How can I "sleep" a Dart program

Question

2019 edition:

In Async Code

await Future.delayed(Duration(seconds: 1));

In Sync Code

import 'dart:io';

sleep(Duration(seconds:1));

Note: This blocks the entire process (isolate), so other async functions will not be processed. It's also not available on the web because Javascript is really async-only.

You can also use the Future.delayed factory to complete a future after a delay. Here is an example of two functions that return a string asynchronously after a delay:

import 'dart:async';

Future sleep1() {
  return new Future.delayed(const Duration(seconds: 1), () => "1");
}

Future sleep2() {
  return new Future.delayed(const Duration(seconds: 2), () => "2");
}

It's not always what you want (sometimes you want Future.delayed), but if you really want to sleep in your Dart command-line app, you can use dart:io's sleep():

import 'dart:io';

main() {
  sleep(const Duration(seconds:1));
}

I found that there are several implementations in Dart to make the code delay execution:

new Future.delayed(const Duration(seconds: 1)); //recommend

new Timer(const Duration(seconds: 1), ()=>print("1 second later."));

sleep(const Duration(seconds: 1)); //import 'dart:io';

new Stream.periodic(const Duration(seconds: 1), (_) => print("1 second later.")).first.then((_)=>print("Also 1 second later."));
//new Stream.periodic(const Duration(seconds: 1)).first.then((_)=>print("Also 1 second later."));

For Dart 2+ syntax , in a async function context:

import 'package:meta/meta.dart'; //for @required annotation

void main() async {
  void justWait({@required int numberOfSeconds}) async {
    await Future.delayed(Duration(seconds: numberOfSeconds));
  }

  await justWait(numberOfSeconds: 5);
} 

This a useful mock that can take an optional parameter to mock an error:

  Future _mockService([dynamic error]) {
    return new Future.delayed(const Duration(seconds: 2), () {
      if (error != null) {
        throw error;
      }
    });
  }

You can use it like this:

  await _mockService(new Exception('network error'));