Cloud Firestore 支持离线数据持久化。这一功能会为您的应用正在使用的 Cloud Firestore 数据缓存一份副本,以便让您的应用可以在设备离线时访问数据。您可以写入、读取、监听和查询缓存的数据。当设备恢复在线状态时,Cloud Firestore 会将您的应用在本地所做的所有更改都同步到 Cloud Firestore 后端。
您无需更改用于访问 Cloud Firestore 数据的代码,即可使用离线持久化功能。启用离线持久化后,Cloud Firestore 客户端库会自动管理在线和离线数据访问,并在设备恢复在线状态时同步本地数据。
配置离线持久化
在初始化 Cloud Firestore 时,您可以启用或停用离线持久化:
- 对于 Android 和 Apple 平台,离线持久化默认处于启用状态。如需停用持久化,请将
PersistenceEnabled
选项设置为false
。 - 对于 Web 应用,离线持久化默认处于停用状态。如需启用持久化,请调用
enablePersistence
方法。在不同会话之间切换时,系统不会自动清除 Cloud Firestore 的缓存。因此,如果您的 Web 应用涉及到敏感信息的处理,请务必在启用持久化之前询问用户其设备是否可信。
Web version 9
// Memory cache is the default if no config is specified.
initializeFirestore(app);
// This is the default behavior if no persistence is specified.
initializeFirestore(app, {localCache: memoryLocalCache()});
// Defaults to single-tab persistence if no tab manager is specified.
initializeFirestore(app, {localCache: persistentLocalCache(/*settings*/{})});
// Same as `initializeFirestore(app, {localCache: persistentLocalCache(/*settings*/{})})`,
// but more explicit about tab management.
initializeFirestore(app,
{localCache:
persistentLocalCache(/*settings*/{tabManager: persistentSingleTabManager()})
});
// Use multi-tab IndexedDb persistence.
initializeFirestore(app,
{localCache:
persistentLocalCache(/*settings*/{tabManager: persistentMultipleTabManager()})
});
Web version 8
firebase.firestore().enablePersistence() .catch((err) => { if (err.code == 'failed-precondition') { // Multiple tabs open, persistence can only be enabled // in one tab at a a time. // ... } else if (err.code == 'unimplemented') { // The current browser does not support all of the // features required to enable persistence // ... } }); // Subsequent queries will use persistence, if it was enabled successfully
Swift
let settings = FirestoreSettings() settings.isPersistenceEnabled = true // Any additional options // ... // Enable offline data persistence let db = Firestore.firestore() db.settings = settings
Objective-C
FIRFirestoreSettings *settings = [[FIRFirestoreSettings alloc] init]; settings.persistenceEnabled = YES; // Any additional options // ... // Enable offline data persistence FIRFirestore *db = [FIRFirestore firestore]; db.settings = settings;
Kotlin+KTX
val settings = firestoreSettings { isPersistenceEnabled = true } db.firestoreSettings = settings
Java
FirebaseFirestoreSettings settings = new FirebaseFirestoreSettings.Builder() .setPersistenceEnabled(true) .build(); db.setFirestoreSettings(settings);
Dart
// Apple and Android db.settings = const Settings(persistenceEnabled: true); // Web await db .enablePersistence(const PersistenceSettings(synchronizeTabs: true));
配置缓存大小
启用持久化后,Cloud Firestore 会缓存从后端接收的每个文档以便在离线状态下进行访问。Cloud Firestore 会为缓存大小设置默认阈值。超出默认值后,Cloud Firestore 会定期尝试清理较旧的未使用文档。您可以配置不同的缓存大小阈值,也可以完全停用清理功能:
Web version 9
import { initializeFirestore, CACHE_SIZE_UNLIMITED } from "firebase/firestore"; const firestoreDb = initializeFirestore(app, { cacheSizeBytes: CACHE_SIZE_UNLIMITED });
Web version 8
firebase.firestore().settings({ cacheSizeBytes: firebase.firestore.CACHE_SIZE_UNLIMITED });
Swift
// The default cache size threshold is 100 MB. Configure "cacheSizeBytes" // for a different threshold (minimum 1 MB) or set to "FirestoreCacheSizeUnlimited" // to disable clean-up. let settings = Firestore.firestore().settings settings.cacheSizeBytes = FirestoreCacheSizeUnlimited Firestore.firestore().settings = settings
Objective-C
// The default cache size threshold is 100 MB. Configure "cacheSizeBytes" // for a different threshold (minimum 1 MB) or set to "kFIRFirestoreCacheSizeUnlimited" // to disable clean-up. FIRFirestoreSettings *settings = [FIRFirestore firestore].settings; settings.cacheSizeBytes = kFIRFirestoreCacheSizeUnlimited; [FIRFirestore firestore].settings = settings;
Kotlin+KTX
// The default cache size threshold is 100 MB. Configure "setCacheSizeBytes" // for a different threshold (minimum 1 MB) or set to "CACHE_SIZE_UNLIMITED" // to disable clean-up. val settings = FirebaseFirestoreSettings.Builder() .setCacheSizeBytes(FirebaseFirestoreSettings.CACHE_SIZE_UNLIMITED) .build() db.firestoreSettings = settings
Java
// The default cache size threshold is 100 MB. Configure "setCacheSizeBytes" // for a different threshold (minimum 1 MB) or set to "CACHE_SIZE_UNLIMITED" // to disable clean-up. FirebaseFirestoreSettings settings = new FirebaseFirestoreSettings.Builder() .setCacheSizeBytes(FirebaseFirestoreSettings.CACHE_SIZE_UNLIMITED) .build(); db.setFirestoreSettings(settings);
Dart
db.settings = const Settings( persistenceEnabled: true, cacheSizeBytes: Settings.CACHE_SIZE_UNLIMITED, );
监听离线数据
当设备离线时,如果您已启用离线持久化,则当本地缓存的数据发生更改时,您的监听器将收到监听事件。您可以监听文档、集合和查询。
如需检查您收到的数据是来自服务器还是来自缓存,可使用快照事件的 SnapshotMetadata
中的 fromCache
属性。如果 fromCache
为 true
,则说明数据来自缓存,可能是过时的或不完整的。如果 fromCache
为 false
,则说明数据是完整的,并且与服务器上的最新更新保持一致。
默认情况下,如果只有 SnapshotMetadata
发生更改,则不会引发任何事件。如果您依赖于 fromCache
值,请在附加监听处理程序时指定 includeMetadataChanges
监听选项。
Web version 9
import { collection, onSnapshot, where, query } from "firebase/firestore"; const q = query(collection(db, "cities"), where("state", "==", "CA")); onSnapshot(q, { includeMetadataChanges: true }, (snapshot) => { snapshot.docChanges().forEach((change) => { if (change.type === "added") { console.log("New city: ", change.doc.data()); } const source = snapshot.metadata.fromCache ? "local cache" : "server"; console.log("Data came from " + source); }); });
Web version 8
db.collection("cities").where("state", "==", "CA") .onSnapshot({ includeMetadataChanges: true }, (snapshot) => { snapshot.docChanges().forEach((change) => { if (change.type === "added") { console.log("New city: ", change.doc.data()); } var source = snapshot.metadata.fromCache ? "local cache" : "server"; console.log("Data came from " + source); }); });
Swift
// Listen to metadata updates to receive a server snapshot even if // the data is the same as the cached data. db.collection("cities").whereField("state", isEqualTo: "CA") .addSnapshotListener(includeMetadataChanges: true) { querySnapshot, error in guard let snapshot = querySnapshot else { print("Error retreiving snapshot: \(error!)") return } for diff in snapshot.documentChanges { if diff.type == .added { print("New city: \(diff.document.data())") } } let source = snapshot.metadata.isFromCache ? "local cache" : "server" print("Metadata: Data fetched from \(source)") }
Objective-C
// Listen to metadata updates to receive a server snapshot even if // the data is the same as the cached data. [[[db collectionWithPath:@"cities"] queryWhereField:@"state" isEqualTo:@"CA"] addSnapshotListenerWithIncludeMetadataChanges:YES listener:^(FIRQuerySnapshot *snapshot, NSError *error) { if (snapshot == nil) { NSLog(@"Error retreiving snapshot: %@", error); return; } for (FIRDocumentChange *diff in snapshot.documentChanges) { if (diff.type == FIRDocumentChangeTypeAdded) { NSLog(@"New city: %@", diff.document.data); } } NSString *source = snapshot.metadata.isFromCache ? @"local cache" : @"server"; NSLog(@"Metadata: Data fetched from %@", source); }];
Kotlin+KTX
db.collection("cities").whereEqualTo("state", "CA") .addSnapshotListener(MetadataChanges.INCLUDE) { querySnapshot, e -> if (e != null) { Log.w(TAG, "Listen error", e) return@addSnapshotListener } for (change in querySnapshot!!.documentChanges) { if (change.type == DocumentChange.Type.ADDED) { Log.d(TAG, "New city: ${change.document.data}") } val source = if (querySnapshot.metadata.isFromCache) { "local cache" } else { "server" } Log.d(TAG, "Data fetched from $source") } }
Java
db.collection("cities").whereEqualTo("state", "CA") .addSnapshotListener(MetadataChanges.INCLUDE, new EventListener<QuerySnapshot>() { @Override public void onEvent(@Nullable QuerySnapshot querySnapshot, @Nullable FirebaseFirestoreException e) { if (e != null) { Log.w(TAG, "Listen error", e); return; } for (DocumentChange change : querySnapshot.getDocumentChanges()) { if (change.getType() == Type.ADDED) { Log.d(TAG, "New city:" + change.getDocument().getData()); } String source = querySnapshot.getMetadata().isFromCache() ? "local cache" : "server"; Log.d(TAG, "Data fetched from " + source); } } });
Dart
db .collection("cities") .where("state", isEqualTo: "CA") .snapshots(includeMetadataChanges: true) .listen((querySnapshot) { for (var change in querySnapshot.docChanges) { if (change.type == DocumentChangeType.added) { final source = (querySnapshot.metadata.isFromCache) ? "local cache" : "server"; print("Data fetched from $source}"); } } });
获取离线数据
如果您在设备离线时获取某个文档,Cloud Firestore 会从缓存中返回数据。
查询集合时,如果没有缓存的文档,系统会返回空结果。提取特定文档时,系统会返回错误。
查询离线数据
查询功能可与离线持久化功能结合使用。您可以直接使用 get 方法或通过监听(如前面的部分所述)来检索查询的结果。当设备离线时,您也可以针对已在本地永久保存的数据创建新的查询,但这些查询最开始只会针对缓存的文档运行。
配置离线查询索引
默认情况下,在执行离线查询时,Firestore SDK 会在其本地缓存中扫描集合内包含的所有文档。鉴于此默认行为,如果用户长时间处于离线状态,离线查询的性能可能会受到影响。
您可以通过配置本地查询索引来提高离线查询的性能:
Swift
Apple 平台 SDK 提供了一个 setIndexConfiguration
方法,可读取用来在服务器上配置索引的相同 JSON 结构配置,并采用相同的索引定义格式。
// You will normally read this from a file asset or cloud storage. let indexConfigJson = """ { indexes: [ ... ], fieldOverrides: [ ... ] } """ // Apply the configuration. Firestore.firestore().setIndexConfiguration(indexConfigJson)
Objective-C
Apple 平台 SDK 提供了一些 setIndexConfiguration
方法,可读取用来在服务器上配置索引的相同 JSON 结构配置,并采用相同的索引定义格式。
// You will normally read this from a file asset or cloud storage. NSString *indexConfigJson = @" { " " indexes: [ " " ... " " ], " " fieldOverrides: [ " " ... " " ] " " } "; // Apply the configuration. [[FIRFirestore firestore] setIndexConfigurationFromJSON:indexConfigJson completion:^(NSError * _Nullable error) { // ... }];
Java
Android SDK 提供了一个 setIndexConfiguration
方法,可读取用来在服务器上配置索引的相同 JSON 结构配置,并采用相同的索引定义格式。
// You will normally read this from a file asset or cloud storage. String indexConfigJson = " { " + " indexes: [ " + " ... " + " ], " + " fieldOverrides: [ " + " ... " + " ] " + " } "; // Apply the configuration. FirebaseFirestore.getInstance().setIndexConfiguration(indexConfigJson);
Kotlin+KTX
Android SDK 提供了一个 setIndexConfiguration
方法,可读取用来在服务器上配置索引的相同 JSON 结构配置,并采用相同的索引定义格式。
// You will normally read this from a file asset or cloud storage. val indexConfigJson = """ { indexes: [ ... ], fieldOverrides: [ ... ] } """ // Apply the configuration. FirebaseFirestore.getInstance().setIndexConfiguration(indexConfigJson)
Dart
Flutter SDK 提供了一个 setIndexConfigurationFromJSON
方法,可读取用来在服务器上配置索引的相同 JSON 结构配置,并采用相同的索引定义格式。
// You will normally read this from a file asset or cloud storage. var indexConfigJson = """ { indexes: [ ... ], fieldOverrides: [ ... ] } """; // Apply the configuration. await FirebaseFirestore.instance.setIndexConfigurationFromJSON(json: indexConfigJson);
或者,您也可以使用 setIndexConfiguration
方法通过基于类的 API 配置索引。
var indexes = [ Index( collectionGroup: "posts", queryScope: QueryScope.collection, fields: [ IndexField(fieldPath: "author", arrayConfig: ArrayConfig.contains), IndexField(fieldPath: "timestamp", order: Order.descending) ], ), ]; await FirebaseFirestore.instance.setIndexConfiguration(indexes: indexes);
具体使用何种离线索引配置,取决于您的应用在离线时会频繁访问哪些集合和文档,以及您对离线性能有何要求。虽然您可以导出后端索引配置以在客户端上使用,但应用的离线访问模式与在线访问模式可能有很大区别,因此您的在线索引配置可能不适合在离线时使用。您需要考虑自己希望应用离线访问哪些集合和文档,同时还兼具高性能。对应用的行为进行分析后,在定义索引时,您还需遵循索引指南中提供的原则。
如需提供离线索引配置以便在客户端应用中加载,请执行以下操作:
- 对这些配置进行编译并通过您的应用进行分发
- 从 CDN 下载配置
- 从 Cloud Storage for Firebase 等存储系统提取配置。
停用和启用网络访问权限
您可以使用以下方法为您的 Cloud Firestore 客户端停用网络访问权限。网络访问权限停用后,所有快照监听器和文档请求都将从缓存中检索结果。写入操作会排入队列,直到网络访问权限重新启用。
Web version 9
import { disableNetwork } from "firebase/firestore"; await disableNetwork(db); console.log("Network disabled!"); // Do offline actions // ...
Web version 8
firebase.firestore().disableNetwork() .then(() => { // Do offline actions // ... });
Swift
Firestore.firestore().disableNetwork { (error) in // Do offline things // ... }
Objective-C
[[FIRFirestore firestore] disableNetworkWithCompletion:^(NSError *_Nullable error) { // Do offline actions // ... }];
Kotlin+KTX
db.disableNetwork().addOnCompleteListener { // Do offline things // ... }
Java
db.disableNetwork() .addOnCompleteListener(new OnCompleteListener<Void>() { @Override public void onComplete(@NonNull Task<Void> task) { // Do offline things // ... } });
Dart
db.disableNetwork().then((_) { // Do offline things });
使用以下方法可重新启用网络访问权限:
Web version 9
import { enableNetwork } from "firebase/firestore"; await enableNetwork(db); // Do online actions // ...
Web version 8
firebase.firestore().enableNetwork() .then(() => { // Do online actions // ... });
Swift
Firestore.firestore().enableNetwork { (error) in // Do online things // ... }
Objective-C
[[FIRFirestore firestore] enableNetworkWithCompletion:^(NSError *_Nullable error) { // Do online actions // ... }];
Kotlin+KTX
db.enableNetwork().addOnCompleteListener { // Do online things // ... }
Java
db.enableNetwork() .addOnCompleteListener(new OnCompleteListener<Void>() { @Override public void onComplete(@NonNull Task<Void> task) { // Do online things // ... } });
Dart
db.enableNetwork().then((_) { // Back online });