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Mobile Fl Dart Async Isolates Interview Questions

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Sample questions

Fl Dart Async IsolatesDifficulty 1
What does an async function in Dart return, even if you don't write it explicitly?
  • aNothing — async functions are always void.
  • bA Stream of the function's result over time.
  • cThe raw result value, computed synchronously.
  • dA Future wrapping the function's result.✓
Explanation:Marking a function async makes it return a Future (or Future<void>) automatically, even if the body has no explicit Future type — the compiler wraps the result.
Fl Dart Async IsolatesDifficulty 1
Future<int> fetchValue() async {
  await Future.delayed(Duration(seconds: 1));
  return 42;
}

What happens when fetchValue() is called?
  • aIt blocks the calling code for one second before returning.
  • bIt returns 42 immediately and runs the delay in the background afterward.
  • cIt returns a Future<int> immediately, resolving to 42 after the delay.✓
  • dIt throws unless the caller also uses await.
Explanation:Calling an async function does not block; it returns a pending Future right away. The caller can await it (or attach .then) to get 42 once the delayed future completes.
Fl Dart Async IsolatesDifficulty 2
A function does final a = await taskA(); final b = await taskB(); where taskA and taskB are independent (don't depend on each other's result). What is the practical downside of writing it this way?
  • ataskB doesn't start until taskA finishes — they run sequentially.✓
  • bDart will refuse to compile this — independent awaits must use Future.wait.
  • ctaskA and taskB are automatically parallelized regardless of the syntax.
  • dThe code throws a runtime exception because b is declared before a resolves.
Explanation:Each await pauses until that specific future completes before moving to the next line. Since taskB() isn't even called until after taskA finishes, they run one after another; Future.wait([taskA(), taskB()]) would let both start immediately.
Fl Dart Async IsolatesDifficulty 1
What is the key structural difference between a single-subscription Stream and a broadcast Stream?
  • aA single-subscription stream allows only one listener over its lifetime, while a broadcast stream allows multiple listeners.✓
  • bBroadcast streams can only emit errors, never data.
  • cSingle-subscription streams emit events synchronously, broadcast streams never do.
  • dThere is no real difference; .asBroadcastStream() is purely cosmetic.
Explanation:A single-subscription Stream buffers/produces events only once a single listener attaches, and calling .listen() twice throws. A broadcast stream supports any number of listeners, but doesn't buffer events for listeners that attach late.
Fl Dart Async IsolatesDifficulty 2
final controller = StreamController<int>();
controller.stream.listen((v) => print(v));
controller.add(1);
controller.add(2);

What does this print, assuming no errors?
  • aNothing, because StreamController requires .broadcast() to emit any events.
  • b2 then 1, because the listener attaches after both add calls are scheduled.
  • c1 then 2, in that order.✓
  • dIt throws because add was called before listen.
Explanation:A default StreamController is single-subscription; since listen is called before any add, the controller has a listener ready and delivers events in the order they were added: 1, then 2.
Fl Dart Async IsolatesDifficulty 1
Why does Dart require actual parallel CPU-bound work to use an Isolate instead of just an async function?
  • aasync functions are physically incapable of calling any CPU-heavy code at all.
  • bA single Dart isolate has one thread and one event loop; async/await doesn't use extra cores.✓
  • cIsolates are just a different syntax for the exact same single-threaded execution as async.
  • dasync functions can only run inside widget build methods.
Explanation:async/await gives concurrency (interleaving) on a single thread, not parallelism. Heavy synchronous computation still blocks that one thread and the event loop with it. An Isolate gets its own thread and memory, enabling true parallel execution.

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