Flutter Interview Questions And Answers

Flutter is an open-source software UI (User Interface) development kit by google to develop cross-platform applications from a single code base. It is a framework for building beautiful, natively compiled, multi-platform applications for Android, iOS, Linux, macOS, Windows, Google Fuchsia and the web.

Flutter is from Dart (a language optimized for fast apps on any platform) that inherits Dart’s Pub package manager and software repository, allowing users to publish, and use custom packages and Flutter-specific plugins.

First described in 2015, Flutter was released in 2017. The first version of the Flutter was Sky, with the stated intention of being able to render consistently at 120 frames per second. With many releases and improvisations, recently, on 30- August 2022, Flutter 3.3 was announced.

Also, Flutter 3.3 features Objective-C and Swift interop and an early preview of a new rendering engine called “Impeller”, which aims to reduce stuttering caused by shader compilation.

Flutter Features 

Flutter is from Google and many leading brands around the world trust it. Also, the platform is maintained by a community of global developers. Top features include:

  • Flutter transforms the app development process. Using Flutter, you can build, test, and deploy mobile, web and embedded apps from a single codebase.
  • Flutter is fast. Flutter code compiles to ARM (Advanced RISC Machine) or Intel machine code, as well as JavaScript, for fast performance on any device.
  •  Flutter offers great productivity. You can build and iterate quickly with Hot Reload. Also, it offers updating code and seeing changes almost instantly, without losing state.
  •  The Flutter platform gives you flexibility. You can control every pixel of the application to create customized, adaptive designs that look and feel great on any screen.
  • It transforms the workflow for any developer. Programmers can take control of the codebase with automated testing, developer tooling, and more to build production-quality apps.

Benefits of using Flutter

Most businesses require an app that can work on different mobile platforms in one go. Hence the role of Flutter comes into the picture. Here are some of the benefits of using Flutter:

  • Cross-platform Operations: Operates on both platforms (iOS and Android) and there is no need for reconfigurations and redesigning.
  • Time Constraint: The time required to launch the app into the market is less as only a single app has to be made, which would work independently of the platform.
  •  Less Need of Developers
  •  Low Development Cost

Since you know the basics of Flutter, let us start with Flutter interview questions and answers.

1 . What Is Flutter?

Flutter is an open-source UI toolkit from Google for crafting beautiful, natively compiled applications for desktop, web, and mobile from a single codebase. Flutter apps are built using the Dart programming language.

2 . List the popular apps that use Flutter?

Flutter has grown in popularity and acceptance as the most widely used and accepted tool for developing apps. The following are some of the most popular Flutter apps.

  • Google Ads
  • Alibaba
  • Coach Yourself
  • Watermaniac
  • Reflect
  • Birch Finance
  • Tencent

3 . How To Implement FlutterError.OnError Correctly?

try {
   throw new UnrecognizedTermException();
 } catch (e) {
   setState(() => _status = e.errMsg());
}

// then in my test
expect(find.text('Could not determine the type of item you scanned'), findsOneWidget);

4 . What Are Packages And Plugins In Flutter?

  • Packages allow you to import new widgets or functionality into your app.
  • There is a small distinction between packages and plugins.
  • Packages are usually new components or code written purely in Dart whereas plugins work to allow more functionality on the device side using native code.
  • Usually on DartPub, both packages and plugins are popular as packages and only while creating a new package is the distinction clearly mentioned.

5 . Explain about Tween Animation?

Tween Animation is the shortened version for in-between animation. In tween animation, you must specify the start and endpoints of the animation. This enables the animation to begin at the beginning and progress through a series of values until it reaches the endpoint. The tween animation also determines the transition’s speed and duration.

6 . How Many Types Of Widgets Are There In Flutter?

There are two types of widgets:

  1. StatelessWidget : A widget that does not require mutable state.
  2. StatefulWidget: A widget that has a mutable state.

7 . What Are The Different Build Modes In Flutter?

  • The Flutter tooling supports three modes when compiling your app, and a headless mode for testing.
  • You choose a compilation mode depending on where you are in the development cycle.
  • The modes are:
    • Debug
    • Profile
    • Release

8 . Differentiate StatelessWidget And StatefulWidget?

Stateless : Widget state creates ONLY ONCE, then it can update values but not state explicitly. That’s why it has only one class which extends with StatelessWidget. They can never re-run build() method again.

Stateful : Widgets can update their STATE (locally) & values multiple times upon event triggered. That’s the reason, the implementation is also different. In this, we have 2 classes, one is StatefulWidget & the other is it’s State implementation handler i.e. State<YourWidget>. So if I say, they can re-run build() method again & again based on events triggered.

  • A StatelessWidget will never rebuild by itself (but can from external events). A StatefulWidget can.
  • A StatelessWidget is static whereas a StatefulWidget is dynamic.

9 . Why Do We Pass Functions To Widgets?

  • Functions are first class objects in Dart that you can pass as parameters to other functions.
  • We pass a function to a widget essentially saying, “invoke this function when something happens”.
  • Callbacks using interfaces like Android (<Java 8) have too much boilerplate code for a simple callback.

10 . What Is The Difference Between “Main()” And “RunApp()” Functions In Flutter?

  • main () function came from Java-like languages so it’s where all programs started, without it, you can’t write any program on Flutter even without UI.
  • runApp() function should return a Widget that would attach to the screen as a root of the Widget Tree that will be rendered.

11 . What Is A “Widget” And Mention Its Importance In Flutter?

  • Widgets are basically the UI components in Flutter.
  • It is a way to describe the configuration of an Element.
  • They are inspired from components in React.

Widgets are important in Flutter because everything within a Flutter application is a Widget, from a simple “Text” to “Buttons” to “Screen Layouts”.

12 . Explain pubspec.yaml file?

The project’s configuration file will work frequently when dealing with the Flutter project. It gives you control over how your app works. It also allows us to set the app’s constraints. This file contains the following information:

  • The name, description, and version of the project are all general project environments.
  • Dependencies on the project.
  • Assets for the project (e.g., images, audio, etc.).

13 . Why is Flutter UI focused?

Flutter is UI-focused since it has an excellent user interface because it uses a design-centric widget, advanced APIs, high-development tools, and many more features.

14 . What is the point of making separate directories for iOS and Android in a flutter project?

ANDROID: This folder is for storing a complete Android project. When creating a Flutter framework for the Android platform, this will come in handy. When you compile Flutter code into native code, it imports into an Android project, resulting in a native Android app.

iOS: This folder is for storing a complete Mac project. When creating a Flutter program for the iOS platform, this will come in handy. It functions in the same way as the Android folder. When you compile Flutter code into native code, it imports into the Android iOS project, resulting in a native Android app. Working with the Xcode IDE and macOS is the only way to build a Flutter program.

15 . Tell us about Hot Reload in flutter?

The hot reload function helps you to experiment with the project quickly and easily. It aids in the creation of user interfaces, the addition of new features, the correction of bugs, and the rapid development of apps. Follow these steps to perform a hot reload of a Flutter app:

  • Run the app in a Flutter editor or terminal window that supports it.
  • Any Dart file in the project can change.
  • Select Save It all or click the Hot Reload button on the toolbar if you’re using an IDE that supports Flutter. You will immediately see the result in your simulator or on a real computer.

16 . What are the advantages of a Flutter Inspector?

  • Choose a widget mode.
  • Toggle the platform to reveal the paint baselines.
  • Debug paint should be visible.
  • Widget refresh
  • It allows slow animations.
  • Activate or deactivate the output overlay.

17 . How to define a generic extension in Dart?

This can be done by adding a template parameter, like the below,

extension MyFancyList<T> on List<T> {
int get doubleLength => length * 2;
List<T> operator -() => reversed.toList();
List<List<T>> split(int at) => <List<T>>[sublist(0, at), sublist(at)];
}

18 . How do you create a factory?

Factory constructors are developed by using the factory keyword. They can return subtypes or even null.

e.g., factory User.fromXml(String xml).

19 . Explain Broadcast streams in Flutter?

This stream is for single messages that you can process one at a time. In a browser, these types of streams are widely for mouse events. This sort of stream is that you can to the list at any time. Multiple listeners can listen at the same time, and you can listen even though your previous subscription has been cancelled.

20 . Tell us about Single subscription streams available in Flutter?

It is a common and widely used stream. It is made up of a set of events that make up a larger whole. All events must be delivered in a specific order, with no single event being missed. When you get a web request or a file, you get this sort of stream. There is only one way to list this stream. Listing it over and over results in missed initial values, and the final stream makes little sense. The data is fetched and given in chunks when the listing begins in this stream.

21 . Create a TextField with an initial value?

We can set a controller with an initial value like TextEditingController(text: “Initial Text”).

22 . How will you declare an async function as a variable in Dart?

Async functions are normal functions having variable type just specifies that it returns a Future:

class Example {
Future Function() asyncFuncVar;
Future asyncFunc() async => print(‘Do async stuff…’);
Example() {
asyncFuncVar = asyncFunc;
asyncFuncVar().then((_) => print(‘Hello’));
}
}
void main() => Example();

23 . How will you use release mode in Flutter?

Release mode helps us to customize the codes and produce them in a completely configured state without any debug data. Many sections of the application’s code will be omitted or rewritten in this mode.

When we’re ready to release the software, we switch to this mode. It allows for optimal optimization and a limited device footprint.

To compile the release mode, run the following command:

flutter run-release

24 . When would you use profile mode in Flutter development?

Our applications’ output is measured using profile mode. In this mode, you can still debug and profile the output of your app. Since the emulator and simulator are not indicative of real-world output, this mode is disabled.

To compile the profile mode, use the following command:

flutter run-profile

25 . Why do you need Mixins?

Multiple inheritances are not supported in Dart. Mixins are important to formulate multiple inheritances in Flutter/Dart. Mixins allow you to write the code for a reusable class in multiple class hierarchies.

26 . Explain Ticker in Flutter?

In Flutter, a ticker represents the rate at which our animation is refreshed. It’s a signal-sending class that sends a signal at a set frequency, such as 60 times per second. Our watch, which tics at regular intervals, helps us grasp it. Ticker offers a callback method for each tick that has the length since the first ticks at each second since it was started. Even if the tickers begin at different times, they are immediately synchronized.

27 . Explain the ‘?’ operator in detail?

The “?” operator calculates and returns the difference in value between two expressions based on the given condition. It can be used in the following ways:

condition ? expr1 : expr2

This operator tests the condition first, then evaluates expr1 and returns its value if it is valid (if the condition is matched). Otherwise, it evaluates expr2 and returns its value.

28 . Explain Null aware pointers?

Dart has some helpful tips for dealing with null values.

1. The “??=” assignment operator, which only assigns a value to a variable if it is null.

int x; // Initial value of a is null.

x??= 2;

print(x); // It will print 2.

2. The null-aware “??” operator evaluates and returns the value between two expressions. It checks expression 1 and returns its value if it is not null; otherwise, it evaluates and returns the value of expression 2:

print(4 ?? 5); // It will print 4.

print(null ?? 5); // It will print 5.

29 . What is the use of the Await function?

Await is only used within async. It puts all currently running functions on hold until pf nature is done.

30 . How can you run code in debug mode only?

To run the code in debug mode only, we must first import the dart foundation as shown below:

import ‘package:flutter/foundation.dart’ as Foundation;
Next, use the kReleaseMode as shown below:
if (Foundation.kReleaseMode){ // is Release Mode ??
print(‘release mode’);
} else {
print(‘debug mode’);
}

31 . What is the Flutter architecture?

Flutter has a three-layered architecture:-

  • Upper Layer: The upper layer consists of the Dart programming language along with the widgets, animations, illustrations, customizations, etc.
  • The middle layer or the Flutter Engine: This layer deals with text display, formatting, layout, etc.
  • Bottom Layer or the built-in service: This layer is for managing plugins or packages.

32 . What are the most useful editors for Flutter apps?

Flutter tools make use of plugins to code mobile phone apps. The plugins can help in Dart code compilation, analysis of the code, and development of apps. Some used Flutter tools for IDE development are

  • Visual Studio
  • Android Studio
  • Xcode
  • IntelliJ IDEA
  • Eclipse
  • Emacs

33 . In What technology is Flutter built?

Flutter is built using C, C++, Skia – 2D rendering engine, and Dart-object-oriented language.

34 . What do you mean by keys in flutter?

Keys in flutter are identifiers for widgets, elements, and semantic nodes, whereas GlobalKeys and LocalKeys are the subclasses of Key.

35 . Uses of keys in Flutter?

  • Keys in flutter are used primarily to preserve the state of modified widgets in the widget trees.
  • It is used to reorganize and modify the collections of widgets having equivalent types and defined states.
  • It does not modify a tree that consists of only stateless widgets.

36 . What is a container class in a flutter?

A container class in flutter is a widget having the capacity to accommodate multiple child widgets and manage them efficiently through dimensions, padding, and background color.

37 . What is the difference between SizedBox and Container?

SizedBox

  • The SizedBox widget in Flutter has a specified size. 
  • In SizedBox, we cannot set the color or decoration for the widget. 
  • We can only use it with a child widget having a specific width or height.

Container

  • The Container in Flutter is a parent widget containing multiple child widgets to manage them efficiently in width, height, padding, background color, etc. 
  • If we have a widget that needs some background styling may be a color, shape, or size constraint that can be wrapped in a container widget.

38 . Is Flutter an SDK?

Yes.

39 . Differentiate between required and optional parameters in Dart

Required Parameters

Dart required parameters are the arguments that are passed to a function and the function or method requires all those parameters to complete its code block.

findVolume(int length, int breath, int height) {

 print('length = $length, breath = $breath, height = $height');

}

findVolume(10,20,30);

Optional Parameters

  • Optional parameters are defined at the end of the parameter list, after any required parameters.
  • In Flutter/Dart, there are 3 types of optional parameters: – Named – Parameters wrapped by { } – eg. getUrl(int color, [int favNum]) – Positional – Parameters wrapped by [ ]) – eg. getUrl(int color, {int favNum}) – Default – Assigning a default value to a parameter. – eg. getUrl(int color, [int favNum = 6])

40 . What is a ScopedModel / BLoC Pattern?

ScopedModel and BLoC (Business Logic Components) are common Flutter app architecture patterns to help separate business logic from UI code and using fewer Stateful Widgets.

  • Scoped Model is a third-party package that is not included in the Flutter framework. It’s a set of utilities that allow you to easily pass a data Model from a parent Widget down to its descendants. In addition, it also rebuilds all of the children that use the model when the model is updated. This library was originally extracted from the Fuchsia codebase.
  • BLoC stands for Business Logic Components. It helps in managing state and making access to data from a central place in your project. The gist of BLoC is that everything in the app should be represented as a stream of events: widgets submit events; other widgets will respond. BLoC sits in the middle, managing the conversation.

41 . Which skills are required to use Flutter?

Applicants should know that hard skills are essential for using the Flutter framework. They may mention some of the following hard skills when responding to this question:

  • SQL knowledge
  • Knowledge of the Dart programming language
  • Knowledge of the Flutter framework
  • Git knowledge
  • Android mobile development skills
  • iOS mobile development skills

42 . Is Flutter Open Source or not?

Indeed, Flutter is a free and open-source UI programming improvement unit from Google that permits engineers to construct cross-stage portable applications effortlessly.

43 . Why does a flutter app usually take a long developing time?

Whenever you first form a Flutter application, it takes significantly longer than expected since Flutter makes a gadget explicit IPA or APK record. Xcode and Gradle are utilized in this interaction to assemble a record, which as a rule requires some investment.

44 . Explain Flutter Provider.

The Flutter provider is assembled utilizing gadgets. You can involve every one of the articles in the supplier as though they were simply aspects of Flutter with the new gadget subclasses it makes. This additionally implies that the supplier isn’t cross-stage. The supplier is the easiest method for dealing with the state of the board. Fundamentally, it chips away at the idea of PUB-SUB i.e., there is one provider and a few supporters.

45 . Name two database packages mostly used in Flutter.

All things considered, the accompanying data set bundles are generally acknowledged and for the most part utilized:

Firebase database: It gives clients admittance to and command over the cloud data set. Firebase fundamentally gives a NoSQL data set to Flutter applications with the capacity to oversee information recovery and capacity through JSON convention. Information sync and fast stacking make it one of the most appropriate choices for Flutter Apps.

SQFlite database: Users can get to and change the SQLite data set utilizing this. With this information base, you have full command over your data set, inquiries, connections, and whatever you might want.

46 . How do you add interactivity to an app?

Stateful gadgets are dynamic gadgets that can be altered to answer client input (intuitiveness). The condition of a stateful gadget can be overseen by either the actual gadget or the parent gadget, or both. Tasteful states are better overseen by the gadget while utilitarian states (like tolerating client information), then, at that point, the gadget’s parent ought to oversee such.

On the other hand, a designer might involve the Gesture Detector to execute intuitiveness for a custom gadget because of activities like tapping and hauling.

47 . What are some drawbacks of Flutter and Dart?

Indeed Flutter is a great platform for mobile app development. However, a smart and competent developer must know the limits of their tools and be able to produce great results within those limits. Therefore, while Flutter interview questions and answers like this seem to be tricky at first, it is far from such.

Flutter, only just released in 2018, is still in the early stages, compared with established platforms such as native technology, Xamarin, and React Native. Therefore, it still needs time to have very rich library support.

Due to its rapid evolution, it can be a challenge for developers to maintain code because changes are being introduced and implemented constantly. On the one hand, it is indicative of growth; yet, on the other, it might make the job of developers more challenging.

Apps developed using Flutter are often storage heavy. Native Java apps can be as small as 539 KB but for Flutter, the minimum is about 4 MB. Larger app sizes result in storage space issues and poor user experience for users.

48 . What is Widget Testing?

Basically, Flutter apps are subject to three main types of tests including unit tests and integration tests, as every competent developer must be familiar with. In Flutter, there is a third: widget test. Unit tests are for individual functions and elements while integration tests are comprehensive app tests.

However, widget tests are implemented for individual widgets. As everything in Flutter is a widget, familiarity with this type of testing is inevitable for successful development. In a non-Flutter framework, ‘widget testing’ would be the equivalent of ‘component testing’.

UnitWidgetIntegration
ConfidenceLowHigherHighest
Maintenance costLowHigherHighest
DependenciesFewMoreMost
Execution speedQuickQuickSlow

Tools for testing widgets include WidgetTester (for controlling widgets within the test environment), the testWidgets() function (for running callbacks in WidgetTester), Finder classes (for searching the widget tree during testing), and Matcher constants.

49 . Explain state management

For stateful widgets, a state is the local data that the widget holds that makes it respond to dynamic context, including interactive input. For the entire app, the state consists of everything contained in the app memory while running.

There are two different types of state: app state and ephemeral state. First, the ephemeral state is the UI state. It is also called the local state because it is usually contained in a single widget. On the other hand, app state refers to a state that cuts across various parts of the app.

That is, it is not ephemeral. This includes shopping carts, user preferences, login info, and so on. Sometimes though, both the app state and the ephemeral state flow into each other that they are indistinguishable.

50 . How does one handle indentation and structure in Flutter code?

First of all, it’s important to notice that Android Studio has tools that make structuring flutter code simple.

Hence, the 2 main aspects are;

Alt + Enter/ Command + Enter

This enables developers to wrap, swap, and take away widgets during a complex hierarchy. To seek out this, direct your cursor to the widget declaration and click on the keys to access more options.

DartFMT

DartFMT is great since it formats your code to make sure a clean identification and hierarchy.

Additionally, it makes your codes easier to figure with by just twiddling with a couple of brackets.