Basic examples of using Futures in Dart

readme.md

Basic Examples: Dart Futures

Basic examples of using Futures in Dart

Find this at dartpad.dartlang.org/ab210bb6bfa5add329df104854838bce.

main.dart

import "dart:async";
import "dart:math" as math;

const asyncDelay = const Duration(seconds: 3);

///Asynchronous method that returns back a random int
Future<num> getNumber(num max) {
  return new Future.delayed(asyncDelay, () => new math.Random().nextInt(max));
}

///Simple async that returns given obj after a delay
Future delay(dynamic z) => new Future.delayed(asyncDelay, () => z);

///Simple method defined as closure
num addToNumber(num x, num y) => x + y;

main() async {
  print(" * Starting *");

  num newNum;
  // Line below is invalid, can't assign a Future to a num
  //newNum = getNumber(4);
  
  //Stop continued execution until the Future completes
  newNum = await getNumber(10);
  print("Awaited async call completed with result |$newNum|");

  Future<num> f; //Futures are plain old objects
  
  //Retreive the Future and continue execution
  f = getNumber(10);
  f.then( (y) => print("Future completed with $y") );
  print("Async call 1 completed with result |$f|");
  
  //Chaining futures
  f = getNumber(10);
  f.then( (m) => addToNumber(7, m))
   .then( (m) => addToNumber(21, m))
   .then( (y) => print("Future completed with $y") );
  print("Async call 2 completed with result |$f|");

  //Chaining futures #2
  Function add100 = (num n) => 100 + n; //Functions can be plain old objects
  Function add1000 = (num n) => 1000 + n; //Functions can be plain old objects
  Function printAndReturn = (num n) { print("  -Num is $n"); return n;};
  getNumber(4).then(printAndReturn)
              .then(add100)
              .then(printAndReturn)
              .then(delay)
              //.then(addToNumber) //error, mismatched Function type  
              .then(add1000)
              .then(printAndReturn);
 
  print(" * Ending * "); 
}