Read and Write Data on Apple platforms

(Optional) Prototype and test with Firebase Local Emulator Suite

Before talking about how your app reads from and writes to Realtime Database, let's introduce a set of tools you can use to prototype and test Realtime Database functionality: Firebase Local Emulator Suite. If you're trying out different data models, optimizing your security rules, or working to find the most cost-effective way to interact with the back-end, being able to work locally without deploying live services can be a great idea.

A Realtime Database emulator is part of the Local Emulator Suite, which enables your app to interact with your emulated database content and config, as well as optionally your emulated project resources (functions, other databases, and security rules).

Using the Realtime Database emulator involves just a few steps:

1. Adding a line of code to your app's test config to connect to the emulator.
2. From the root of your local project directory, running firebase emulators:start.
3. Making calls from your app's prototype code using a Realtime Database platform SDK as usual, or using the Realtime Database REST API.

A detailed walkthrough involving Realtime Database and Cloud Functions is available. You should also have a look at the Local Emulator Suite introduction.

Get a FIRDatabaseReference

To read or write data from the database, you need an instance of FIRDatabaseReference:

var ref: DatabaseReference!

ref = Database.database().reference()

@property (strong, nonatomic) FIRDatabaseReference *ref;

self.ref = [[FIRDatabase database] reference];

Write data

This document covers the basics of reading and writing Firebase data.

Firebase data is written to a Database reference and retrieved by attaching an asynchronous listener to the reference. The listener is triggered once for the initial state of the data and again anytime the data changes.

Basic write operations

For basic write operations, you can use setValue to save data to a specified reference, replacing any existing data at that path. You can use this method to:

• Pass types that correspond to the available JSON types as follows:
  • NSString
  • NSNumber
  • NSDictionary
  • NSArray

For instance, you can add a user with setValue as follows:

self.ref.child("users").child(user.uid).setValue(["username": username])

[[[self.ref child:@"users"] child:authResult.user.uid]
    setValue:@{@"username": username}];

Using setValue in this way overwrites data at the specified location, including any child nodes. However, you can still update a child without rewriting the entire object. If you want to allow users to update their profiles you could update the username as follows:

self.ref.child("users/\(user.uid)/username").setValue(username)

[[[[_ref child:@"users"] child:user.uid] child:@"username"] setValue:username];

Read data

Read data by listening for value events

To read data at a path and listen for changes, use the observeEventType:withBlock of FIRDatabaseReference to observe FIRDataEventTypeValue events.

You can use the FIRDataEventTypeValue event to read the data at a given path, as it exists at the time of the event. This method is triggered once when the listener is attached and again every time the data, including any children, changes. The event callback is passed a snapshot containing all data at that location, including child data. If there is no data, the snapshot will return false when you call exists() and nil when you read its value property.

The following example demonstrates a social blogging application retrieving the details of a post from the database:

refHandle = postRef.observe(DataEventType.value, with: { snapshot in
  // ...
})

_refHandle = [_postRef observeEventType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot * _Nonnull snapshot) {
  NSDictionary *postDict = snapshot.value;
  // ...
}];

The listener receives a FIRDataSnapshot that contains the data at the specified location in the database at the time of the event in its value property. You can assign the values to the appropriate native type, such as NSDictionary. If no data exists at the location, the value is nil.

Read data once

Read once using getData()

The SDK is designed to manage interactions with database servers whether your app is online or offline.

Generally, you should use the value events techniques described above to read data to get notified of updates to the data from the backend. Those technique reduce your usage and billing, and are optimized to give your users the best experience as they go online and offline.

If you need the data only once, you can use getData() to get a snapshot of the data from the database. If for any reason getData() is unable to return the server value, the client will probe the local storage cache and return an error if the value is still not found.

The following example demonstrates retrieving a user's public-facing username a single time from the database:

do {
  let snapshot = try await ref.child("users/\(uid)/username").getData()
  let userName = snapshot.value as? String ?? "Unknown"
} catch {
  print(error)
}

NSString *userPath = [NSString stringWithFormat:@"users/%@/username", uid];
[[ref child:userPath] getDataWithCompletionBlock:^(NSError * _Nullable error, FIRDataSnapshot * _Nonnull snapshot) {
  if (error) {
    NSLog(@"Received an error %@", error);
    return;
  }
  NSString *userName = snapshot.value;
}];

Unnecessary use of getData() can increase use of bandwidth and lead to loss of performance, which can be prevented by using a realtime listener as shown above.

Read data once with an observer

In some cases you may want the value from the local cache to be returned immediately, instead of checking for an updated value on the server. In those cases you can use observeSingleEventOfType to get the data from the local disk cache immediately.

This is useful for data that only needs to be loaded once and isn't expected to change frequently or require active listening. For instance, the blogging app in the previous examples uses this method to load a user's profile when they begin authoring a new post:

let userID = Auth.auth().currentUser?.uid
ref.child("users").child(userID!).observeSingleEvent(of: .value, with: { snapshot in
  // Get user value
  let value = snapshot.value as? NSDictionary
  let username = value?["username"] as? String ?? ""
  let user = User(username: username)

  // ...
}) { error in
  print(error.localizedDescription)
}

NSString *userID = [FIRAuth auth].currentUser.uid;
[[[_ref child:@"users"] child:userID] observeSingleEventOfType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot * _Nonnull snapshot) {
  // Get user value
  User *user = [[User alloc] initWithUsername:snapshot.value[@"username"]];

  // ...
} withCancelBlock:^(NSError * _Nonnull error) {
  NSLog(@"%@", error.localizedDescription);
}];

Updating or deleting data

Update specific fields

To simultaneously write to specific children of a node without overwriting other child nodes, use the updateChildValues method.

When calling updateChildValues, you can update lower-level child values by specifying a path for the key. If data is stored in multiple locations to scale better, you can update all instances of that data using data fan-out. For example, a social blogging app might want to create a post and simultaneously update it to the recent activity feed and the posting user's activity feed. To do this, the blogging application uses code like this:

guard let key = ref.child("posts").childByAutoId().key else { return }
let post = ["uid": userID,
            "author": username,
            "title": title,
            "body": body]
let childUpdates = ["/posts/\(key)": post,
                    "/user-posts/\(userID)/\(key)/": post]
ref.updateChildValues(childUpdates)

NSString *key = [[_ref child:@"posts"] childByAutoId].key;
NSDictionary *post = @{@"uid": userID,
                       @"author": username,
                       @"title": title,
                       @"body": body};
NSDictionary *childUpdates = @{[@"/posts/" stringByAppendingString:key]: post,
                               [NSString stringWithFormat:@"/user-posts/%@/%@/", userID, key]: post};
[_ref updateChildValues:childUpdates];

This example uses childByAutoId to create a post in the node containing posts for all users at /posts/$postid and simultaneously retrieve the key with getKey(). The key can then be used to create a second entry in the user's posts at /user-posts/$userid/$postid.

Using these paths, you can perform simultaneous updates to multiple locations in the JSON tree with a single call to updateChildValues, such as how this example creates the new post in both locations. Simultaneous updates made this way are atomic: either all updates succeed or all updates fail.

Add a Completion Block

If you want to know when your data has been committed, you can add a completion block. Both setValue and updateChildValues take an optional completion block that is called when the write has been committed to the database. This listener can be useful for keeping track of which data has been saved and which data is still being synchronized. If the call was unsuccessful, the listener is passed an error object indicating why the failure occurred.

do {
  try await ref.child("users").child(user.uid).setValue(["username": username])
  print("Data saved successfully!")
} catch {
  print("Data could not be saved: \(error).")
}

[[[_ref child:@"users"] child:user.uid] setValue:@{@"username": username} withCompletionBlock:^(NSError *error, FIRDatabaseReference *ref) {
  if (error) {
    NSLog(@"Data could not be saved: %@", error);
  } else {
    NSLog(@"Data saved successfully.");
  }
}];

Delete data

The simplest way to delete data is to call removeValue on a reference to the location of that data.

You can also delete by specifying nil as the value for another write operation such as setValue or updateChildValues. You can use this technique with updateChildValues to delete multiple children in a single API call.

Detach listeners

Observers don't automatically stop syncing data when you leave a ViewController. If an observer isn't properly removed, it continues to sync data to local memory. When an observer is no longer needed, remove it by passing the associated FIRDatabaseHandle to the removeObserverWithHandle method.

When you add a callback block to a reference, a FIRDatabaseHandle is returned. These handles can be used to remove the callback block.

If multiple listeners have been added to a database reference, each listener is called when an event is raised. In order to stop syncing data at that location, you must remove all observers at a location by calling the removeAllObservers method.

Calling removeObserverWithHandle or removeAllObservers on a listener does not automatically remove listeners registered on its child nodes; you must also keep track of those references or handles to remove them.

Save data as transactions

When working with data that could be corrupted by concurrent modifications, such as incremental counters, you can use a transaction operation. You give this operation two arguments: an update function and an optional completion callback. The update function takes the current state of the data as an argument and returns the new desired state you would like to write.

For instance, in the example social blogging app, you could allow users to star and unstar posts and keep track of how many stars a post has received as follows:

ref.runTransactionBlock({ (currentData: MutableData) -> TransactionResult in
  if var post = currentData.value as? [String: AnyObject],
    let uid = Auth.auth().currentUser?.uid {
    var stars: [String: Bool]
    stars = post["stars"] as? [String: Bool] ?? [:]
    var starCount = post["starCount"] as? Int ?? 0
    if let _ = stars[uid] {
      // Unstar the post and remove self from stars
      starCount -= 1
      stars.removeValue(forKey: uid)
    } else {
      // Star the post and add self to stars
      starCount += 1
      stars[uid] = true
    }
    post["starCount"] = starCount as AnyObject?
    post["stars"] = stars as AnyObject?

    // Set value and report transaction success
    currentData.value = post

    return TransactionResult.success(withValue: currentData)
  }
  return TransactionResult.success(withValue: currentData)
}) { error, committed, snapshot in
  if let error = error {
    print(error.localizedDescription)
  }
}

[ref runTransactionBlock:^FIRTransactionResult * _Nonnull(FIRMutableData * _Nonnull currentData) {
  NSMutableDictionary *post = currentData.value;
  if (!post || [post isEqual:[NSNull null]]) {
    return [FIRTransactionResult successWithValue:currentData];
  }

  NSMutableDictionary *stars = post[@"stars"];
  if (!stars) {
    stars = [[NSMutableDictionary alloc] initWithCapacity:1];
  }
  NSString *uid = [FIRAuth auth].currentUser.uid;
  int starCount = [post[@"starCount"] intValue];
  if (stars[uid]) {
    // Unstar the post and remove self from stars
    starCount--;
    [stars removeObjectForKey:uid];
  } else {
    // Star the post and add self to stars
    starCount++;
    stars[uid] = @YES;
  }
  post[@"stars"] = stars;
  post[@"starCount"] = @(starCount);

  // Set value and report transaction success
  currentData.value = post;
  return [FIRTransactionResult successWithValue:currentData];
} andCompletionBlock:^(NSError * _Nullable error,
                       BOOL committed,
                       FIRDataSnapshot * _Nullable snapshot) {
  // Transaction completed
  if (error) {
    NSLog(@"%@", error.localizedDescription);
  }
}];

Using a transaction prevents star counts from being incorrect if multiple users star the same post at the same time or the client had stale data. The value contained in the FIRMutableData class is initially the client's last known value for the path, or nil if there is none. The server compares the initial value against its current value and accepts the transaction if the values match, or rejects it. If the transaction is rejected, the server returns the current value to the client, which runs the transaction again with the updated value. This repeats until the transaction is accepted or too many attempts have been made.

Atomic server-side increments

In the above use case we're writing two values to the database: the ID of the user who stars/unstars the post, and the incremented star count. If we already know that user is starring the post, we can use an atomic increment operation instead of a transaction.

let updates = [
  "posts/\(postID)/stars/\(userID)": true,
  "posts/\(postID)/starCount": ServerValue.increment(1),
  "user-posts/\(postID)/stars/\(userID)": true,
  "user-posts/\(postID)/starCount": ServerValue.increment(1)
] as [String : Any]
Database.database().reference().updateChildValues(updates)

NSDictionary *updates = @{[NSString stringWithFormat: @"posts/%@/stars/%@", postID, userID]: @TRUE,
                        [NSString stringWithFormat: @"posts/%@/starCount", postID]: [FIRServerValue increment:@1],
                        [NSString stringWithFormat: @"user-posts/%@/stars/%@", postID, userID]: @TRUE,
                        [NSString stringWithFormat: @"user-posts/%@/starCount", postID]: [FIRServerValue increment:@1]};
[[[FIRDatabase database] reference] updateChildValues:updates];

This code does not use a transaction operation, so it does not automatically get re-run if there is a conflicting update. However, since the increment operation happens directly on the database server, there is no chance of a conflict.

If you want to detect and reject application-specific conflicts, such as a user starring a post that they already starred before, you should write custom security rules for that use case.

Work with data offline

If a client loses its network connection, your app will continue functioning correctly.

Every client connected to a Firebase database maintains its own internal version of any active data. When data is written, it's written to this local version first. The Firebase client then synchronizes that data with the remote database servers and with other clients on a "best-effort" basis.

As a result, all writes to the database trigger local events immediately, before any data is written to the server. This means your app remains responsive regardless of network latency or connectivity.

Once connectivity is reestablished, your app receives the appropriate set of events so that the client syncs with the current server state, without having to write any custom code.

We'll talk more about offline behavior in Learn more about online and offline capabilities.

Next Steps

• Working with lists of data
• Learn how to structure data
• Learn more about online and offline capabilities