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?