What is this operator "?." in Dart?

Question

i work with flutter framework this part of the code use a operation "?." but idont understand

 if (state is WeatherLoaded) {
          final weather = state.weather;
          final themeBloc = BlocProvider.of<ThemeBloc>(context);
          themeBloc.dispatch(WeatherChanged(condition: weather.condition));

          _refreshCompleter?.complete();
          _refreshCompleter = Completer();

all code this link

Solution 1

The best way to demonstrate this is a simple example.
I have an object SomeObject with one method username.

I have made 2 instances of it:

• aeonObject which is not null
• someOtherObject which is null

class SomeObject {
  String username() => "aeon";
}

void main() {
  final aeonObject = SomeObject();
  print(aeonObject.username());

  SomeObject someOtherObject;
  print(someOtherObject.username());
}

If I execute this snippet you'll see the following output.
The program will crash because we tried to execute a method on a null reference.

dart lib/main.dart lib/main.dart: Warning: Interpreting this as package URI, 'package:sample/main.dart'.

aeon

Unhandled exception: NoSuchMethodError: The method 'username' was called on null.

Receiver: null Tried calling: username()

However if I call the print statement with the ?. aka Conditional member access operator.

print(someOtherObject?.username());

We instead get.

null

Solution 2

Check this link: Language tour

?.

Conditional member access

Like ., but the leftmost operand can be null; example: foo?.bar selects property bar from expression foo unless foo is null (in which case the value of foo?.bar is null)