flutter - Dart, Identifier with exclamation mark in the back

Question

Recently I have been developing mobile aplication with flutter, when I looking at the source code for TickerProvider I see these lines:

mixin SingleTickerProviderStateMixin<T extends StatefulWidget> on State<T> implements TickerProvider {
Ticker? _ticker;

  @override
  Ticker createTicker(TickerCallback onTick) {
    ...
    _ticker = Ticker(onTick, debugLabel: kDebugMode ? 'created by $this' : null);
    return _ticker!;
  }

...
}

I'm interested with this line:

return _ticker!;

I have seen boolean identifier with exclamation mark in the front meaning that it will return the opposite value of it, but I never see this one. Can someone tell me what this does?

Answer 1

It's part of the null safety that Dart have.

You can read about it here

If you’re sure that an expression with a nullable type isn’t null, you can add ! to make Dart treat it as non-nullable

Example:

int? aNullableInt = 2;
int value = aNullableInt!; // `aNullableInt!` is an int.
// This throws if aNullableInt is null.