Unlocking the Mysteries of Dart Completers

Syed Abdul Basit
3 min readNov 30, 2023

Unlocking Asynchronous Magic in Flutter with Dart’s Completers

Ever stumbled upon a coding challenge in Dart or Flutter that left you wishing for a magic wand to handle Future objects or callback values? What if I told you that such a ‘magic wand’ exists but is often overlooked? Welcome to the enigmatic world of the Completer.

Dive in to uncover the secret powers of Completer, and discover how they can transform the way you handle asynchronous operations. Are you ready to unlock a hidden gem in your Flutter toolkit?

What is a Completer in Dart?

Imagine you’re a chef, but instead of food, you’re cooking up asynchronous operations in Dart. A Completer is like your kitchen assistant who tells the diners (your program) that the dish (result of an asynchronous operation) is ready to be served. It's a way to produce Future objects and to signal when their results are ready.

Why Use a Completer?

Completers are the unsung heroes when dealing with asynchronous operations that don’t naturally return a Future. They allow you to:

  • Convert callback-based code to Future-based code.
  • Resolve a Future at a specific time.
  • New asynchronous primitive which means you can use it with Timer, scheduleMicrotask, and Zone.

Let’s see Completers in action with some examples.

Basic Completer

import 'dart:async';

void main() {
Completer completer = Completer<String>();
Future<String> future = completer.future;
future.then((String value) => print(value));
completer.complete('Hello, Flutter Developers!');

In this snippet, we create a Completer<String> and access its future. When we call complete with a string, the future is resolved, printing "Hello, Flutter Developers!".

Asynchronous Operation

Imagine you’re fetching data from an API:

Completer completer = Completer();
fetchDataFromAPI() {
// Imagine fetching data here
Timer(Duration(seconds: 3), () {
completer.complete('Data fetched successfully');
return completer.future;

Here, the Completer completes after a simulated delay, mimicking an API call.

Future<ApiResponse> fetchData(String endpoint) {
final Completer<ApiResponse> completer = Completer<ApiResponse>();

apiClient.request(endpoint, (response) {
}, (error) {

return completer.future;

In this example, fetchData represents a function that makes an asynchronous request to an API endpoint using an imaginary apiClient. The function returns a Future that will be completed with the API response or an error.

Tips for Using Completers

  • Don’t overuse Completers. They’re great for specific cases but aren’t always the answer.
  • Handle errors appropriately by using completer.completeError.
  • Remember, a Completer can complete only once. It’s like telling a joke; it doesn’t have the same effect the second time.


Completers in Dart are a potent tool for handling asynchronous operations in Flutter development. They bridge gaps in async programming, making your code cleaner and more intuitive. So next time you’re juggling asynchronous tasks in Flutter, remember that Completers might just be the secret ingredient you need!

Remember, keep coding fun and your semicolons in check! Happy Fluttering!

I welcome your feedback or corrections in the comments below. I would also appreciate the opportunity to connect with you on LinkedIn!