Flutter MCQ

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Flutter is Google's open-source UI toolkit that allows developers to build natively compiled applications for mobile, web, and desktop from a single Dart codebase. It uses its own rendering engine (Skia/Impeller) to draw widgets.

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Flutter applications are written in Dart, a language developed by Google. Dart is strongly typed, supports both AOT and JIT compilation, and has a syntax familiar to Java and JavaScript developers.

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In Flutter, everything is a Widget. Widgets are the basic building blocks of the Flutter UI — from layout elements like Row and Column to interactive elements like Button and TextField.

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StatelessWidget describes a part of the UI that does not depend on any mutable state. StatefulWidget is paired with a State object that holds mutable data and can trigger rebuilds by calling setState().

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Calling setState() inside a StatefulWidget tells the Flutter framework that the internal state of this object has changed. Flutter will then schedule a call to the build() method to update the UI with the new state.

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pubspec.yaml is the project configuration file for a Flutter/Dart project. It defines the project name, version, dependencies (packages), dev_dependencies, and asset paths (images, fonts, etc.).

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MaterialApp is typically the root widget of a Flutter Material Design application. It configures the top-level Navigator, theme, routes, and locale settings for the entire app.

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Scaffold implements the basic Material Design visual layout structure. It provides slots for AppBar, body, floatingActionButton, drawer, bottomNavigationBar, snackBar, and more.

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Hot Reload injects updated source code into the running Dart VM and rebuilds the widget tree while preserving state. Hot Restart fully restarts the Dart VM, losing all current state but picking up new initializations like changed global variables.

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BuildContext is a handle to the location of a widget in the widget tree. It is used to look up inherited widgets (like Theme), show dialogs, navigate, and access data that is propagated down the widget tree.

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The Column widget arranges its children along the vertical axis (top to bottom). Its counterpart, Row, arranges children along the horizontal axis. Both use MainAxisAlignment and CrossAxisAlignment properties.

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The Row widget lays out its children in a horizontal array. You can control spacing and alignment using mainAxisAlignment and crossAxisAlignment properties on the Row.

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Container is a convenience widget that combines common painting, positioning, and sizing widgets. It can apply padding, margins, colors, borders, and box shadows, and can also constrain or size its child widget.

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The Text widget displays a string of text with a single style. You can customize the font, size, color, alignment, and overflow behavior using TextStyle and other properties of the Text widget.

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The Padding widget insets its child by the given padding specified via EdgeInsets (e.g., EdgeInsets.all(8.0) or EdgeInsets.symmetric(horizontal: 16)). Container can also apply padding via its padding property.

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The Center widget centers its child both horizontally and vertically within itself. It expands to fill the available space from its parent and places its child in the middle.

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Expanded expands a child of a Row, Column, or Flex to fill the available space along the main axis. If multiple children are expanded, the space is divided proportionally according to their flex factors.

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Navigator manages a stack of Route objects and provides methods for navigating between screens. Navigator.push() adds a route to the stack and Navigator.pop() removes the top route to go back to the previous screen.

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To navigate to a new screen in Flutter, you use Navigator.push() with a MaterialPageRoute. The builder callback returns the widget to display on the new screen. Navigator.pop() takes you back.

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A Future in Dart represents a potential value or error that will be available at some time in the future. It is the basis of asynchronous programming in Dart — similar to a Promise in JavaScript.

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The await keyword pauses the execution of an async function until the Future it is waiting on completes, then resumes with the result. The function must be marked with the async keyword to use await.

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ListView is a scrollable, linear list of widgets. For large or infinite lists, use ListView.builder() which lazily builds items on demand rather than building all items upfront into memory.

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GestureDetector is a widget that detects gestures without any visual feedback. It has callbacks for tap, double tap, long press, pan, scale, and more. For Material ink ripple feedback, use InkWell instead.

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FloatingActionButton is a circular icon button that hovers over content. Per Material Design guidelines, it represents the primary action of a screen and is placed in Scaffold via the floatingActionButton parameter.

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AppBar displays a Material Design app bar — a toolbar at the top of the screen. It typically shows the page title, a leading icon (back button or hamburger menu), and trailing action icons on the right.

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BottomNavigationBar is a Material Design widget that displays a row of small widgets at the bottom for selecting among a small number of views (2-5 tabs). It is set in the Scaffold's bottomNavigationBar parameter.

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A Drawer in Flutter is a Material Design panel that slides in from the edge of the screen. It is typically used for navigation — containing a header and a list of navigation items. Added to the Scaffold via the drawer parameter.

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TextField is the standard widget for accepting text input in Flutter. It can be configured with a TextEditingController, InputDecoration, keyboard type, validation, and focus handling via FocusNode.

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The Form widget is a container for grouping FormField widgets (like TextFormField). It provides a GlobalKey<FormState> that allows calling validate(), save(), and reset() on all form fields at once.

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Card is a Material Design sheet with slightly rounded corners and an elevation shadow. It is used to display related information in a contained, visually distinct panel. It takes a single child, typically a Column or ListTile.

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Stack allows you to overlay widgets on top of each other. Children can be positioned using Positioned widget. The last child in the children list appears on top. Useful for overlays, badges, and floating elements.

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SizedBox creates a fixed-size box. It is commonly used as a gap/spacer between widgets (e.g., SizedBox(height: 16)) or to give a specific width/height to a child. SizedBox.shrink() creates a zero-size invisible box.

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Every Flutter application starts with the main() function in lib/main.dart. Inside main(), you call runApp() with your root widget to attach it to the screen and start the Flutter rendering engine.

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runApp() takes the given Widget and makes it the root of the widget tree. It inflates the widget and attaches it to the screen. The widget's build() method is called to produce the initial UI.

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The build() method must return a Widget. Flutter calls build() whenever the widget needs to be rendered or rebuilt (e.g., after setState()). It should be a pure function of the widget's properties and state.

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initState() is called exactly once when the State object is first created. It is the right place to perform one-time initialization: subscribing to streams, initializing AnimationControllers, or fetching initial data.

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dispose() is called when the State object is permanently removed from the tree. Override it to release resources acquired in initState(): TextEditingControllers, AnimationControllers, StreamSubscriptions, and FocusNodes.

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In a Column (vertical main axis), CrossAxisAlignment controls how children are aligned on the horizontal (cross) axis. Options: start, end, center, stretch, and baseline. mainAxisAlignment controls the vertical distribution.

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CircularProgressIndicator is a Material Design circular progress indicator (spinner). It can be determinate (with a value 0.0–1.0) or indeterminate (spinning indefinitely). LinearProgressIndicator provides a horizontal bar variant.

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Image.network() displays an image loaded from a URL. Flutter downloads and caches the image. Other constructors: Image.asset() for bundled assets, Image.file() for local file images, and Image.memory() for in-memory bytes.

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Both Flexible and Expanded use a flex factor to allocate space in Row/Column. Expanded forces the child to fill all allocated space. Flexible allows the child to be smaller — it can be its natural size if fit is FlexFit.loose.

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Provider is a popular Flutter state management package that wraps InheritedWidget to make it simpler. It allows you to expose data (ChangeNotifier, Streams, etc.) to any widget in the tree without manually threading it through constructors.

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InheritedWidget propagates data down the widget tree without passing it through constructors. Descendants access it via BuildContext.dependOnInheritedWidgetOfExactType(). It is the foundation for Provider, Theme, and MediaQuery.

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BLoC (Business Logic Component) is an architectural pattern that separates business logic from the UI. It uses Streams for inputs (events) and outputs (states), making the UI reactive and highly testable.

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FutureBuilder takes a Future and a builder function, rebuilding the widget whenever the Future's state changes. The builder receives an AsyncSnapshot with connectionState (waiting/done/error) and the data or error.

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StreamBuilder listens to a Stream and rebuilds its subtree whenever a new event is emitted. Useful for real-time data like Firestore updates, BLoC state streams, and WebSocket messages.

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GlobalKey uniquely identifies a widget in the entire app. It allows accessing a widget's State (GlobalKey<FormState> for form.validate()), its RenderObject size/position, or a specific widget across the tree.

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MediaQuery.of(context) returns a MediaQueryData object with: screen size (size.width/height), devicePixelRatio, textScaleFactor, padding (safe area insets for notches), and system UI visibility info.

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ThemeData defines the visual properties of a Material app: color scheme, typography, button styles, input decoration themes, and more. It is set in MaterialApp's theme parameter and accessed anywhere via Theme.of(context).

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AnimationController generates values between 0.0 and 1.0 over a given duration. It can play forward, reverse, and repeat. It requires a vsync (TickerProvider) to sync with screen refresh, typically via SingleTickerProviderStateMixin.

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A Tween defines the range of values an animation produces. Tween<double>(begin: 0, end: 300) maps 0.0–1.0 to 0–300. You can tween Colors, Offsets, sizes, and custom types using the animate() method with an AnimationController.

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Hero animations create shared element transitions between routes. Wrap a widget in Hero(tag: 'myTag') on both screens. Flutter smoothly animates it between the two routes when navigating. Tags must match exactly on both screens.

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ListView takes a children list and builds all widgets upfront — suitable for small, fixed lists. ListView.builder lazily builds only the visible items using an itemBuilder function. This makes it memory-efficient for long or infinite lists.

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GridView.builder lazily builds grid items. SliverGridDelegateWithFixedCrossAxisCount specifies fixed column count; SliverGridDelegateWithMaxCrossAxisExtent specifies max item width. Great for photo galleries and product grids.

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The shared_preferences package provides persistent key-value storage. It wraps NSUserDefaults on iOS and SharedPreferences on Android. It supports String, int, double, bool, and List<String> values.

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MethodChannel enables communication between Flutter (Dart) and native platform code (Kotlin/Java on Android, Swift/ObjC on iOS). Call invokeMethod() from Dart and register a MethodCallHandler in the native layer.

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final variables are assigned once (runtime) and cannot change. const variables are compile-time constants — values must be known at compile time. const also makes objects deeply immutable and enables object canonicalization.

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Mixins in Dart allow code reuse across class hierarchies using the with keyword. Unlike single inheritance, a class can use multiple mixins. Mixins cannot have constructors. Common Flutter examples: SingleTickerProviderStateMixin, WidgetsBindingObserver.

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SingleChildScrollView makes a single child widget scrollable. It is ideal when screen content might overflow (e.g., a form on a small screen). For large item lists, prefer ListView.builder for better performance.

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LayoutBuilder provides the parent widget's constraints (maxWidth, maxHeight, etc.) to the builder function, allowing different UIs based on available space. This is the primary tool for building responsive Flutter layouts.

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Riverpod is a state management library from the same author as Provider. Improvements include: providers are global (no context needed), compile-time safe, testable, and support async out of the box with AsyncValue.

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Dart Isolates are independent workers that run concurrently with their own memory. Unlike threads, they do not share memory — they communicate via SendPort/ReceivePort message passing. Use compute() for a simple one-off Isolate.

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CustomPainter allows drawing directly on a Canvas via the CustomPaint widget. Implement paint() to draw shapes, paths, images, and text. shouldRepaint() controls when the painting should re-execute.

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Equatable overrides == and hashCode based on a props list you define, enabling value equality without boilerplate. It is widely used in BLoC to prevent duplicate state emissions when the state's values have not actually changed.

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GestureDetector detects gestures with no visual feedback. InkWell extends this with Material Design ink ripple animations on tap. Use InkWell inside Material widgets for visual feedback; use GestureDetector when you control the visual response yourself.

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sqflite is a Flutter plugin for SQLite, enabling local relational database storage on Android and iOS. It supports SQL queries, transactions, and batch operations. For desktop/web support, consider the drift package.

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Dart's sound null safety means types are non-nullable by default. You must explicitly declare nullable types with ? (e.g., String?). This eliminates null reference exceptions at compile time and improves app reliability.

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A factory constructor uses the factory keyword and does not always create a new instance — it can return a cached instance, a subtype, or compute the value. Commonly used for singletons, fromJson() deserializers, and named constructors.

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go_router is Flutter's officially recommended routing package. It provides declarative, URL-based navigation built on Navigator 2.0, with named routes, path parameters, query parameters, redirects, and deep linking support.

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Hive is a fast, lightweight key-value NoSQL database for Flutter/Dart. It is written in pure Dart (no native dependencies), supports custom objects via TypeAdapters, and is significantly faster than shared_preferences for larger or structured data.

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AnimatedBuilder rebuilds only the widget returned by its builder function when the animation changes. The optional child parameter lets you pass a static sub-widget that is built once and passed into the builder without rebuilding.

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Dart extension methods allow you to add new functionality to existing types — even types you don't own — without subclassing. Defined with the extension keyword. Example: adding .isNullOrEmpty getter to String.

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Dio is a feature-rich HTTP client for Dart/Flutter. It supports interceptors (for auth headers, logging), request cancellation, file uploads/downloads, response caching, and FormData — making it more powerful than the basic http package.

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Completer<T> gives you control over a Future. You create one with Completer<T>(), expose completer.future, and later call completer.complete(value) or completer.completeError(error). Useful when adapting callback-based APIs to Futures.

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The spread operator (...) inserts all elements of a List, Set, or Map into another collection literal. Example: [...list1, ...list2]. The null-aware spread (...?) safely handles nullable collections.

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Navigator 2.0 introduces a declarative navigation API: the Router widget, RouteInformationParser (parses URLs), and RouterDelegate (controls the page stack). It enables full deep link support and URL synchronization on Flutter web.

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NotificationListener<T> intercepts Notification objects (like ScrollNotification, OverscrollNotification) that bubble up through the widget tree. Return true from onNotification to stop the notification from propagating further.

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TextEditingController lets you read (controller.text), set (controller.text = '...'), and listen to changes in a TextField. Assign it to TextField's controller parameter. Always dispose it in dispose() to free resources.

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WillPopScope wraps a route and intercepts pop events (Android back button, navigation gesture). The onWillPop callback returns a Future<bool> — returning false prevents the pop. In Flutter 3.12+, prefer PopScope with canPop and onPopInvoked.

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CustomScrollView assembles multiple Slivers (SliverAppBar, SliverList, SliverGrid, SliverPersistentHeader) into a single scrollable. It is the foundation for complex scroll effects like collapsing toolbars combined with pinned headers and lazy-loaded grids.

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Flutter maintains three trees: Widget Tree (immutable configuration), Element Tree (mutable instantiation that reconciles widget changes and holds references to State), and RenderObject Tree (handles layout and painting).

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RenderObject is the base class for the render tree. It handles performLayout (sizing/positioning), paint (drawing to canvas), and hit testing. RenderBox is the most common subclass, using a 2D Cartesian coordinate system.

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Slivers are portions of a scrollable area that can be individually laid out with different scroll behaviors. They power SliverAppBar (collapsing headers), SliverList, SliverGrid, and SliverPersistentHeader inside a CustomScrollView.

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Tree shaking removes unreferenced code and assets at compile time. The Dart AOT compiler eliminates unused functions and classes. Flutter also removes unused icon font glyphs (e.g., from material_icons), significantly reducing app size.

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JIT (Just-In-Time) compilation runs during development, enabling hot reload. AOT (Ahead-Of-Time) compilation is used for release builds, producing native ARM/x64 code. AOT removes startup compilation overhead, resulting in faster launch and better performance.

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compute() runs a top-level function in a new Isolate and returns a Future with the result. It is used for CPU-intensive work (JSON parsing large payloads, image processing) that would block the main thread and cause UI jank.

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RepaintBoundary creates a separate GPU compositing layer. When the subtree inside it repaints, the rest of the screen is unaffected. Use it around frequently animating widgets (animated overlays, scrolling chat lists) to avoid full-screen repaints.

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WidgetsBindingObserver receives lifecycle notifications (didChangeAppLifecycleState), theme/locale changes (didChangePlatformBrightness, didChangeLocales), and low memory warnings (didHaveMemoryPressure). Register with WidgetsBinding.instance.addObserver(this).

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Golden tests capture a widget screenshot and compare it pixel-by-pixel against a pre-approved reference image. They catch unintended visual regressions. Use the matchesGoldenFile() matcher and run flutter test --update-goldens to regenerate references.

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pumpWidget() builds the widget into the test environment. pump() triggers a frame (optionally advancing time) to process animations, timers, or async work. pumpAndSettle() keeps pumping frames until the tree is stable with no pending animations.

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Flutter's rendering has three phases: Layout (constraints flow down, sizes flow up — each RenderObject determines its size), Paint (RenderObjects draw on picture layers), Composite (the engine rasterizes and combines layers for the GPU display).

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Dart's event loop processes the microtask queue (scheduleMicrotask, Future.then callbacks) completely before handling the next item in the event queue (Timer, I/O completions). This ensures promise-like chains resolve before I/O events.

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InheritedModel extends InheritedWidget to support aspect-based dependency. Dependents register which aspect they depend on and only rebuild when that aspect changes — unlike InheritedWidget which rebuilds all dependents on any change.

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A Flutter plugin uses MethodChannel with a unique channel name. In Dart, call channel.invokeMethod('methodName'). On Android, implement FlutterPlugin and MethodCallHandler in Kotlin. On iOS, implement FlutterPlugin in Swift. The channel name must match exactly.

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The Flutter Engine is a portable C++ runtime hosting the Dart VM, rendering (Skia/Impeller), platform channels, and input processing. On Android it lives inside a FlutterActivity; on iOS inside a FlutterViewController.

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shouldRepaint() is called whenever a new CustomPainter instance is provided during a rebuild. Return true if the visual output depends on changed data (causing a repaint). Return false if nothing changed. Always returning true is correct but wasteful.

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Add-to-app integrates a Flutter module into an existing native Android (Activity/Fragment) or iOS (ViewController) app. The host app manages a FlutterEngine and attaches a FlutterFragment/FlutterViewController to display Flutter UI.

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Impeller pre-compiles a fixed set of shaders at build time, eliminating the shader compilation stutters (jank) that Skia experiences at runtime. It is the default renderer on iOS and progressively rolling out on Android.

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@override indicates the method is intentionally overriding a superclass or interface method. If no such method exists in the superclass, the Dart analyzer produces a warning, helping catch typos and method signature mismatches.

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ChangeNotifier is a simple class that implements the observable pattern. It stores state and calls notifyListeners() when state changes. Used with Provider (ChangeNotifierProvider), all listening widgets rebuild when notifyListeners() is called.