Advanced interview questions in Flutter & Dart
What are Dart extension methods? Implement toIntOrDefault on String.
Tests Dart extension syntax and static resolution. A strong answer defines an extension on String, uses int.tryParse with a fallback, and notes extensions do not mutate the original type. Red flag: using try-catch over tryParse or claiming dynamic dispatch.
Explain Dart switch exhaustiveness for enums versus sealed classes
This tests Dart 3 exhaustiveness. A strong answer notes that enums and sealed classes both require exhaustive switches, but sealed classes allow distinct payloads because subclasses stay in the same library. A red flag is calling them just enums with methods.
Explain Dart's event loop, microtask queue, event queue, and await behavior
Tests Dart single-threaded event loop model. Strong answer: microtasks drain before event queue tasks; await suspends the function and schedules its resumption via the event loop when the Future completes. Red flag: claiming await blocks the thread.
How does Dart resolve mixin method conflicts and application order?
Tests understanding of Dart's mixin linearization. Answer: Dart applies mixins left-to-right, building a superclass chain where the last mixin wins conflicts. Red flag: claiming first mixin wins or confusing with multiple inheritance.
Explain StreamController, write a broadcast stream example, and why close it?
This tests Dart stream lifecycle and memory safety. A strong answer uses StreamController.broadcast(), adds data and errors, closes it, and explains unclosed controllers leak memory.
Explain the lifecycle of a State object
List initState through dispose, emphasizing didUpdateWidget reacts to parent config changes.
What are Keys in Flutter and why are they critical?
Tests widget identity during reconciliation. Keys let Flutter distinguish moved widgets from changed data; without ValueKeys, ReorderableListView leaves state at old positions, e.g., a checkbox swap. Red flag: calling them performance tools.
How does const on a widget constructor impact build and reconciliation?
Tests widget canonicalization and reconciliation. Strong answers: const creates identical widget instances; Element.updateChild skips subtree rebuild when oldWidget == newWidget, saving elements and GC.
Explain Flutter's 'constraints down, sizes up' rule and unbounded height error
Parents constrain down, children size up. ListView in Column throws unbounded height as Column gives infinite maxHeight; fix with Expanded or shrinkWrap.
LayoutBuilder versus MediaQuery for responsive widgets
MediaQuery reports the whole window; LayoutBuilder gives the parent's actual constraints.
What problem does IntrinsicWidth solve and what are its performance costs?
This tests Flutter constraints and intrinsic sizing. A great answer covers: capping width to the child's max intrinsic width, aligning Column children, and the speculative pass that costs O(N²) in tree depth. A red flag is omitting performance cost.
Implement efficient async username validation in Flutter
Tests async field hygiene in Flutter. Outline: debounce 300ms, cancel inflight requests, decouple loading UI from errors, use reactive_forms async validators or manual streams. Red flag: API calls per keystroke without cancellation, loading treated as errors.
Explain Flutter's GestureArena and overlapping detector scenario
The arena opens on pointer down; recognizers accept or reject; the last accepted member wins.
Custom fade-and-scale transition with PageRouteBuilder and GoRoute
Tests bridging imperative transitions to declarative routers. Use PageRouteBuilder with FadeTransition and ScaleTransition, set globally in theme or per route via GoRoute pageBuilder with CustomTransitionPage. Red flag: omitting child rebuilds the page.
Immutable bloc state versus mutable ChangeNotifier
Immutable gives predictable diffs, easy debugging and replayability at the cost of boilerplate; mutable is terse but error-prone.
Explain Dio interceptors and automatic token refresh
Define interceptors as hooks; queue concurrent 401s during refresh; retry with Bearer header via token manager.
Implement an API caching layer with offline support and storage trade-offs
Use in-memory for hot data and disk for offline, pick Hive or SQLite by shape, and use TTL with a sync queue.
Why are stale search requests problematic and how do you cancel them?
This tests race conditions and resource waste in async UI. Strong answers note stale requests waste bandwidth and overwrite newer results; cancel the previous call with a CancelToken before issuing the next.
How do you safely add a non-nullable column in sqflite?
Tests onCreate for fresh installs vs onUpgrade for existing data. Outline: bump version; in onUpgrade use ALTER TABLE ADD COLUMN NOT NULL DEFAULT for existing rows; keep onCreate as latest schema. Red flag: only changing onCreate so old users crash.
Implement staggered animations using a single AnimationController and Interval
This tests multiplexing one ticker into sequential animations. Use one AnimationController, map each Tween to an Interval on distinct 0-to-1 sub-ranges, and rebuild with AnimatedBuilder. Red flag: multiple controllers or manual forward chaining.
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