List use of double dot (.) in dart?

It's the cascade operator of Dart

var l1 = new List<int>()..add(0)..addAll([1, 2, 3]);

results in l1 being a list [0, 1, 2, 3]

var l1 = new List<int>().add(0).addAll([1, 2, 3]);

results in an error, because .add(0) returns void

.. (in the former example) refers to new List(), while . (in the later) refers to the return value of the previous part of the expression.

.. was introduced to avoid the need to return this in all kinds of methods like add() to be able to use an API in a fluent way.

.. provides this out of the box for all classes.

.. is known as cascade notation. It allows you to not repeat the same target if you want to call several methods on the same object.

List list = [];
list.add(color1);
list.add(color2);
list.add(color3);
list.add(color4);

// with cascade

List list = [];
list
  ..add(color1)
  ..add(color2)
  ..add(color3)
  ..add(color4);

.. Is known as the cascading operator in dart.

It allows you to use more than one subsequence operation:

Examples:

banerad..load()..show().

List coursename;
coursename..add("java")..add("flutter" )..add("dart");

Here is another example

Cascades (..) allow you to make a sequence of operations on the same object. read doc for details

querySelector('#confirm') // Get an object.
  ..text = 'Confirm' // Use its members.
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'));

The previous example is equivalent to:

var button = querySelector('#confirm');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));