Repository pattern in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a repository interface in Flutter.

Flutter

abstract class UserRepository {
  Future<List<User>> [1]();
}

Drag options to blanks, or click blank then click option'

AgetUsers

BfetchUsers

CloadUsers

DretrieveUsers

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names that are too vague or uncommon.

Not using Future for asynchronous data fetching.

The method name getUsers clearly indicates fetching users, which is a common naming convention in repository interfaces.

2fill in blank

medium

Complete the code to implement the repository interface with a remote data source.

Flutter

class RemoteUserRepository implements UserRepository {
  final ApiClient apiClient;

  RemoteUserRepository(this.apiClient);

  @override
  Future<List<User>> [1]() {
    return apiClient.fetchUsers();
  }
}

Drag options to blanks, or click blank then click option'

AgetUsers

BretrieveUsers

CloadUsers

DfetchUsers

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name than the interface.

Not overriding the method properly.

The method getUsers overrides the interface method and calls the API client to fetch users.

3fill in blank

hard

Fix the error in the repository method to handle exceptions properly.

Flutter

Future<List<User>> getUsers() async {
  try {
    final users = await apiClient.fetchUsers();
    return users;
  } catch ([1]) {
    throw Exception('Failed to load users');
  }
}

Drag options to blanks, or click blank then click option'

Aerror

Bexception

Ce

Derr

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid variable names.

Not catching the exception variable at all.

The variable e is commonly used to catch exceptions in Dart try-catch blocks.

4fill in blank

hard

Fill both blanks to create a repository that caches data locally after fetching remotely.

Flutter

class CachedUserRepository implements UserRepository {
  final UserRepository remoteRepo;
  final UserCache cache;

  CachedUserRepository(this.remoteRepo, this.cache);

  @override
  Future<List<User>> getUsers() async {
    final cachedUsers = cache.[1]();
    if (cachedUsers != null) {
      return cachedUsers;
    }
    final users = await remoteRepo.[2]();
    cache.save(users);
    return users;
  }
}

Drag options to blanks, or click blank then click option'

AgetCachedUsers

BfetchUsers

CgetUsers

DloadUsers

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing method names between cache and remote repository.

Not checking for null before returning cached data.

The cache method getCachedUsers retrieves stored users, and the remote repository method getUsers fetches fresh data.

5fill in blank

hard

Fill all three blanks to define a repository pattern with interface, implementation, and error handling.

Flutter

abstract class [1] {
  Future<List<User>> getUsers();
}

class [2] implements [1] {
  final ApiClient apiClient;

  [2](this.apiClient);

  @override
  Future<List<User>> getUsers() async {
    try {
      return await apiClient.fetchUsers();
    } catch (e) {
      throw Exception('Error fetching users');
    }
  }
}

Drag options to blanks, or click blank then click option'

AUserRepository

BRemoteUserRepository

CUserService

DUserApi

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent class names.

Not implementing the interface correctly.

The interface is named UserRepository, and the implementation class is RemoteUserRepository which implements the interface and handles errors.

Practice

1. What is the main purpose of the Repository pattern in Flutter app development?

easy

A. To manage app navigation

B. To create UI widgets faster

C. To handle user input events

D. To separate data access logic from UI code

Solution

1. Step 1: Understand the role of Repository pattern

   The Repository pattern is designed to separate data access from the UI, so the UI does not directly handle data fetching or storage.

2. Step 2: Identify the correct purpose

   Among the options, only separating data access logic from UI code matches the Repository pattern's purpose.

3. Final Answer:

   To separate data access logic from UI code -> Option D

4. Quick Check:

   Repository pattern = separate data access from UI [OK]

Hint: Repository = data access separated from UI [OK]

Common Mistakes:

• Confusing Repository with UI widget creation
• Thinking Repository manages navigation
• Assuming Repository handles user input

2. Which of the following is the correct way to declare a repository interface in Dart for fetching a list of users?

easy

A. abstract class UserRepository { Future fetchUsers(); }

B. class UserRepository { List<User> fetchUsers() async {} }

C. interface UserRepository { List<User> fetchUsers(); }

D. abstract UserRepository { Future fetchUsers() List<User>; }

Solution

1. Step 1: Recall Dart syntax for abstract classes

   In Dart, interfaces are created using abstract classes with method signatures but no bodies.

2. Step 2: Check method signature for async fetching

   The method should return a Future to indicate asynchronous fetching.

3. Final Answer:

   abstract class UserRepository { Future fetchUsers(); } -> Option A

4. Quick Check:

   Dart interface = abstract class with Future method [OK]

Hint: Use abstract class with Future return for async methods [OK]

Common Mistakes:

• Using 'interface' keyword which Dart doesn't have
• Missing Future for async methods
• Incorrect method syntax or return type

3. Given this repository implementation snippet, what will be printed?

class UserRepositoryImpl implements UserRepository {
  @override
  Future fetchUsers() async {
    return ['Alice', 'Bob'];
  }
}

void main() async {
  var repo = UserRepositoryImpl();
  var users = await repo.fetchUsers();
  print(users.length);
}

medium

A. 2

B. ['Alice', 'Bob']

C. Error: fetchUsers is not async

D. 0

Solution

1. Step 1: Understand the fetchUsers method

   The method returns a Future with a list containing two strings: 'Alice' and 'Bob'.

2. Step 2: Analyze the main function output

   The code awaits the fetchUsers call, then prints the length of the list, which is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   List length = 2 [OK]

Hint: Count list items returned by async method [OK]

Common Mistakes:

• Confusing printed list with its length
• Ignoring async/await causing errors
• Assuming fetchUsers returns empty list

4. Identify the error in this repository implementation:

class UserRepositoryImpl implements UserRepository {
  Future fetchUsers() {
    return ['Alice', 'Bob'];
  }
}

medium

A. No error, code is correct

B. fetchUsers must return Future, but returns List<String>

C. UserRepositoryImpl should not implement UserRepository

D. Missing async keyword before fetchUsers method

Solution

1. Step 1: Check return type of fetchUsers

   The method signature says it returns Future, but the returned value is a List<String> without wrapping in Future.

2. Step 2: Identify type mismatch and missing Future

   Returning a plain list instead of a Future causes a type mismatch error.

3. Final Answer:

   fetchUsers must return Future, but returns List<String> -> Option B

4. Quick Check:

   Return type mismatch = fetchUsers must return Future, but returns List<String> [OK]

Hint: Check method return matches declared Future type [OK]

Common Mistakes:

• Forgetting to wrap return value in Future
• Ignoring type mismatch between User and String
• Assuming async keyword is always required

5. You want to combine data from a local database and a remote API in your repository. Which approach best follows the Repository pattern principles?

hard

A. Use only remote API and ignore local DB to simplify code

B. Let UI call local DB and remote API separately and merge data itself

C. Create a repository that fetches from local DB first, then updates from remote API, exposing a single method to UI

D. Create two separate repositories and let UI decide which to use

Solution

1. Step 1: Understand Repository pattern goal

   The pattern centralizes data access, hiding multiple sources behind one interface.

2. Step 2: Evaluate options for combining data sources

   Create a repository that fetches from local DB first, then updates from remote API, exposing a single method to UI combines local and remote data inside one repository method, exposing a simple interface to UI, which matches the pattern.

3. Step 3: Reject options that expose data sources to UI

   Options B and D let UI handle data sources, breaking separation. Use only remote API and ignore local DB to simplify code ignores local data, losing benefits.

4. Final Answer:

   Create a repository that fetches from local DB first, then updates from remote API, exposing a single method to UI -> Option C

5. Quick Check:

   Repository hides multiple sources behind one interface [OK]

Hint: Repository hides data sources, exposes one method to UI [OK]

Common Mistakes:

• Letting UI handle multiple data sources
• Ignoring local cache benefits
• Splitting data sources into separate repositories without abstraction