What is the use of curly braces in Dart method signature

Question

class TapboxA extends StatefulWidget {

 TapboxA({Key key}) : super(key: key);

}

This part :

TapboxA({Key key}) 

The second half

super(key: key);

I understand, (a call to super class constructor).

But what is the

{Key key}

syntax doing?

Answer 1

It's not something special with constructors only.

For all Dart methods we have the option of named arguments as in languages like Python

Using a curly brace syntax, you can define optional parameters that have names.

So with the method signature using the curly braces,

TapboxA({Key key}) :

you can call this constructor in two different ways

The usual method, without named parameter

tapboxA1 = TapboxA(keyObject)

The extra nicety of named parameter

tapboxA2 = TapboxA(key: keyObject)

In other words the usefulness comes at the time of calling the method, not in the method itself.

Answer 2

In Dart constructors ( and other methods ) can have optional named parameters :

MyClass({String namedParam}){//...}

In the case of a Flutter widget constructor :

TapboxA({Key key})  // TapboxA constructor defines a named parameters `key`
: super(key: key); //which is used within the super constructor call (which also has `key` as named parameter )

You can find more information about optional named parameters in the Dart language tour

Answer 3

Dart provides an option on constructors. By default, when you instantiate a class with a constructor you are forced to provide the parameters defined.

Therefore, TapboxA({Key key}) means you can instantiate the class without providing the arguments.