Project design
- Dart course
- Advanced
- Lesson 79/83
- 6 min
Design the skeleton of the advanced final mini app.
Project design and the Task model
In this project we will build a small task API. First we define the Task model: id and title fields. With final we make the fields immutable; with required we make them mandatory in the constructor.
Then we write the abstract class TaskService interface; it declares the List<Task> list() method. The interface is the contract that concrete classes implement. This way in-memory, database or fake services can be swapped through the same interface. This is the basis of dependency injection and makes the code testable.
InMemoryTaskService
InMemoryTaskService is the in-memory service that implements the TaskService interface. It is declared with implements TaskService. The list() method returns a fixed list of tasks.
Example: class InMemoryTaskService implements TaskService { @override List<Task> list() => [Task(id: 1, title: 'task 1')]; }. @override says the interface method is overridden. This service is used for fast tests; the data is not persistent. In the next lessons we will add database and isolate based services to the same interface; the main code will keep working unchanged.
Practice Task model and service
Write the Task class: final int id and final String title fields, a Task({required this.id, required this.title}) constructor. Write abstract class TaskService declaring List<Task> list(). Write InMemoryTaskService implements TaskService with @override List<Task> list() => [Task(id: 1, title: 'task 1')]. In main add final svc = InMemoryTaskService(); and print('Design: ${svc.list().length} task').
Starter code
// write the Task class (id, title)
// write abstract TaskService and InMemoryTaskService
void main() {
// print the task count
}Sample solution
class Task {
final int id;
final String title;
Task({required this.id, required this.title});
}
abstract class TaskService {
List<Task> list();
}
class InMemoryTaskService implements TaskService {
@override
List<Task> list() => [Task(id: 1, title: 'task 1')];
}
void main() {
final svc = InMemoryTaskService();
print('Design: ${svc.list().length} task');
}Expected output
Design: 1 task
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does TaskService define?
- Database
- HTTP client
- Service interface Correct answer
- DOM
Explanation: TaskService defines a service interface (contract).
2 The implements interface
class InMemoryTaskService implements ____ { @override List<Task> list() => []; }class InMemoryTaskService implements TaskService { @override List<Task> list() => []; }Explanation: InMemoryTaskService implements the TaskService interface.
3 Match the project parts
| Term | Meaning |
|---|---|
Task |
Model |
TaskService |
Interface |
InMemoryTaskService |
Implementation |
list |
List method |