(Flutter) StreamBuilder returns only null - flutter

Question

I am trying to create a "CategoryStream" to update the UI based on the users choice.
This is my stream:
import 'dart:async';
class CategoryStream {
StreamController<String> _categoryStreamController =
StreamController<String>();
closeStream() {
_categoryStreamController.close();
}
updateCategory(String category) {
print("Update category in stream was called");
print("the new category is: " + category);
_categoryStreamController.sink.add(category);
}
Stream<String> get stream => _categoryStreamController.stream;
}
And my StreamBuilder looks like this:
return StreamBuilder<String>(
stream: CategoryStream().stream,
builder: (context, snapshot) {
return Container(
color: Colors.white,
child: Center(
child: Text(snapshot.data.toString()),
),
);
},
);
So when the User choses a new category, i try to update the Stream like this:
CategoryStream().updateCategory(currentChosenCategory);
Whatever i do, the result is always null. Although the right category is displayed in the print() function...
What am i missing?
Maybe i need to work with a StreamProvider? Not a StreamBuilder? Because i am adding the data from a Parent-Widget to a Child-Widget of a Child-Widget..

Answer 1

By default, the value of a StreamController is null. You need to set at initialData or add data to the stream before you call snapshot.data in your StreamBuilder:
final CategoryStream _categoryStream = CategoryStream();
return StreamBuilder<String>(
stream: _categoryStream.stream,
initialData: "",
builder: (context, snapshot) {
return Container(
color: Colors.white,
child: Center(
child: Text(snapshot.data), //toString() is unnecessary, your data is already a string
),
);
},
);