AnimatedOpacity({Key key, Widget child, double opacity, Curve curve: Curves.linear, @required Duration duration })

Creates a widget that animates its opacity implicitly.

The opacity argument must not be null and must be between 0.0 and 1.0, inclusive. The curve and duration arguments must not be null.

Source 

AnimatedOpacity({
  Key key,
  this.child,
  this.opacity,
  Curve curve: Curves.linear,
  @required Duration duration,
}) : super(key: key, curve: curve, duration: duration) {
  assert(opacity != null && opacity >= 0.0 && opacity <= 1.0);
}