Flex({Key key, @required Axis direction, MainAxisAlignment mainAxisAlignment: MainAxisAlignment.start, MainAxisSize mainAxisSize: MainAxisSize.max, CrossAxisAlignment crossAxisAlignment: CrossAxisAlignment.center, TextBaseline textBaseline, List<Widget> children: const [] })

Creates a flex layout.

The direction is required.

The direction, mainAxisAlignment, and crossAxisAlignment arguments must not be null. If crossAxisAlignment is CrossAxisAlignment.baseline, then textBaseline must not be null.

Source 

Flex({
  Key key,
  @required this.direction,
  this.mainAxisAlignment: MainAxisAlignment.start,
  this.mainAxisSize: MainAxisSize.max,
  this.crossAxisAlignment: CrossAxisAlignment.center,
  this.textBaseline,
  List<Widget> children: const <Widget>[],
}) : super(key: key, children: children) {
  assert(direction != null);
  assert(mainAxisAlignment != null);
  assert(mainAxisSize != null);
  assert(crossAxisAlignment != null);
  assert(crossAxisAlignment != CrossAxisAlignment.baseline || textBaseline != null);
}