Stop Guessing When It's Done —
Just Complete It!

"Needed to wait for multiple async tasks in Flutter — BLoC events, downloads, DB inserts — and that's when I discovered the power of Completer."

Overview

In Flutter development, we often trigger multiple Future-based tasks — from API calls, database writes, to Cubit events — but we don't always have a clean way to know exactly when all of them are done.

Enter Completer — Dart's most powerful tool for manual control over async flow.

Why Use Completer?

• Flutter's Cubit/BLoC events usually return void
• You can't await multiple functions if they don't return a Future
• You often need to show a loader or dialog only after everything is done
• You want custom control after async logic completes

Benefits of Completer

• Works with any function, not just Cubit
• Combine multiple async events and wait cleanly
• Trigger logic after animation, file parsing, user action, API chain, etc.
• Build robust flows in large-scale Flutter apps
• Testable, clean, and scalable

Top Use Cases

Real Example: Wait for Multiple Async Events

Step 1 — Use Completer in your Cubit or service

download_cubit.dart
void download({required Completer<void> completer}) async {
  try {
    await fetchFromApi();
    await saveToLocalDB();
    completer.complete();
  } catch (e) {
    completer.completeError(e);
  }
}

Step 2 — Trigger in UI and wait

download_page.dart
final cubit1 = Completer<void>();
final cubit2 = Completer<void>();
final cubit3 = Completer<void>();

context.read<Cubit1>().download(completer: cubit1);
context.read<Cubit2>().download(completer: cubit2);
context.read<Cubit3>().download(completer: cubit3);

Future.wait([
  cubit1.future,
  cubit2.future,
  cubit3.future,
]).then((_) {
  showDialog(context: context, builder: (_) => SuccessDialog());
}).catchError((e) {
  showToast("❌ Failed: $e");
});

Reusable Utility: Wait for Multiple Completers

utils.dart
Future<void> waitForAll(List<Completer<void>> completers) {
  return Future.wait(completers.map((c) => c.future));
}

// Usage
final c1 = Completer<void>();
final c2 = Completer<void>();
myService1.download(completer: c1);
myService2.upload(completer: c2);
await waitForAll([c1, c2]);

Where to Use Completer

• Inside Cubit/BLoC void functions
• In async services, repositories, controllers
• Around isolate logic or parsing operations
• Inside UI flows waiting on multiple steps
• Custom logic layers that coordinate between APIs, DBs, or sensors

Final Thought

Think of Completer as your async completion notifier. It doesn't replace state management — it completes it.

Whether you're building enterprise-level apps, coordinating offline sync, or waiting for a background isolate, Completer gives you the full control you need.

🎯 Next time you're stuck waiting for something to finish — don't guess. Complete it.

By Rohan Kumar Chaudhary | Flutter Developer | Clean Code Advocate 🧼💙
Happy Coding! 💙

Rohan Kumar Chaudhary

Flutter · iOS · Mobile Developer

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