Expo Background Tasks Overview

Overview

This lecture covers the Expo Background Task module, explaining how to run logic when your app is not open, including setup, use cases, constraints, and implementation in a React Native app.

Background Tasks: Concepts & Constraints

• Background tasks allow apps to run custom logic when not in the foreground (e.g., prefetch data, sync, OTA updates).
• Tasks can be scheduled with a minimum interval of 15 minutes, but iOS often schedules them less frequently (like overnight).
• Task execution depends on device state (network, battery, user activity) and may be skipped if resources are low.
• Users can disable background refresh or kill the app, which stops background tasks.
• Best practices: keep tasks light, batch operations, avoid heavy processing.

Use Cases & Examples

• Sync user data, backup, upload logs, or download updates automatically.
• Prefetch content to ensure fast load times when users return to the app.
• The demo app fetches and saves daily quotes in the background for instant display.

Setup & Dependencies

• Install dependencies: expo install react-native-async-storage, expo-task-manager, expo-background-task.
• Configure Expo for iOS/Android; Expo background task depends on Expo Task Manager.
• Only test on real iOS devices; Android emulators support background tasks.

Implementation Steps

• Define constants: task identifier, minimum interval (e.g., 15 min), async storage key, and maximum history size.
• Create TypeScript types for quotes and quote history.
• Implement core functions:
  • initializeBackgroundTask: registers background task and ensures it runs only after app JS loads.
  • Background task logic: fetches a quote, adds a timestamp, maintains a max history, and stores it.
  • Use an “inner app mounted” promise to ensure AsyncStorage is accessible.
• Use React hooks to call initialization, load stored quotes, and set up state.
• Display quote history in the UI.
• Add a debug button to manually trigger background tasks for testing.
• Use AppState to detect when the app returns to foreground and reload quote history.
• Clean up AppState subscriptions on component unmount.

Key Terms & Definitions

• Background Task — A process running logic while the app is in the background.
• AsyncStorage — Local persistent storage for React Native apps.
• Expo Task Manager — Expo module for managing long-running/background tasks.
• AppState — React Native API to track app foreground/background status.
• Minimum Interval — Shortest possible time between background task executions (15 min).

Action Items / Next Steps

• Review Expo Background Task and Task Manager documentation.
• Test background tasks on a real iOS device.
• Clean up AppState event listeners when components unmount.