What is widgets in Flutter?

Flutter is Google’s UI toolkit for crafting beautiful, natively compiled iOS and Android apps from a single code base. To build any application we start with widgets – The building block of flutter applications. Widgets describe what their view should look like given their current configuration and state. It includes a text widget, row widget, column widget, container widget, and many more. 

Widgets: Each element on a screen of the Flutter app is a widget. The view of the screen completely depends upon the choice and sequence of the widgets used to build the app. And the structure of the code of an app is a tree of widgets. 

Category of Widgets:

There are mainly 14 categories in which the flutter widgets are divided. They are mainly segregated on the basis of the functionality they provide in a flutter application.

1. Accessibility: These are the set of widgets that make a flutter app more easily accessible.
2. Animation and Motion: These widgets add animation to other widgets.
3. Assets, Images, and Icons: These widgets take charge of assets such as display images and show icons.
4. Async: These provide async functionality in the flutter application.
5. Basics: These are the bundle of widgets that are absolutely necessary for the development of any flutter application.
6. Cupertino: These are the ios designed widgets.
7. Input: This set of widgets provides input functionality in a flutter application.
8. Interaction Models: These widgets are here to manage touch events and route users to different views in the application.
9. Layout: This bundle of widgets helps in placing the other widgets on the screen as needed.
10. Material Components: This is a set of widgets that mainly follow material design by Google.
11. Painting and effects: This is the set of widgets that apply visual changes to their child widgets without changing their layout or shape.
12. Scrolling: This provides scrollability of to a set of other widgets that are not scrollable by default.
13. Styling: This deals with the theme, responsiveness, and sizing of the app.
14. Text: This displays text.

Types of Widgets:

There are broadly two types of widgets in the flutter: 

1. Stateless Widget
2. Stateful Widget

Example: The Layout Tree of basic app screen using Stateless Widgets: 

Dart

import 'package:flutter/material.dart';

// function to trigger build pricess
void main() => runApp(const GeeksforGeeks());

class GeeksforGeeks extends StatelessWidget {
  const GeeksforGeeks({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.lightGreen,
        appBar: AppBar(
          backgroundColor: Colors.green,
          title: const Text("GeeksforGeeks"),
        ), // AppBar
        body: Container(
          child: const Center(
            child: Text("Hello Geeks!!"),
          ), // Center
        ), // Container
      ), // Scaffold
    ); // MaterialApp
  }
}

Example:  The Layout Tree of basic app screen using Stateful Widgets. This also produces the same results as the above code.

Dart

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  // ignore: library_private_types_in_public_api
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.lightGreen,
        appBar: AppBar(
          backgroundColor: Colors.green,
          title: const Text("GeeksforGeeks"),
        ), // AppBar
        body: const Center(
          child: Text("Hello Geeks!!"),
        ), // Container
      ), // Scaffold
    );// MaterialApp
  }
}

Description of the widgets used are as follows: 

• Scaffold – Implements the basic material design visual layout structure.
• AppBar – To create a bar at the top of the screen.
• Text  To write anything on the screen.
• Container – To contain any widget.
• Center – To provide center alignment to other widgets.

Output: