Widget re-render when I focus on a TextField - Bloc Pattern - dart

Question

I'm using a BLoC to keep state between two nested FullScreenDialogs.
I'm initializing the bloc when I push the first screen, like so
return FloatingActionButton(
child: Icon(Icons.add),
onPressed: () {
Navigator.of(context).push(MaterialPageRoute(
builder: (BuildContext context) => ProductBlocProvider(child: ProductEntryScreen()),
fullscreenDialog: true
));
},
);
ProductEntryScreen has a bunch of TextFields and a button than opens a new FullScreenDialog. This new Screen also has TextFields.
The problem I'm having is that every time I write on a TextField on the second FullScreenDialog, the onPressed function where I start the ProductBlocProvider runs again.
And that re-run is causing the Bloc to create a new instance, so I end up loosing the state.
What I want to do?
Maybe I'm doing it wrong so I'll explain what I'm trying to achieve.
I want to keep state between the two FullScreenDialogs while I fill all the fields, and when I'm done I want to press a button that send all of the data (both screens) to a database.

Answer 1

The problem is that I was creating the instance of the bloc inside the provider in the builder function of the MaterialPageRoute.
That builder function was being called repeatedly, and creating a new instance of the bloc every time. The solution was to take out from the builde function the creation of the bloc instance, like this:
return FloatingActionButton(
child: Icon(Icons.add),
onPressed: () {
//Here I create the instance
var _bloc = ProductBloc();
Navigator.of(context).push(MaterialPageRoute(
//And I pass the bloc instance to the provider
builder: (BuildContext context) => ProductBlocProvider(bloc: _bloc, child: ProductEntryScreen()),
fullscreenDialog: true
));
},
);

Answer 2

The package get_it may be of help to you. get_it is a service locator library, and uses a Map to store the registered objects; therefore, it provides access at a complexity of O(1), which means it's incredibly fast. The package comes with a singleton GetIt which you can use like so,
// Create a global variable (traditionally called sl or locator)
final sl = GetIt.instance; // There is also a shorthand GetIt.i
// ...
// Then, maybe in a global function called initDi(),
// you could register your dependencies.
sl.registerLazySingleton(() => ProductBloc());
registerLazySingleton() or registerSingleton() will always
return the same instance; lazily (i.e., when first called)
or at app start-up respectively.
If you want to create a new instance every time, use registerFactory() instead (I put this here even though it's not exactly what you want).
For example,
sl.registerFactory(() => ValidatorCubit());
And it could be accessed like this,
MultiBlocProvider(
providers: [
// The type is inferred here
BlocProvider<AuthenticationBloc>(create: (_) => sl()),
// The type is explicitly given here
BlocProvider(create: (_) => sl<ProductsBloc>()),
],
child: ProductsScreen(),
),
This example primarily shows you how it can be done with the flutter_bloc library, but get_it works anywhere, even in non-flutter dart projects.
If you need more functionality, do make sure to read the docs for this package. It is well documented, and contains (almost) every feature you might need, including scoping.
Also, this approach allows you to use the interface pattern, making the code much more maintainable and testable, as you will have to change just one place to use a different implementation.