Constructors in dart [duplicate]

Question

I have this constructor in my class. Now when it's like this, I get

The type parameter icon is annotated with @required
but only named parameters without default value can be annotated with it.

-

const Category(
    @required this.name,
    @required this.icon,
    @required this.color
  ) : assert(name != null),
      assert(icon != null),
      assert(color != null);

And when calling the constructor like this:

        Category(name: _categoryName, icon: _categoryIcon, color: _categoryColor),

It's an error.

All of this goes away when I surround my constructor arguments with {}.

What does this mean?

Answer 1

{} are missing to make them named parameters

const Category({
    @required this.name,
    @required this.icon,
    @required this.color
  }) : assert(name != null),
      assert(icon != null),
      assert(color != null);

or just remove @required

Without {} they are positional parameters which are required anyway.

Category('foo', someIcon, Colors.white)

vs

Category(name: 'foo', icon: someIcon, color: Colors.white)

[] makes them optional positional parameters.

Positional (non-optional) need to be declared first, optional parameters come at the end.

Optional positional and optional named parameters can not be used together.

Optional parameters (positional and named) can have default values

this.name = 'foo'

Default values need to be compile-time constants.