Flutter interview questions and answers

1. Flutter hot reload vs restart

You changed a text color inside a Flutter build method and want to see it while keeping the current screen and the counter value you already set. What should you use?

1. A.Hot restart
2. B.Hot reload
3. C.A full rebuild and reinstall
4. D.flutter clean, then run again

2. Dart final versus const

In Dart, what is the main difference between a `final` variable and a `const` variable?

1. A.final must be known at compile time; const is set once at runtime
2. B.const can be reassigned later; final can never change
3. C.const must be known at compile time; final is set once at runtime
4. D.There is no difference; both are aliases of var

3. Reading the screen width

You need the available screen width inside a Flutter widget to switch between a phone and tablet layout. Which call gives it?

1. A.Theme.of(context).width
2. B.Navigator.of(context).size.width
3. C.Scaffold.of(context).screenWidth
4. D.MediaQuery.sizeOf(context).width

4. ListView.builder for a long feed

A feed has 5,000 items. Why is ListView.builder preferred over ListView(children: [...])?

1. A.It loads all 5,000 items in a background isolate first
2. B.It builds only the items that are on or near the screen
3. C.It caches every item as an image for faster scrolling
4. D.It is the only ListView that supports scrolling

5. Future or Stream

A chat screen must react to every new message that arrives over a socket. Which Dart type models this?

1. A.A Future, because it delivers many values over time
2. B.A Stream, because it delivers many values over time
3. C.A List, because messages arrive in order
4. D.A Completer, because it repeats automatically

6. Validating a Flutter form

A login screen uses a Form with two TextFormFields that have validators. How do you run all validators when Submit is tapped?

1. A.Give the Form a GlobalKey<FormState> and call currentState!.validate()
2. B.Call validate() on each TextEditingController in turn
3. C.Wrap the whole form in a FutureBuilder and rebuild it
4. D.Set autovalidateMode to disabled and then call setState

7. Cleaning up a TextEditingController

A StatefulWidget creates a TextEditingController in initState. What should happen when the screen is removed?

1. A.Nothing; Dart garbage collects it immediately
2. B.Set the controller to null in build
3. C.Call controller.dispose() in the State's dispose method
4. D.Call controller.clear() in initState

8. Animating a size change simply

A card should smoothly grow from 100 to 200 pixels tall when tapped, with no custom controller. Which widget fits best?

1. A.CustomPainter with a Timer
2. B.AnimationController with a Tween and a listener
3. C.A Stack with two Containers
4. D.AnimatedContainer with a duration

9. What flutter doctor reports

A new team member cannot build the app. What does running `flutter doctor` tell them?

1. A.Which Dart files have lint warnings that block the build
2. B.Which packages in pubspec.yaml have newer versions on pub.dev
3. C.Whether the SDK, Android toolchain, Xcode and devices are set up
4. D.How long each widget takes to build on the connected phone

10. How Flutter and React Native draw UI

What is the key difference in how Flutter and React Native put UI on screen?

1. A.Flutter uses native platform views; React Native paints into a canvas
2. B.Both render HTML inside a hidden WebView on the device
3. C.Flutter paints its own widgets; React Native renders native platform views
4. D.Both convert their code into Swift and Kotlin at build time

11. Integer division in Dart

In Dart, what is the value of `7 ~/ 2`?

1. A.3.5
2. B.4
3. C.1
4. D.3

12. String interpolation with an expression

What does this Dart code print when items has 3 elements?

final items = ['a', 'b', 'c'];
print('You have ${items.length} items');

1. A.You have $items.length items
2. B.You have [a, b, c].length items
3. C.You have 3 items
4. D.You have items.length items

13. Required named parameters

What does `required` mean in this constructor?

class Header extends StatelessWidget {
  const Header({super.key, required this.title});
  final String title;
}

1. A.Callers must pass title by name, or the code does not compile
2. B.title defaults to an empty string when it is not passed
3. C.title must be passed as the first positional argument
4. D.title is checked at runtime and may still be null

14. Material 3 theming from one colour

A designer gives one brand colour and wants a full, consistent Material 3 palette in Flutter. What is the simplest approach?

1. A.Set the brand colour on every widget's color property
2. B.Create 40 const Color fields and pick them by hand
3. C.Use a CupertinoTheme with the brand colour
4. D.ThemeData(colorScheme: ColorScheme.fromSeed(seedColor: brand))

15. Platform-looking switches

You want a switch that looks like a Cupertino switch on iOS and a Material switch on Android. What is the quickest option?

1. A.Switch with a custom thumbColor
2. B.CupertinoSwitch on both platforms
3. C.A Checkbox styled as a toggle
4. D.Switch.adaptive

16. Measuring test coverage in Flutter

How do you produce a coverage report from Flutter unit and widget tests?

1. A.flutter build --coverage, which writes a report in the APK
2. B.flutter run --profile, then open DevTools coverage
3. C.dart compile coverage main.dart
4. D.flutter test --coverage, which writes coverage/lcov.info

17. const constructors in Flutter

Why does Flutter recommend const constructors such as const Text('Hello') wherever possible?

1. A.const widgets are created once and reused, so Flutter can skip rebuilding them when the parent rebuilds
2. B.const widgets are drawn only by the GPU
3. C.const tells Flutter to paint the widget once into a cached bitmap and reuse that image on every later frame, even if its text changes
4. D.Without const, a widget cannot be stateless

18. FutureBuilder refetching on rebuild

What is the bug in this Flutter code?

@override
Widget build(BuildContext context) {
  return FutureBuilder<User>(
    future: api.fetchUser(),
    builder: (context, snapshot) => UserCard(snapshot.data),
  );
}

1. A.FutureBuilder cannot be used inside build
2. B.A new request starts on every rebuild; create the future once (for example in initState) and pass the stored future
3. C.fetchUser must return a Stream, because FutureBuilder only listens during the first frame and ignores results that arrive later
4. D.The builder function must be marked async

19. Reading a late variable too early

A Dart class declares `late String token;` and a method reads `token` before any code has assigned it. What happens?

1. A.A LateInitializationError is thrown at runtime
2. B.The variable silently returns an empty string
3. C.The analyzer refuses to compile the class
4. D.The variable returns null because it is nullable

20. Yellow and black overflow stripes

A Row with a long product name shows the debug message "A RenderFlex overflowed by 48 pixels on the right". What is the right fix?

1. A.Increase the font size so Flutter recalculates the layout
2. B.Wrap the whole Row in a Center widget
3. C.Wrap the Text in Expanded or Flexible so it can shrink or wrap
4. D.Switch the app to release mode, where overflow is ignored

21. ListView inside a Column

Placing a ListView directly inside a Column throws "Vertical viewport was given unbounded height". What is the usual fix?

1. A.Give the Column a mainAxisAlignment of center
2. B.Replace the Column with a Stack
3. C.Set the ListView physics to NeverScrollableScrollPhysics
4. D.Wrap the ListView in Expanded so it gets the remaining height

22. Caret version range in pubspec

pubspec.yaml contains `http: ^1.2.0`. Which versions can pub pick?

1. A.Any version from 1.2.0 up to, but not including, 2.0.0
2. B.Exactly 1.2.0 and nothing else
3. C.Any version from 1.2.0 up to, but not including, 1.3.0
4. D.Any version at all, including 2.x and later

23. Reusing behaviour with a mixin

Several unrelated widgets' State classes need the same logging helpers. What does `class _HomeState extends State<Home> with LoggerMixin` do?

1. A.It makes LoggerMixin the parent class of State
2. B.It creates a separate LoggerMixin object inside each State
3. C.It lets _HomeState implement LoggerMixin with no method bodies
4. D.It copies LoggerMixin's members into _HomeState without a second superclass

24. Factory constructor for a singleton

Why use a `factory` constructor in this Dart class?

class ApiClient {
  static final ApiClient _instance = ApiClient._internal();
  factory ApiClient() => _instance;
  ApiClient._internal();
}

1. A.It can return an existing instance instead of always creating a new one
2. B.It makes the constructor body run on a separate background isolate
3. C.It is required for any class that declares a static field
4. D.It makes every field of the class const automatically

25. Calling native code from Flutter

A Flutter app needs a value from an Android-only SDK that has no plugin. What is the standard mechanism?

1. A.A MethodChannel that calls Kotlin code and returns the result asynchronously
2. B.A dart:html import that runs the Kotlin SDK code directly
3. C.A const constructor that statically links the native library
4. D.A new isolate that loads the whole Android SDK into Dart memory

26. Passing config with dart-define

A build uses `--dart-define=API_URL=https://staging.example.com`. How does Dart code read it?

1. A.const String.fromEnvironment('API_URL')
2. B.Platform.environment['API_URL'] at runtime
3. C.dotenv.load() from the device file system
4. D.It cannot be read; it only configures Gradle

27. Adding Flutter to an existing app

A bank has a large native Android and iOS app and wants to build one new loan module in Flutter. What is the supported approach?

1. A.Rewriting the whole app in Flutter before shipping anything
2. B.Loading the Flutter module inside a WebView on each screen
3. C.Shipping the loan module as a separate store app only
4. D.Add-to-app, embedding a Flutter module inside the native apps

28. What an async* function returns

What does this function return?

Stream<int> countdown() async* {
  for (var i = 1; i <= 3; i++) {
    await Future.delayed(const Duration(seconds: 1));
    yield i;
  }
}

1. A.A Future<int> that completes with 3 after three seconds
2. B.A Stream<int> that emits 1, 2 and 3 with a second between them
3. C.A List<int> containing 1, 2 and 3 straight away
4. D.A Future<List<int>> that completes once with all values

29. Closing a StreamController

A ViewModel class creates a StreamController for search results. What should its dispose method do?

1. A.Call controller.close() so listeners finish and get freed
2. B.Set the controller to null and let it keep running
3. C.Call controller.add(null) as the very last event
4. D.Nothing; streams close by themselves on every rebuild

30. Protecting routes with go_router

With go_router, logged-out users who open /orders should be sent to /login. Where is this best handled?

1. A.In every screen's build method with a Navigator.push
2. B.In the router's redirect callback, based on the auth state
3. C.In main() before runApp, by checking the URL once
4. D.In the AndroidManifest.xml intent filters

31. Localizing a Flutter app

How does a Flutter app typically handle English and Arabic strings?

1. A.ARB files per locale, generated into an AppLocalizations class
2. B.An if statement around every Text widget in the app
3. C.A JSON file read from the server on each screen
4. D.Separate Git branches for each of the languages

32. setState called during build

A widget calls `setState(() => count++)` directly inside its build method and the app shows "setState() or markNeedsBuild() called during build". Why is this wrong?

1. A.build must not change state, or rebuilds would loop
2. B.setState is only allowed inside a StatelessWidget
3. C.count must be declared as a const value first
4. D.setState needs to be awaited inside build

33. Swapped Flutter tiles keep colors

Two StatefulWidget tiles in a Row store a random color in their State. When you swap their order in the list, the colors do not move with them. Why?

1. A.Flutter matches widgets to existing elements by type and position, so without keys the State objects stay in place; give each tile a key such as ValueKey(tile.id)
2. B.setState was not called after the swap
3. C.Row caches the first children it was given and keeps painting them, so a reordered list is ignored until the screen is pushed again
4. D.StatefulWidgets cannot be reordered in Flutter

34. Dart microtask and event order

In what order does this Dart program print its letters?

import 'dart:async';

void main() {
  print('A');
  Future(() => print('B'));
  scheduleMicrotask(() => print('C'));
  Future.value().then((_) => print('D'));
  print('E');
}

1. A.A B C D E
2. B.A E B C D
3. C.A E C D B
4. D.A C D E B

35. Widget, element and render trees

Flutter keeps three trees: widgets, elements and render objects. Which statement about them is correct?

1. A.Widgets are long-lived and mutable; elements are rebuilt from scratch on every frame
2. B.Render objects are created again for every build, while elements are never reused
3. C.Only the widget tree exists at runtime; the other two are debug-only views
4. D.Widgets are cheap immutable configs; elements are long-lived and link widgets to render objects

36. Overriding == without hashCode

A Product class overrides `operator ==` to compare ids but does not override `hashCode`. Two products with the same id are added to a Set. What is likely?

1. A.The Set throws a runtime error because hashCode is missing
2. B.The Set always keeps only one product, since == is checked first
3. C.The Set may keep both, because equal objects have different hash codes
4. D.The analyzer refuses to compile the Set literal with these objects

37. Why teams use Pigeon

A team has dozens of MethodChannel calls with string method names and untyped maps, and bugs keep coming from typos. What does Pigeon offer?

1. A.A faster rendering engine that replaces platform channels
2. B.Automatic conversion of Kotlin code into Dart
3. C.A way to share memory between Dart and native threads
4. D.Generated, type-safe Dart, Kotlin and Swift code for the channel API

38. Calling a C library from Dart

You need to call a fast C image-processing library synchronously from Dart on mobile. Which approach avoids platform-channel message overhead?

1. A.A MethodChannel with a binary codec
2. B.dart:ffi bindings to the compiled C library
3. C.An EventChannel that streams pixels
4. D.A WebView running the C code through WebAssembly

39. Reading the performance overlay

The Flutter performance overlay shows the raster (GPU) graph spiking red while the UI graph stays green. Where is the problem most likely?

1. A.Slow Dart code running inside build methods
2. B.Costly painting such as saveLayer, heavy clips or big blurs
3. C.Too many network calls made from the UI isolate
4. D.Slow JSON parsing inside a FutureBuilder callback

40. shrinkWrap lists inside a scroll view

A screen nests five `ListView(shrinkWrap: true)` sections of 200 items each inside a SingleChildScrollView. Scrolling is slow. Why?

1. A.shrinkWrap forces each of the lists to run on its own isolate
2. B.SingleChildScrollView repaints only the first list it contains
3. C.shrinkWrap lays out every item to measure the list, so nothing is lazy
4. D.Nested lists disable hardware acceleration for the whole screen

41. Obfuscating a Flutter release

You build with `flutter build apk --obfuscate --split-debug-info=build/symbols`. What must you keep to read crash stack traces later?

1. A.Nothing; obfuscated traces are readable by default
2. B.The symbol files written to build/symbols for that exact build
3. C.Only the AndroidManifest.xml of the build
4. D.The debug APK built from the same commit

42. pumpAndSettle times out

A widget test calls `await tester.pumpAndSettle()` on a screen with a looping CircularProgressIndicator and fails with a timeout. What is the fix?

1. A.Increase the default test timeout to ten full minutes
2. B.Use tester.pump(duration), since the animation never settles
3. C.Run the same widget test in release mode instead
4. D.Call setState inside the test body to stop the frames

43. List<dynamic> is not a List<String>

Decoding `{"tags": ["a", "b"]}` and writing `List<String> tags = json['tags'];` throws a type error at runtime. What fixes it?

1. A.Build a typed list with List<String>.from(json['tags'])
2. B.Declare the variable as final instead of using a type
3. C.Call jsonDecode twice on the same input string
4. D.Wrap the assignment in a try block and ignore it

44. Flutter's layout rule

Which sentence correctly describes Flutter's layout protocol?

1. A.Sizes go down, constraints go up, and the child sets its position
2. B.Each widget picks any size and the screen scrolls to fit
3. C.Constraints go down, sizes go up, and the parent sets the position
4. D.The root widget measures every leaf before any layout

45. Container ignores its width

Why does this Container fill the whole screen instead of being 100 × 100?

void main() => runApp(
  Container(width: 100, height: 100, color: Colors.red),
);

1. A.Container ignores width and height unless a color is set
2. B.MaterialApp scales every child up to the screen size
3. C.The width is in centimetres, not logical pixels
4. D.The screen gives tight constraints, forcing the child to be full size

46. Blocking back with unsaved changes

On Android 14 and later, predictive back gestures need to know in advance whether a route may pop. Which Flutter widget handles an "unsaved changes" guard correctly?

1. A.WillPopScope returning a Future from onWillPop
2. B.PopScope with canPop set from the form state
3. C.GestureDetector listening for a left swipe
4. D.A WillPopScope inside a SafeArea widget

47. Turning a callback API into a Future

An old SDK reports results through onSuccess and onError callbacks. What does this Dart code use to expose it as a Future?

Future<String> scan() {
  final c = Completer<String>();
  sdk.scan(onSuccess: c.complete, onError: c.completeError);
  return c.future;
}

1. A.A StreamController that is never closed
2. B.A Timer that polls the SDK every second
3. C.An Isolate that blocks until a result arrives
4. D.A Completer, completed from the callbacks