Cloud Storage for files in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Uploading a file to Firebase Storage

What will happen when this Flutter code runs and the user selects a file to upload?

Flutter

final storageRef = FirebaseStorage.instance.ref().child('uploads/myfile.txt');
final file = File('/path/to/file.txt');
await storageRef.putFile(file);
final url = await storageRef.getDownloadURL();
print(url);

A The file is uploaded to the root folder, ignoring the 'uploads/' path.

B The code throws a runtime error because 'putFile' requires a Uint8List, not a File.

C The file is uploaded to Firebase Storage under 'uploads/myfile.txt' and the download URL is printed.

D The file is uploaded but the download URL is null because 'getDownloadURL' is not awaited.

💡 Hint

Check how the storage reference path and file upload method are used.

The code correctly creates a reference to 'uploads/myfile.txt', uploads the file, then fetches and prints the download URL. 'putFile' accepts a File object, and 'await' is used properly.

❓ lifecycle

intermediate

Handling upload progress in Flutter

Which code snippet correctly listens to upload progress events from Firebase Storage in Flutter?

Flutter

final storageRef = FirebaseStorage.instance.ref('uploads/photo.png');
final uploadTask = storageRef.putFile(file);
// Which code below correctly listens to progress?

A

uploadTask.onProgress.listen((progress) {
  print('Progress: $progress');
});

B

uploadTask.addListener(() {
  print('Upload progress');
});

C

uploadTask.events.listen((event) {
  print('Progress event');
});

D

uploadTask.snapshotEvents.listen((TaskSnapshot snapshot) {
  final progress = snapshot.bytesTransferred / snapshot.totalBytes;
  print('Progress: $progress');
});

💡 Hint

Look for the official property that streams upload snapshots.

The 'snapshotEvents' stream provides TaskSnapshot objects with bytesTransferred and totalBytes, allowing progress calculation.

🔧 Debug

advanced

Fixing a permission denied error on upload

A Flutter app using Firebase Storage gets a 'permission denied' error when uploading files. Which Firebase Storage security rule causes this?

Flutter

rules_version = '2';
service firebase.storage {
  match /b/{bucket}/o {
    match /{allPaths=**} {
      allow read;
      allow write: if false;
    }
  }
}

A The rule allows writes only if the user is authenticated, but the app is unauthenticated.

B The rule denies all write operations because 'allow write: if false;' blocks uploads.

C The rule syntax is invalid and causes all operations to fail.

D The rule allows writes only on files smaller than 1MB, but the file is larger.

💡 Hint

Check the condition after 'allow write'.

The rule explicitly denies all writes by using 'if false', so uploads fail with permission denied.

🧠 Conceptual

advanced

Understanding Firebase Storage download URLs

What is true about the download URL returned by Firebase Storage's getDownloadURL() method?

A It is a public URL that anyone can use to access the file without authentication.

B It requires the user to be authenticated to access the file.

C It only works inside the Firebase app and cannot be used in browsers.

D It expires after 1 hour and must be refreshed.

💡 Hint

Think about how Firebase Storage URLs are designed for sharing.

The download URL is a long, unique link that allows public access to the file without authentication until the file or rules change.

❓ navigation

expert

Managing file upload state across app navigation

In a Flutter app, a file upload starts on Screen A. The user navigates to Screen B before upload finishes. How to ensure upload progress updates still show on Screen B?

A Use a shared state management solution (like Provider or Riverpod) to hold the upload task and listen to its progress from both screens.

B Start a new upload task on Screen B to replace the one from Screen A.

C Use Navigator.popUntil to return to Screen A and show progress there only.

D Upload progress cannot be tracked after navigation; show a static message instead.

💡 Hint

Think about how to share data between screens in Flutter.

Using shared state management allows multiple screens to listen to the same upload task and update UI accordingly, even after navigation.