Consultas com base na localização

Muitos apps têm documentos que são indexados por locais físicos. Por exemplo, o app pode permitir que os usuários vejam os itens das lojas perto da localização atual deles.

Solução: Geohashes

O Geohash é um sistema para codificar um par de (latitude, longitude) em uma única string Base32. Nesse sistema, o mundo é dividido em uma grade retangular. Cada caractere de uma string de Geohash especifica uma das 32 subdivisões do hash de prefixo. Por exemplo, o Geohash abcd é um dos 32 hashes de quatro caracteres contidos inteiramente no Geohash maior abc.

Quanto maior for o prefixo compartilhado entre dois hashes, mais próximos eles estarão um do outro. Por exemplo, abcdef está mais próximo de abcdeg do que de abcdff. No entanto, o número de caracteres diferentes não significa que a distância é maior. Duas áreas podem estar muito próximas mesmo com Geohashes bem diferentes:

É possível usar Geohashes para armazenar e consultar documentos por posição no Cloud Firestore com uma eficiência razoável. Para fazer isso, é necessário apenas um único campo indexado.

Instalar uma biblioteca auxiliar

Criar e analisar Geohashes envolve alguns cálculos complicados. Por isso, criamos bibliotecas auxiliares para simplificar as partes mais difíceis no Android, Apple e Web:

Web

// Install from NPM. If you prefer to use a static .js file visit
// https://github.com/firebase/geofire-js/releases and download
// geofire-common.min.js from the latest version
npm install --save geofire-common

Web

// Install from NPM. If you prefer to use a static .js file visit
// https://github.com/firebase/geofire-js/releases and download
// geofire-common.min.js from the latest version
npm install --save geofire-common

Swift

Observação: este produto não está disponível para watchOS e destinos de clipes de apps.

// Adicione isso ao pod do Podfile 'GeoFire/Utils'

Kotlin+KTX

// Add this to your app/build.gradle
implementation 'com.firebase:geofire-android-common:3.2.0'

Java

// Add this to your app/build.gradle
implementation 'com.firebase:geofire-android-common:3.1.0'

Armazenar Geohashes

Para cada documento que você quiser indexar por localização, será preciso armazenar um campo de Geohash:

Web

import { doc, updateDoc } from 'firebase/firestore';

// Compute the GeoHash for a lat/lng point
const lat = 51.5074;
const lng = 0.1278;
const hash = geofire.geohashForLocation([lat, lng]);

// Add the hash and the lat/lng to the document. We will use the hash
// for queries and the lat/lng for distance comparisons.
const londonRef = doc(db, 'cities', 'LON');
await updateDoc(londonRef, {
  geohash: hash,
  lat: lat,
  lng: lng
});
fs_geo_add_hash.js

Web

// Compute the GeoHash for a lat/lng point
const lat = 51.5074;
const lng = 0.1278;
const hash = geofire.geohashForLocation([lat, lng]);

// Add the hash and the lat/lng to the document. We will use the hash
// for queries and the lat/lng for distance comparisons.
const londonRef = db.collection('cities').doc('LON');
londonRef.update({
  geohash: hash,
  lat: lat,
  lng: lng
}).then(() => {
  // ...
});
test.solution-geoqueries.js

Swift

Observação: este produto não está disponível para watchOS e destinos de clipes de apps.

// Compute the GeoHash for a lat/lng point
let latitude = 51.5074
let longitude = 0.12780
let location = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)

let hash = GFUtils.geoHash(forLocation: location)

// Add the hash and the lat/lng to the document. We will use the hash
// for queries and the lat/lng for distance comparisons.
let documentData: [String: Any] = [
  "geohash": hash,
  "lat": latitude,
  "lng": longitude
]

let londonRef = db.collection("cities").document("LON")
londonRef.updateData(documentData) { error in
  // ...
}
SolutionGeoPointViewController.swift

Kotlin+KTX

// Compute the GeoHash for a lat/lng point
val lat = 51.5074
val lng = 0.1278
val hash = GeoFireUtils.getGeoHashForLocation(GeoLocation(lat, lng))

// Add the hash and the lat/lng to the document. We will use the hash
// for queries and the lat/lng for distance comparisons.
val updates: MutableMap<String, Any> = mutableMapOf(
    "geohash" to hash,
    "lat" to lat,
    "lng" to lng,
)
val londonRef = db.collection("cities").document("LON")
londonRef.update(updates)
    .addOnCompleteListener {
        // ...
    }
SolutionGeoqueries.kt

Java

// Compute the GeoHash for a lat/lng point
double lat = 51.5074;
double lng = 0.1278;
String hash = GeoFireUtils.getGeoHashForLocation(new GeoLocation(lat, lng));

// Add the hash and the lat/lng to the document. We will use the hash
// for queries and the lat/lng for distance comparisons.
Map<String, Object> updates = new HashMap<>();
updates.put("geohash", hash);
updates.put("lat", lat);
updates.put("lng", lng);

DocumentReference londonRef = db.collection("cities").document("LON");
londonRef.update(updates)
        .addOnCompleteListener(new OnCompleteListener<Void>() {
            @Override
            public void onComplete(@NonNull Task<Void> task) {
                // ...
            }
        });
SolutionGeoqueries.java

Consultar Geohashes

Os Geohashes nos permitem aproximar as consultas de área ao agrupar um conjunto de consultas no campo de Geohash para depois filtrar alguns falsos positivos:

Web

import { collection, query, orderBy, startAt, endAt, getDocs } from 'firebase/firestore';

// Find cities within 50km of London
const center = [51.5074, 0.1278];
const radiusInM = 50 * 1000;

// Each item in 'bounds' represents a startAt/endAt pair. We have to issue
// a separate query for each pair. There can be up to 9 pairs of bounds
// depending on overlap, but in most cases there are 4.
const bounds = geofire.geohashQueryBounds(center, radiusInM);
const promises = [];
for (const b of bounds) {
  const q = query(
    collection(db, 'cities'), 
    orderBy('geohash'), 
    startAt(b[0]), 
    endAt(b[1]));

  promises.push(getDocs(q));
}

// Collect all the query results together into a single list
const snapshots = await Promise.all(promises);

const matchingDocs = [];
for (const snap of snapshots) {
  for (const doc of snap.docs) {
    const lat = doc.get('lat');
    const lng = doc.get('lng');

    // We have to filter out a few false positives due to GeoHash
    // accuracy, but most will match
    const distanceInKm = geofire.distanceBetween([lat, lng], center);
    const distanceInM = distanceInKm * 1000;
    if (distanceInM <= radiusInM) {
      matchingDocs.push(doc);
    }
  }
}
fs_geo_query_hashes.js

Web

// Find cities within 50km of London
const center = [51.5074, 0.1278];
const radiusInM = 50 * 1000;

// Each item in 'bounds' represents a startAt/endAt pair. We have to issue
// a separate query for each pair. There can be up to 9 pairs of bounds
// depending on overlap, but in most cases there are 4.
const bounds = geofire.geohashQueryBounds(center, radiusInM);
const promises = [];
for (const b of bounds) {
  const q = db.collection('cities')
    .orderBy('geohash')
    .startAt(b[0])
    .endAt(b[1]);

  promises.push(q.get());
}

// Collect all the query results together into a single list
Promise.all(promises).then((snapshots) => {
  const matchingDocs = [];

  for (const snap of snapshots) {
    for (const doc of snap.docs) {
      const lat = doc.get('lat');
      const lng = doc.get('lng');

      // We have to filter out a few false positives due to GeoHash
      // accuracy, but most will match
      const distanceInKm = geofire.distanceBetween([lat, lng], center);
      const distanceInM = distanceInKm * 1000;
      if (distanceInM <= radiusInM) {
        matchingDocs.push(doc);
      }
    }
  }

  return matchingDocs;
}).then((matchingDocs) => {
  // Process the matching documents
  // ...
});
test.solution-geoqueries.js

Swift

Observação: este produto não está disponível para watchOS e destinos de clipes de apps.

// Find cities within 50km of London
let center = CLLocationCoordinate2D(latitude: 51.5074, longitude: 0.1278)
let radiusInM: Double = 50 * 1000

// Each item in 'bounds' represents a startAt/endAt pair. We have to issue
// a separate query for each pair. There can be up to 9 pairs of bounds
// depending on overlap, but in most cases there are 4.
let queryBounds = GFUtils.queryBounds(forLocation: center,
                                      withRadius: radiusInM)
let queries = queryBounds.map { bound -> Query in
  return db.collection("cities")
    .order(by: "geohash")
    .start(at: [bound.startValue])
    .end(at: [bound.endValue])
}

@Sendable func fetchMatchingDocs(from query: Query,
                       center: CLLocationCoordinate2D,
                       radiusInMeters: Double) async throws -> [QueryDocumentSnapshot] {
  let snapshot = try await query.getDocuments()
  // Collect all the query results together into a single list
  return snapshot.documents.filter { document in
    let lat = document.data()["lat"] as? Double ?? 0
    let lng = document.data()["lng"] as? Double ?? 0
    let coordinates = CLLocation(latitude: lat, longitude: lng)
    let centerPoint = CLLocation(latitude: center.latitude, longitude: center.longitude)

    // We have to filter out a few false positives due to GeoHash accuracy, but
    // most will match
    let distance = GFUtils.distance(from: centerPoint, to: coordinates)
    return distance <= radiusInM
  }
}

// After all callbacks have executed, matchingDocs contains the result. Note that this code
// executes all queries serially, which may not be optimal for performance.
do {
  let matchingDocs = try await withThrowingTaskGroup(of: [QueryDocumentSnapshot].self) { group -> [QueryDocumentSnapshot] in
    for query in queries {
      group.addTask {
        try await fetchMatchingDocs(from: query, center: center, radiusInMeters: radiusInM)
      }
    }
    var matchingDocs = [QueryDocumentSnapshot]()
    for try await documents in group {
      matchingDocs.append(contentsOf: documents)
    }
    return matchingDocs
  }

  print("Docs matching geoquery: \(matchingDocs)")
} catch {
  print("Unable to fetch snapshot data. \(error)")
}
SolutionGeoPointViewController.swift

Kotlin+KTX

// Find cities within 50km of London
val center = GeoLocation(51.5074, 0.1278)
val radiusInM = 50.0 * 1000.0

// Each item in 'bounds' represents a startAt/endAt pair. We have to issue
// a separate query for each pair. There can be up to 9 pairs of bounds
// depending on overlap, but in most cases there are 4.
val bounds = GeoFireUtils.getGeoHashQueryBounds(center, radiusInM)
val tasks: MutableList<Task<QuerySnapshot>> = ArrayList()
for (b in bounds) {
    val q = db.collection("cities")
        .orderBy("geohash")
        .startAt(b.startHash)
        .endAt(b.endHash)
    tasks.add(q.get())
}

// Collect all the query results together into a single list
Tasks.whenAllComplete(tasks)
    .addOnCompleteListener {
        val matchingDocs: MutableList<DocumentSnapshot> = ArrayList()
        for (task in tasks) {
            val snap = task.result
            for (doc in snap!!.documents) {
                val lat = doc.getDouble("lat")!!
                val lng = doc.getDouble("lng")!!

                // We have to filter out a few false positives due to GeoHash
                // accuracy, but most will match
                val docLocation = GeoLocation(lat, lng)
                val distanceInM = GeoFireUtils.getDistanceBetween(docLocation, center)
                if (distanceInM <= radiusInM) {
                    matchingDocs.add(doc)
                }
            }
        }

        // matchingDocs contains the results
        // ...
    }
SolutionGeoqueries.kt

Java

// Find cities within 50km of London
final GeoLocation center = new GeoLocation(51.5074, 0.1278);
final double radiusInM = 50 * 1000;

// Each item in 'bounds' represents a startAt/endAt pair. We have to issue
// a separate query for each pair. There can be up to 9 pairs of bounds
// depending on overlap, but in most cases there are 4.
List<GeoQueryBounds> bounds = GeoFireUtils.getGeoHashQueryBounds(center, radiusInM);
final List<Task<QuerySnapshot>> tasks = new ArrayList<>();
for (GeoQueryBounds b : bounds) {
    Query q = db.collection("cities")
            .orderBy("geohash")
            .startAt(b.startHash)
            .endAt(b.endHash);

    tasks.add(q.get());
}

// Collect all the query results together into a single list
Tasks.whenAllComplete(tasks)
        .addOnCompleteListener(new OnCompleteListener<List<Task<?>>>() {
            @Override
            public void onComplete(@NonNull Task<List<Task<?>>> t) {
                List<DocumentSnapshot> matchingDocs = new ArrayList<>();

                for (Task<QuerySnapshot> task : tasks) {
                    QuerySnapshot snap = task.getResult();
                    for (DocumentSnapshot doc : snap.getDocuments()) {
                        double lat = doc.getDouble("lat");
                        double lng = doc.getDouble("lng");

                        // We have to filter out a few false positives due to GeoHash
                        // accuracy, but most will match
                        GeoLocation docLocation = new GeoLocation(lat, lng);
                        double distanceInM = GeoFireUtils.getDistanceBetween(docLocation, center);
                        if (distanceInM <= radiusInM) {
                            matchingDocs.add(doc);
                        }
                    }
                }

                // matchingDocs contains the results
                // ...
            }
        });
SolutionGeoqueries.java

Limitações

O uso de Geohashes para consultar locais nos oferece novos recursos, mas também inclui um conjunto próprio de limitações:

• Falsos positivos: a consulta por Geohash não é exata, e é necessário filtrar os resultados falsos positivos no lado do cliente. Essas leituras extras adicionam custo e latência ao seu app.
• Casos extremos ("edge cases"): esse método de consulta depende da estimativa da distância entre as linhas de longitude/latitude. A precisão dessa estimativa diminui à medida que os pontos se aproximam do Polo Norte ou do Polo Sul, o que significa que as consultas de Geohash têm mais falsos positivos em latitudes extremas.