounts all children at once; FlatList virtualizes, rendering only on-screen items plus a buffer. Use ScrollView for small/fixed content, FlatList for long/dynamic data.
Type-safe navigation with TypeScript
Define a ParamList mapping screen names to param types, type Screen via NativeStackScreenProps, and type useNavigation with the navigation prop generic so route names and params autocomplete.
How react-native-screens optimizes a deep stack
React-native-screens uses native container views so off-screen screens are detached from the view hierarchy and can be freed; enabled by default and powers the native-stack navigator.
Configuring deep linking to a parameterized screen
Register native URL schemes/intent filters, define a linking config with prefixes and screen-to-path mapping including params, pass it to NavigationContainer.
Setting screen options statically versus dynamically
Options prop on Screen (can read route) versus navigation.setOptions inside the screen for state-driven changes.
Accessing navigation outside a screen component
Use the useNavigation hook inside any component under NavigationContainer; for navigating outside React, use a navigationRef.
Refetching data when a screen gains focus
Use useFocusEffect with a useCallback callback; refetch on focus; clean up to abort stale requests.
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