Sample code demonstrating Asynchronous programming with Futures

async-with-futures.dart

import 'dart:convert';
import 'dart:html';
import 'dart:async';

main() {
  // Basic example
  var result = Future(() => 'Hello World!');
  print(result);
  result.then((data) => print(data));
  
  // Delayed example
  var delayedResult = Future.delayed(Duration(seconds: 2), () => 'Displayed after 2 seconds');
  
  delayedResult.then((result) => print(result));
  
  // Error handling & Chainable methods
  var showError = false;
  Future(() => showError ? throw 'There was an error.' : '{"data": "Success!"}')
    .then((data) => json.decode(data))
    .then((result) => print(result['data']))
    .catchError((error) => print(error));
  
  // Dart library example
  HttpRequest.getString('https://swapi.co/api/people/1')
     .then(print)
     .catchError((err) => print('There was a problem: $err'));
  
  // Callback signatures in a Future context
  lookupVersion(cb) => Timer(Duration(seconds: 2), () => cb('v2.1.0'));
  lookupVersion((version) => print('Got the version: $version'));
  
  Future lookupVersionAsFuture() {
    var completer = Completer();
    
    lookupVersion((version) => completer.complete('Got `Future` version: $version'));
//     lookupVersion((_) => completer.completeError('There was a problem'));
      
    return completer.future;
  }
  
  lookupVersionAsFuture()
    .then((version) => print('Got the version as a Future result: $version'))
    .catchError(print);
  
  // Async/await
  lookupVersionAsAsyncAwait() async {
    var version = await lookupVersionAsFuture();
    print('With async/await: $version');
  }
  lookupVersionAsAsyncAwait();
}