Flutter/Dart - Difference between () {} and () => {}

Question

In Flutter/Dart the examples sometimes show fat arrow and sometimes dont. Here are examples:

RaisedButton(
  onPressed: () {
    setState(() {
      _myTxt = \"         


        

Answer 1

There seems to be one difference at least in case of Dart version 2.10:

If the expression to be executed is a Future, then the execution order is not the same.

=>

new Future(() => print('future #1 of 2'))
  .then((_) => new Future(() => print('future #1a (a new future)')))
  .then((_) => print('future #1b'));

new Future(() => print('future #2 of 2'))
  .then((_) => new Future(() => print('future #2a (aa new futue)' )))
  .then((_) => print('future #2b'));

The result is:

future #1 of 2
future #2 of 2
future #1a (a new future)
future #1b
future #2a (aa new futue)
future #2b`

{} :

new Future(() => print('future #1 of 2'))
  .then((_) => new Future(() => print('future #1a (a new future)')))
  .then((_) => print('future #1b'));

new Future(() => print('future #2 of 2'))
  .then((_) { new Future(() => print('future #2a (aa new futue)' )); })
  .then((_) => print('future #2b'));

The result is

future #1 of 2
future #2 of 2
future #2b
future #1a (a new future)
future #1b
future #2a (a new futue)