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React Native Academy · レッスン

expo-locationによるGPS位置情報の取得

フォアグラウンド位置情報の権限をリクエストし、getCurrentPositionAsyncで現在の座標を取得して、画面に緯度と経度を表示します

「expo-locationによるGPS位置情報の取得」はCoddyKit上の無料React Native Academyレッスンです。 これはレッスン2/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはReact Native Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 React Native Academyコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

What Is expo-location?

expo-location provides an API to read the device's geographic position using GPS, Wi-Fi triangulation, and cell tower data. It works on both iOS and Android without writing native code. You can get a one-time position fix, watch the location as the user moves, or geocode an address to coordinates and vice versa.

Installing expo-location

Use the Expo install command to get the SDK-compatible version of expo-location. In managed Expo projects, the native module is linked automatically. For bare React Native projects, you must also run pod install on iOS and update AndroidManifest.xml with the location permission declarations.

npx expo install expo-location

Requesting Foreground Location Permission

Location access requires explicit user permission. Call Location.requestForegroundPermissionsAsync() to ask for permission to read location while the app is in the foreground. Check the status field of the returned object — only proceed if it equals 'granted'. The hook version Location.useForegroundPermissions() simplifies this in functional components.

import * as Location from 'expo-location';

const [permission, requestPermission] = Location.useForegroundPermissions();

if (!permission) return <ActivityIndicator />;

if (!permission.granted) {
  return (
    <View>
      <Text>Location access is needed to show your position.</Text>
      <Button title='Allow Location' onPress={requestPermission} />
    </View>
  );
}

Getting the Current Position Once

Location.getCurrentPositionAsync() performs a single GPS fix and returns a LocationObject containing coords.latitude, coords.longitude, coords.accuracy (in meters), and a timestamp. Use this for one-time lookups like 'find nearby restaurants' where you do not need continuous tracking.

const [location, setLocation] = React.useState(null);

async function fetchLocation() {
  const loc = await Location.getCurrentPositionAsync({
    accuracy: Location.Accuracy.Balanced,
  });
  setLocation(loc);
  console.log('Lat:', loc.coords.latitude);
  console.log('Lng:', loc.coords.longitude);
  console.log('Accuracy:', loc.coords.accuracy, 'meters');
}

Accuracy Levels

expo-location provides several accuracy presets via Location.Accuracy. Use Lowest or Low for coarse location with minimal battery impact, Balanced for general use, and High or BestForNavigation when you need precise GPS coordinates for turn-by-turn navigation. Higher accuracy drains battery faster, so choose appropriately for your use case.

// Coarse — fast, low battery, uses Wi-Fi/cell
Location.Accuracy.Lowest      // ~3 km
Location.Accuracy.Low         // ~1 km
Location.Accuracy.Balanced    // ~100 m — good default

// Precise — uses GPS, more battery
Location.Accuracy.High        // ~10 m
Location.Accuracy.Highest     // ~1 m
Location.Accuracy.BestForNavigation // highest + sensors

Watching the Location Continuously

Location.watchPositionAsync starts a subscription that calls a callback every time the device's position changes by a specified distance or time interval. It returns a subscription object with a remove() method. Always call remove() in a useEffect cleanup function to stop tracking when the component unmounts and save battery.

useEffect(() => {
  let subscription: Location.LocationSubscription;

  async function startWatching() {
    subscription = await Location.watchPositionAsync(
      {
        accuracy: Location.Accuracy.High,
        distanceInterval: 10, // update every 10 meters
        timeInterval: 5000,   // or every 5 seconds
      },
      (loc) => setLocation(loc)
    );
  }

  startWatching();
  return () => subscription?.remove(); // cleanup on unmount
}, []);

Displaying Coordinates on Screen

Once you have a location object, display the coordinates in a human-readable format. Round latitude and longitude to 5-6 decimal places for readability — that corresponds to roughly 1-meter precision. Display the accuracy value so users understand how precise the reading is, especially on devices that cannot get a good GPS fix indoors.

export default function LocationDisplay() {
  const [location, setLocation] = React.useState(null);
  // ... permission check and fetch ...

  if (!location) return <Button title='Get Location' onPress={fetchLocation} />;

  return (
    <View>
      <Text>Latitude: {location.coords.latitude.toFixed(6)}</Text>
      <Text>Longitude: {location.coords.longitude.toFixed(6)}</Text>
      <Text>Accuracy: {location.coords.accuracy?.toFixed(0)} m</Text>
    </View>
  );
}

Reverse Geocoding: Coordinates to Address

Location.reverseGeocodeAsync converts latitude and longitude coordinates into a human-readable address. It returns an array of LocationGeocodedAddress objects with fields like street, city, region, country, and postalCode. Use the first result — it is the most likely match for the given coordinates.

async function getAddress(latitude: number, longitude: number) {
  const results = await Location.reverseGeocodeAsync({ latitude, longitude });
  if (results.length > 0) {
    const addr = results[0];
    return [addr.street, addr.city, addr.region, addr.country]
      .filter(Boolean)
      .join(', ');
  }
  return 'Unknown location';
}

Forward Geocoding: Address to Coordinates

Location.geocodeAsync converts a text address into latitude and longitude. This is useful for features like 'search nearby' where users type a location name. The function returns an array of results (multiple matches are possible for ambiguous addresses) each containing latitude, longitude, and altitude.

async function searchAddress(address: string) {
  const results = await Location.geocodeAsync(address);
  if (results.length > 0) {
    const { latitude, longitude } = results[0];
    console.log('Coordinates:', latitude, longitude);
    return { latitude, longitude };
  }
  return null;
}

Handling Location Errors

Location requests can fail when the user is indoors, when the device's GPS is disabled, or on simulators that do not have a simulated location set. Wrap location calls in try-catch and show a fallback UI. Check Location.hasServicesEnabledAsync() before requesting a position to detect when the device's location services are turned off in system settings.

async function fetchLocationSafe() {
  const enabled = await Location.hasServicesEnabledAsync();
  if (!enabled) {
    Alert.alert('Location Off', 'Please enable Location Services in Settings.');
    return;
  }
  try {
    const loc = await Location.getCurrentPositionAsync({
      accuracy: Location.Accuracy.Balanced,
    });
    setLocation(loc);
  } catch (err) {
    Alert.alert('Error', 'Could not get your location. Please try again.');
  }
}

Background Location Tracking

Tracking location when the app is in the background requires background location permission (requestBackgroundPermissionsAsync) and a background task defined with expo-task-manager. This is only needed for apps like fitness trackers or delivery apps. Apple and Google have strict review requirements for apps that use background location, so only request it if the feature is core to your app.

Quick Check

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

Lesson Recap

In this lesson you learned: requestForegroundPermissionsAsync must be called and granted before any location reading is possible, getCurrentPositionAsync performs a one-time position fix while watchPositionAsync tracks continuous movement, and reverseGeocodeAsync converts coordinates to a human-readable address. Next up we explore scheduling local notifications.

よくある質問

「expo-locationによるGPS位置情報の取得」レッスンは無料ですか？

はい。「expo-locationによるGPS位置情報の取得」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、React Native Academyコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 React Native Academyコースには全4レッスンが含まれています。

「expo-locationによるGPS位置情報の取得」で何を学びますか？

フォアグラウンド位置情報の権限をリクエストし、getCurrentPositionAsyncで現在の座標を取得して、画面に緯度と経度を表示します ブラウザで直接実行するハンズオンコードでReact Native Academyを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

React Native Academyを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのReact Native Academyは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン2/4です。

「expo-locationによるGPS位置情報の取得」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このReact Native Academyレッスンでコードを書いて実行できますか？

はい。すべてのReact Native Academyレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. expo-cameraによるカメラアクセス
  2. expo-locationによるGPS位置情報の取得
  3. ローカル通知のスケジュール設定
  4. 権限の適切な処理
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