DEV Community: oganao

[Not solved]Difference among value, sync, microtask and async on Future

oganao — Sat, 24 Dec 2022 08:12:33 +0000

I am still learning Dart. Now I am focusing on Future this week. I made toy codes to understand Future. Syntax of Future is fine for me, but I still don't comprehend Future in detail.

For example, following snippets seem to be the same, but they are not.

Future.value and Future.sync destruct a local value, but Future.microtask and async function do not.

I guess a reddit article could give me a clue. But it seems to me this is not the case for me.

void main() async{
  print('--work01--');
  await work01();
  print('--work02--');
  await work02();
  print('--work03--');
  await work03();
  print('--work04--');
  await work04();
}

work01() async {
  int c = 2;
  Future<int> x = Future.value(++c); // 3->3
  print(c);
  print(await x);
}

work02() async {
  int c = 2;
  Future<int> x = Future.sync(()=>++c); // 3->3
  print(c);
  print(await x);
}

work03() async {
  int c = 2;
  Future<int> x = Future.microtask(()=>++c); // 2->3
  print(c);
  print(await x);
}

work04() async {
  Future<int> foo(int x) async {
    return ++x;
  }
  int c = 2;
  Future<int> x = foo(c); // 2->3
  print(c);
  print(await x);
}

Console

--work01--
3
3
--work02--
3
3
--work03--
2
3
--work04--
2
3

A compile error of await for in Dart

oganao — Sat, 17 Dec 2022 07:44:39 +0000

People love dart programming language because of its easy-to-use asynchronous feature. I definitely agree with it.

I came across the following error the other day.

The type 'Stream' used in the 'for' loop must implement Iterable. (view docs)

My code snippest was :

void main() async {

  for(String v in countStream('a b c d')) { // *** compile error ***
    print(v);
  };

 }

Stream<String> countStream(String text) async* {
  var words = text.split(' ');
  for (int i = 0; i <= words.length; i++) {
    await Future.delayed(const Duration(milliseconds: 300));
    yield words.getRange(0, i).join(' ');
  }
}

You'll soon notice I forgot "await" before "for". But I couldn't figure out that. I went the wrong way.

The compiler requested Iterable, so I was wondering if Stream is Iterable and googled how to make Stream into Iterable. It was a wast of time.

In dart, "await for" is said to be "Asynchronous For-in" in the language specification. It is different from "for", which is used for asynchronous.

The compiler can't infer which feature a programmer intends to use, even thought one codes async*.

Applying reduce to a list with no element. Oops.

oganao — Thu, 17 Nov 2022 01:55:21 +0000

I have the following code. Getting datetimes from a list of a model, removing a datetime if the date is today, and gaining the max value, is simple functional programming. It works fine, but sometimes 'No element' exception occurs.

var maxDate = list //removing today's data. .where((e) => !Util.isSameDay(e.datetime, DateTime.now())) .map((e) => e.datetime) // getting the max value. .reduce((v, e) => v.isAfter(e) ? v : e);

At first, I didn't figure out what the problem is. I spent some time trying to solve it.

The reason is simple. The reduce document says

The iterable must have at least one element

So, following the above code, we sometimes have a list with no element before we apply the reduce function.

I know how to use the reduce function, but I completely forgot the specification when I coded.

I use Android Studio, which is really awesome, but I hope the code inspector warns of such the code.