What is the use of future.microtask with perfect example to understand the use case of it?

Overview

Future.microtask()

Future

The Dart Event Loop

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Synchronous code → Microtask queue → Event queue (Futures, timers, I/O)

• Synchronous code runs first
• Microtask queue (Future.microtask, scheduleMicrotask) runs next — before any events
• Event queue (Future, Timer, I/O callbacks) runs last

Basic Example

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void main() {
  print('1 - Sync start');

  Future(() => print('4 - Future (event queue)'));

  Future.microtask(() => print('3 - Microtask'));

  print('2 - Sync end');
}

// Output:
// 1 - Sync start
// 2 - Sync end
// 3 - Microtask       ← runs before the regular Future
// 4 - Future (event queue)

Practical Use Case 1: Defer work to after build

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@override
void initState() {
  super.initState();

  // ❌ Cannot call setState during initState synchronously
  // setState(() { ... }); // Throws error

  // ✅ Use microtask to run after initState completes
  Future.microtask(() {
    setState(() {
      _data = 'Loaded';
    });
  });
}

Practical Use Case 2: Priority async work

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void processPayment() {
  // Schedule critical validation as microtask (highest priority)
  Future.microtask(() => validateCard());

  // Schedule non-critical log (lower priority)
  Future(() => logPaymentAttempt());

  print('Payment initiated');
}

// Output order:
// Payment initiated
// validateCard()     ← microtask first
// logPaymentAttempt() ← regular Future after

Practical Use Case 3: Avoid setState during build

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@override
Widget build(BuildContext context) {
  // ❌ Never call setState directly in build
  
  // ✅ Defer it to after the current build frame
  Future.microtask(() {
    if (mounted) setState(() => _count++);
  });

  return Text('$_count');
}

Future.microtask vs scheduleMicrotask

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// These are equivalent
Future.microtask(() => doWork());
scheduleMicrotask(() => doWork());

Execution Order Summary

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void main() {
  print('A');                               // 1st
  Timer(Duration.zero, () => print('E'));   // 5th (timer = event queue)
  Future(() => print('D'));                 // 4th (Future = event queue)
  Future.microtask(() => print('C'));       // 3rd (microtask)
  scheduleMicrotask(() => print('B+'));     // also microtask
  print('B');                               // 2nd
}
// Output: A → B → B+ → C → D → E

When to Use Future.microtask

Future.delayed

Isolate

compute

Key Rule:

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Future.microtask

runs after current sync code but before any queued events. It is ideal for deferring work that must happen before the next event loop cycle but after the current execution context completes.