Cloud Firestore 支持离线数据持久化。这一功能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任何更改同步到 Cloud Firestore 的后端。
您不必对访问 Cloud Firestore 数据时所用的代码进行任何更改，就可以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，并在设备重新在线时同步本地数据。
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 iOS 版应用，离线持久化默认为启用状态。如需停用持久化，请将
PersistenceEnabled选项设置为false。 - 对于 Web 应用，离线持久化默认为停用状态。如需启用持久化，可调用
enablePersistence方法。在各个会话之间切换时，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
firebase.firestore().enablePersistence()
.catch(function(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;
Java
FirebaseFirestoreSettings settings = new FirebaseFirestoreSettings.Builder()
.setPersistenceEnabled(true)
.build();
db.setFirestoreSettings(settings);
Kotlin
val settings = FirebaseFirestoreSettings.Builder()
.setPersistenceEnabled(true)
.build()
db.firestoreSettings = settings
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进行离线访问。Cloud Firestore 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过程：
Web
// The default cache size threshold is 40 MB. Configure "cacheSizeBytes"
// for a different threshold (minimum 1 MB) or set to "CACHE_SIZE_UNLIMITED"
// to disable clean-up.
firebase.firestore().settings({
cacheSizeBytes: firebase.firestore.CACHE_SIZE_UNLIMITED
});
firebase.firestore().enablePersistence()
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;
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);
Kotlin
// 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
侦听离线数据
当设备离线时，如果已启用离线持久化，则当本地缓存的数据发生更改时，您的侦听器将收到侦听事件。您可以侦听文档、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，可能会过时或不完整。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。如果您需要侦听 fromCache 值的变化，请在附加侦听处理程序时指定 includeMetadataChanges 侦听选项。
Web
db.collection("cities").where("state", "==", "CA")
.onSnapshot({ includeMetadataChanges: true }, function(snapshot) {
snapshot.docChanges().forEach(function(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);
}];
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);
}
}
});
Kotlin
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")
}
}
获取离线数据
如果您在设备离线时获取文档，Cloud Firestore 会从缓存中返回数据。如果缓存不包含该文档的数据，或该文档不存在，则 get 调用会返回一个错误。
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 或通过侦听（如前面的部分中所述）来检索查询的结果。当设备离线时，您也可以针对已在本地持久化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。
停用和启用网络访问
您可以使用以下方法为您的 Cloud Firestore 客户端停用网络访问。网络访问停用后，所有快照侦听器和文档请求会从缓存中检索结果。写入操作会排入队列，直到网络访问重新启用。
Web
firebase.firestore().disableNetwork()
.then(function() {
// Do offline actions
// ...
});
Swift
Firestore.firestore().disableNetwork { (error) in
// Do offline things
// ...
}
Objective-C
[[FIRFirestore firestore] disableNetworkWithCompletion:^(NSError *_Nullable error) {
// Do offline actions
// ...
}];
Java
db.disableNetwork()
.addOnCompleteListener(new OnCompleteListener<Void>() {
@Override
public void onComplete(@NonNull Task<Void> task) {
// Do offline things
// ...
}
});
Kotlin
db.disableNetwork().addOnCompleteListener {
// Do offline things
// ...
}
使用以下方法可重新启用网络访问：
Web
firebase.firestore().enableNetwork()
.then(function() {
// Do online actions
// ...
});
Swift
Firestore.firestore().enableNetwork { (error) in
// Do online things
// ...
}
Objective-C
[[FIRFirestore firestore] enableNetworkWithCompletion:^(NSError *_Nullable error) {
// Do online actions
// ...
}];
Java
db.enableNetwork()
.addOnCompleteListener(new OnCompleteListener<Void>() {
@Override
public void onComplete(@NonNull Task<Void> task) {
// Do online things
// ...
}
});
Kotlin
db.enableNetwork().addOnCompleteListener {
// Do online things
// ...
}