In Dart, check if a class inherits parent class with type checking

Question

I am try to find a safe way to use Bloc patterns in Flutter with a very strong type checking.

Let's say we have this simple Bloc code:

// States 

abstract class CourseState {}

class CourseInitialized extends CourseState {}

// Events 

abstract class CourseEvent {}

class CourseFetch extends CourseEvent {}

class CourseLoaded extends CourseEvent {}

// Bloc (somewhere in the Bloc)

final event = CourseFetch();

In dart we can use the is operator to check if the event variable is of type CourseFetch or CourseEvent:

event is CourseFetch // true
event is CourseEvent // true

But nothing forbid me to check if event is of type CourseState or even of type a num, String, etc.

event is CourseInitialized // false
event is String // false

Of course is this case event can't be a String, because it has been initialized with CourseFetch() class.

I am try to find a way to forbid programmers (us) to wrongly write if statement that can never be evaluate to true.

In my case scenario I would like to have a way that stop me to check if a variable event of a certain type state in the IDE and give me a red squiggly line.

if (event is CourseInitialized) { 
// the above line should give me a warning, this variable event is not a State
}

Any ideas? Linting tools or syntax that can help?

UPDATE: I follow Rémi suggestion and try out Freezed, it makes the code simple and add up a safe type checking. Here you can find a complete implementation of Bloc with Freezed and Built Value.

Solution

There is no such thing (nor is it possible to know that you checked all the possible cases).

What you can instead do is replace the is operator with a function that does it for you.

One example of this is what the code-generator Freezed does.

Instead of:

abstract class CourseEvent {}

class CourseFetch extends CourseEvent {}

class CourseLoaded extends CourseEvent {}

You would write:

@freezed
abstract class CourseEvent with _$CourseEvent {
  factory CourseEvent.fetch() = _Fetch;
  factory CourseEvent.loaded() = _Loaded;
}

Which is then used as:

CourseEvent event;

event.when(
  fetch: () => print('Fetch event'),
  loaded: () => print('Loaded event'),
);

Comments

[Thomas Gazzoni]

Hi @rémi-rousselet, thanks to point it out Freezed library, it works very well. I made this simple project definitely it simply a lot the Bloc code.