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. 

What are Widgets?

Each element on the 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 apps. And the structure of the code of apps is a tree of widgets. 

Category of Widgets

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

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

Types of Widgets

There are broadly two types of widgets in the flutter: 

• Stateless Widget
• Stateful Widget

1. Stateless Widget

Stateless Widget is a type of widget which once built , then it’s properties and state can’t be changed. These widgets are immutable, once created can’t be modified.

Note: These are used for static content or UI content that don’t need a change after time.

Key Characterstics of Stateless Widgets are: Immutable , No State and Lightweight.

Examples: Display Text , Icons, Images, etc.

2. Stateful Widget

Stateful Widgets is a type of widget that can change state. It can maintain and update the appearance in the response to change in state.

Note: These are used for dynamic change in the properties and appearance over the time.

Key Characterstics of Stateful Widgets are: Mutable State , State Lifecycle and Dynamic Updates.

Examples: Buttons, Sliders, Text Fields, etc.

Implementation of Stateful and Stateless Widgets

Below is the Image with showing the Layout Tree:

Description of the widgets used are as follows: 

• Scaffold – Implements the basic material design visual layout structure.
• App-Bar – 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.

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

Dart

import 'package:flutter/material.dart';

// function to trigger build process
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
  }
}

Output: