Cloud Firestore in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Firestore Data Fetching Behavior

You have this Swift code snippet to fetch a document from Firestore and update a label. What will be the label's text if the document does not exist?

iOS Swift

let db = Firestore.firestore()
db.collection("users").document("user123").getDocument { document, error in
  if let document = document, document.exists {
    label.text = document.get("name") as? String ?? "No Name"
  } else {
    label.text = "Document not found"
  }
}

A "Document not found"

B "No Name"

C App crashes with nil error

D "" (empty string)

💡 Hint

Check the condition that tests if the document exists.

If the document does not exist, the else block runs and sets the label text to "Document not found".

🧠 Conceptual

intermediate

Firestore Offline Persistence

What happens when Firestore offline persistence is enabled and the device loses internet connection?

A Firestore caches data locally and serves reads from cache until connection is restored

B Firestore stops all reads and writes until internet is back

C Firestore deletes all cached data immediately

D Firestore automatically switches to a backup server

💡 Hint

Think about how offline apps should behave to keep working.

Enabling offline persistence allows Firestore to cache data locally and serve reads from cache when offline, syncing changes when back online.

📝 Syntax

advanced

Correct Firestore Document Update Syntax

Which option correctly updates the "age" field to 30 in the Firestore document "users/user123"?

iOS Swift

let db = Firestore.firestore()
db.collection("users").document("user123").updateData(["age": 30]) { error in
  if let error = error {
    print("Error updating document: \(error)")
  } else {
    print("Document successfully updated")
  }
}

A db.collection("users").document("user123").update(["age": 30])

B db.collection("users").document("user123").setData(["age": 30])

C db.collection("users").document("user123").updateData(["age": 30]) { error in ... }

D db.collection("users").document("user123").updateData("age", 30)

💡 Hint

Look for the method that updates fields without overwriting the whole document.

updateData is the correct method to update specific fields. setData overwrites the whole document. update and updateData with wrong parameters cause errors.

❓ lifecycle

advanced

Firestore Listener Behavior on App Backgrounding

If you add a Firestore snapshot listener in your iOS app, what happens to the listener when the app goes to the background?

A Listener automatically stops and resumes when app returns to foreground

B Listener continues to receive updates until explicitly removed

C Listener causes app crash in background

D Listener pauses and never resumes

💡 Hint

Think about how Firestore listeners behave unless you remove them.

Firestore listeners remain active in background unless you remove them, which can affect battery and data usage.

🔧 Debug

expert

Diagnosing Firestore Permission Denied Error

You get a "Permission denied" error when trying to read a Firestore collection in your iOS app. Which is the most likely cause?

A The app is missing internet permission in Info.plist

B The Firestore database is offline

C The document ID is invalid

D Firestore security rules do not allow read access to the collection

💡 Hint

Check your Firestore security rules first when you see permission errors.

Permission denied errors usually mean Firestore security rules block access. Network or ID issues cause different errors.