Repository pattern in Flutter

Introduction

The repository pattern helps organize how your app gets data. It keeps data fetching and storage separate from the UI, making your app easier to manage and test.

When your app needs to get data from different places like a web server and local storage.

When you want to change where data comes from without changing the UI code.

When you want to write tests for your data logic without involving the UI.

When your app grows and data handling becomes complex.

When you want to keep your code clean and easy to understand.

Syntax

Flutter

abstract class Repository {
  Future<String> fetchData();
}

class ApiRepository implements Repository {
  @override
  Future<String> fetchData() async {
    // Simulate fetching data from API
    return 'Data from API';
  }
}

class LocalRepository implements Repository {
  @override
  Future<String> fetchData() async {
    // Simulate fetching data from local storage
    return 'Data from Local Storage';
  }
}

The Repository is an abstract class or interface that defines data methods.

Concrete classes like ApiRepository and LocalRepository implement these methods to get data from different sources.

Examples

This class gets data from an API source.

Flutter

class ApiRepository implements Repository {
  @override
  Future<String> fetchData() async {
    return 'Data from API';
  }
}

This class gets data from local storage.

Flutter

class LocalRepository implements Repository {
  @override
  Future<String> fetchData() async {
    return 'Data from Local Storage';
  }
}

Sample App

This Flutter app uses the repository pattern to get data. You can switch between ApiRepository and LocalRepository by changing one line. The UI shows the data fetched.

Flutter

import 'package:flutter/material.dart';

abstract class Repository {
  Future<String> fetchData();
}

class ApiRepository implements Repository {
  @override
  Future<String> fetchData() async {
    await Future.delayed(Duration(seconds: 1));
    return 'Data from API';
  }
}

class LocalRepository implements Repository {
  @override
  Future<String> fetchData() async {
    await Future.delayed(Duration(seconds: 1));
    return 'Data from Local Storage';
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  late Repository repository;
  String data = 'Loading...';

  @override
  void initState() {
    super.initState();
    // Change repository here to switch data source
    repository = ApiRepository();
    loadData();
  }

  void loadData() async {
    String result = await repository.fetchData();
    setState(() {
      data = result;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Repository Pattern Example')),
        body: Center(child: Text(data, style: TextStyle(fontSize: 24))),
      ),
    );
  }
}

Important Notes

Use the repository pattern to keep your UI code clean and focused only on showing data.

You can easily add more data sources by creating new repository classes.

Testing is easier because you can mock the repository without touching UI code.

Summary

The repository pattern separates data access from UI code.

It helps manage multiple data sources in one place.

It makes your app easier to test and maintain.

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