dart event loop confused me - dart

Question

Recently, I'm learning dart asynchronous programming. When it comes to event loop part.
it have some code like below:
new Future(() => print('future #3 of 4'))
.then((_) => new Future(() => print('future #3a a new future')))
.then((_) => print('future #3b'))
.then((_) => print('future #3c'));
new Future(() => print('future #4 of 4'))
.then((_) { new Future(() => print('future #4a')); })
.then((_) => print('future #4b'))
.then((_) => print('future #4c'));
It's results like below:
future #3 of 4
future #4 of 4
future #4b
future #4c
future #3a a new future
future #3b
future #3c
future #4a
What make me confused have sth like below:
what's the difference below
(_) => new Future(() => print('future #3a a new future')))
and
(_) { new Future(() => print('future #4a')); }
why results as above.
I'm new to dart. Any explanation is appreciated.

Answer 1

The difference between the two syntax is that this:
(_) => new Future(() => print('future #3a a new future')))
Is a shorthand syntax for doing this:
(_) {
return new Future(() => print('future #3a a new future'));
})
Note the return which are not part of your second example. The problem with your code is that by omitting the return you change the flow of your code since the spawned Future no longer are getting a Future it should wait on.
By adding the missing return your code will output:
future #3 of 4
future #4 of 4
future #3a a new future
future #3b
future #3c
future #4a
future #4b
future #4c