20 Flutter BLoC Interview Questions and Answers

Flutter is a mobile app SDK for building high-quality native applications on iOS and Android. The BLoC (Business Logic Component) pattern is a common way to manage state in Flutter apps. If you’re interviewing for a position that involves Flutter development, you may be asked questions about the BLoC pattern. In this article, we review some common BLoC questions and how you can answer them.

Flutter BLoC Interview Questions and Answers

Here are 20 commonly asked Flutter BLoC interview questions and answers to prepare you for your interview:

1. What is BLoC and how does it differ from other reactive frameworks?

BLoC is a reactive programming framework that uses streams to emit data in response to events. This is different from other frameworks like ReactJS, which use a more traditional event-based model. The advantage of using a stream-based approach is that it allows for a more declarative style of programming, where the developer does not need to worry about explicitly handling events.

2. How do you use the StreamProvider to manage state within a Flutter application?

The StreamProvider is a widget that provides a stream of data to its children. In order to use it, you will need to create a stream of data and then pass that stream to the StreamProvider. The StreamProvider will then handle listening to the stream and updating its children accordingly.

3. What are some common Bloc patterns that are used when developing an application in Flutter?

Some common Bloc patterns that are used when developing an application in Flutter include the following:

-The use of a StreamController to emit events that will be processed by the Bloc.
-The use of a StreamBuilder to listen for events emitted by the StreamController and rebuild the UI in response to those events.
-The use of a Sink to provide input to the Bloc from outside sources.
-The use of a StatefulWidget to manage the state of the Bloc.

4. Is there any difference between RxDart and vanilla Dart streams? If yes, then what are they?

Yes, there are some differences between RxDart and vanilla Dart streams. RxDart streams are able to work with data types that are not just simple streams of data. This allows for more complex data structures and operations to be performed on the stream. Additionally, RxDart streams can be used to perform operations such as filtering, mapping, and reducing, which are not possible with vanilla Dart streams.

5. Can you explain what sinks and streams are?

In a nutshell, streams are a way of handling asynchronous data, while sinks are a way of handling data that is being added to a stream. Streams can be thought of as a pipe, through which data flows. Sinks are like the tap that controls the flow of data into the stream.

6. Why should we not make `setState` calls inside our Bloc classes?

The main reason for this is that `setState` calls are asynchronous, which means that they can’t be guaranteed to happen in a specific order. This can lead to unexpected results, especially when multiple `setState` calls are made in quick succession. Additionally, `setState` calls can cause performance issues if they are called too frequently.

7. What’s the best way to fire events into your Bloc? Should the bloc class have access to the event stream directly or should it go through a repository?

The best way to fire events into your Bloc is through a repository. This will allow you to keep your Bloc class decoupled from the event stream, and will also make it easier to unit test your Bloc.

8. What kinds of design patterns can be leveraged with BLoC?

BLoC can be used with a variety of design patterns, but it is most commonly used with the Model-View-Controller (MVC) and Model-View-Presenter (MVP) patterns.

9. Can you give me examples of commonly used widgets such as `StreamBuilder`, `ListView`, `Form`, etc.?

Some commonly used widgets in Flutter include:

– `StreamBuilder`: A widget that builds itself based on the latest snapshot of a stream.
– `ListView`: A scrollable list of widget items.
– `Form`: A widget that collects user input in a form.
– `TextField`: A widget that allows the user to input text.

10. What are some advantages of using constructs like TextField, CheckBox, RadioButton, etc. over standard HTML elements like input, select, checkbox, radio button, etc.?

Some advantages of using these constructs over standard HTML elements are that they provide more flexibility and functionality. For example, the TextField construct can be used to create text input fields that can be used to validate data, while the CheckBox construct can be used to create checkboxes that can be used to track selections.

11. What is the easiest way to search for data in a ListView?

The easiest way to search for data in a ListView is to use the built-in search functionality provided by the ListView class. This can be accessed by calling the search() method on a ListView instance.

12. What is the purpose of a form widget?

A form widget allows you to create a form that can be used to submit information to a server. This is useful for creating contact forms, surveys, or any other type of form that needs to be submitted.

13. What are some ways to handle errors generated by async functions in your flutter code?

There are a few different ways that you can handle errors that are generated by async functions in your flutter code. One way is to use the try/catch method. Another way is to use the Future.catchError() method. Finally, you can also use the onError callback function.

14. Can you provide more details about error handling in Flutter?

There are a few different ways to handle errors in Flutter. The first is to use the try/catch method. This involves wrapping your code in a try block and then catching any errors that occur in the catch block. You can also use the assert method to check for errors. This will cause the program to stop if the condition is not met. Finally, you can use the onError callback. This will allow you to handle errors in a specific way.

15. What is the best practice for making network requests in Flutter?

The best practice for making network requests in Flutter is to use the built-in HttpClient class. This class provides a simple way to make HTTP requests and receive responses from a web server.

16. What is the purpose of FutureBuilders in Flutter?

FutureBuilders are used to build widgets that depend on data from a Future. When the Future completes, the widget is rebuilt with the new data. This is useful when you need to perform an asynchronous operation to obtain data before building a widget.

17. When would you use a StatefulWidget vs a StatelessWidget?

A StatefulWidget is used when the widget needs to be able to change its state, e.g. when a user interacts with it. A StatelessWidget is used when the widget will always look the same, e.g. a simple button.

18. Can you explain the difference between `async/await` and `Future` in Dart?

`async/await` is a way of writing asynchronous code that is easier to read and understand than using Futures. `async/await` allows you to write code as if it were synchronous, even though it is actually asynchronous. `Future` is a way of representing a value that may not be available yet. Futures are used in Dart to represent asynchronous operations.

19. What are some ways to optimize performance while building UIs in Flutter?

One way to optimize performance while building UIs in Flutter is to use the BLoC pattern. This pattern helps to separate the business logic from the UI, so that the UI can be rebuilt without having to recalculate the business logic. This can lead to a more responsive and performant UI. Other ways to optimize performance include using caching and lazy loading, as well as avoiding unnecessary rebuilds.

20. How would you define the term “widget” in the context of Flutter apps?

A widget is a piece of a Flutter app’s user interface. Widgets can be used to build up larger user interface elements, or they can be used on their own.