Stateful Widget & Stateless Widget In Flutter

Stateful Widget & Stateless Widget In Flutter :

Content Overview

  • Types of main widgets in flutter
  • What is a StatefulWidget
  • What is a StatelessWidgets
  • How to use StatelessWidgets in your application.
  • How to use StatefulWidget in flutter
  • How to create StatelessWidgets in flutter.
  • How to create StatefulWidget in flutter.
  • How to create re-rendered value in flutter
  • What is state in flutter
  • What does State mean? What is setState()?
  • How to set new value in flutter?
  • What is builder?
  • What is MaterialApp VS Material?
  • Stateful Widgets be gone: Stateful Builder.

Flutter all the UI components are known as widgets. There are two types: 1) stateful 2) stateless

Let’s dissucss how to use stateful and stateless widgets.

Stateless :
Flutter Stateless widgets also known immutable widgets.
Stateless widgets can’t change the state runtime your application, which means the widgets value not redrawm. For example: App title. It is extends StatelessWidget.

The build method can be called only once while app is in run. It is responsiblity for drawing the layout on screen. build method takes in a ‘BuildContext’ as the parameter and returns a widget.

Stateful :

  • It is mutable and change the value single or multiple times and lifetime.
  • It is the responsibility of the widget implementer to ensure that the State is promptly notified when such state changes, using the State.setState method.

What does State mean? What is setState()?
State in a flutter a group of the variable values of own widget. For example counter, text etc. This is called the State. For exmple belows the program counter, when press button + or – that time counter get the new value and disply your scree.

Flutter StatefulWidget

import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
   // This widget is the root of your application.
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
       title: 'Flutter Demo',
       theme: ThemeData(
         // This is the theme of your application.
         //
         // Try running your application with "flutter run". You'll see the
         // application has a blue toolbar. Then, without quitting the app, try
         // changing the primarySwatch below to Colors.green and then invoke
         // "hot reload" (press "r" in the console where you ran "flutter run",
         // or simply save your changes to "hot reload" in a Flutter IDE).
         // Notice that the counter didn't reset back to zero; the application
         // is not restarted.
         primarySwatch: Colors.blue,
       ),
       home: MyHomePage(title: 'StatefulWidget Example'),
     );
   }
 }
class MyHomePage extends StatefulWidget {
   MyHomePage({Key key, this.title}) : super(key: key);
// This widget is the home page of your application. It is stateful, meaning
// that it has a State object (defined below) that contains fields that affect
// how it looks.
// This class is the configuration for the state. It holds the values (in this
// case the title) provided by the parent (in this case the App widget) and
// used by the build method of the State. Fields in a Widget subclass are
// always marked "final".

final String title;
@override
   _MyHomePageState createState() => _MyHomePageState();
 }
class _MyHomePageState extends State {
   int _counter = 0;
void _incrementCounter() {
     setState(() {
       // This call to setState tells the Flutter framework that something has
       // changed in this State, which causes it to rerun the build method below
       // so that the display can reflect the updated values. If we changed
       // _counter without calling setState(), then the build method would not be
       // called again, and so nothing would appear to happen.
       _counter++;
     });
   }
@override
   Widget build(BuildContext context) {
     // This method is rerun every time setState is called, for instance as done
     // by the _incrementCounter method above.
     //
     // The Flutter framework has been optimized to make rerunning build methods
     // fast, so that you can just rebuild anything that needs updating rather
     // than having to individually change instances of widgets.
     return Scaffold(
       appBar: AppBar(
         // Here we take the value from the MyHomePage object that was created by
         // the App.build method, and use it to set our appbar title.
         title: Text(widget.title),
       ),
       body: Center(
         // Center is a layout widget. It takes a single child and positions it
         // in the middle of the parent.
         child: Column(
           // Column is also layout widget. It takes a list of children and
           // arranges them vertically. By default, it sizes itself to fit its
           // children horizontally, and tries to be as tall as its parent.
           //
           // Invoke "debug painting" (press "p" in the console, choose the
           // "Toggle Debug Paint" action from the Flutter Inspector in Android
           // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
           // to see the wireframe for each widget.
           //
           // Column has various properties to control how it sizes itself and
           // how it positions its children. Here we use mainAxisAlignment to
           // center the children vertically; the main axis here is the vertical
           // axis because Columns are vertical (the cross axis would be
           // horizontal).
           mainAxisAlignment: MainAxisAlignment.center,
           children: [
             Text(
               'You have pushed the button this many times:',
             ),
             Text(
               '$_counter',
               style: Theme.of(context).textTheme.display1,
             ),
           ],
         ),
       ),
       floatingActionButton: FloatingActionButton(
         onPressed: _incrementCounter,
         tooltip: 'Increment',
         child: Icon(Icons.add),
       ), // This trailing comma makes auto-formatting nicer for build methods.
     );
   }
 }

Builder
builder property is used to have a widget wrap all Navigator routes.

What is MaterialApp VS Material in flutter?
You should have a MaterialApp near the root of your app and then use widgets that introduce a Material instance (such as Scaffold, Appbar, Dialog etc). When you want to use Text or InkWell.

Stateful Builder :
StatefulWidget means rebuilding the entire widget tree every time setState is called.

StatefulBuilder Screenshot :

  • Allows you to inline state in your build method. (inline means your program inside the declare)
  • typedef StatefulWidgetBuilder = Widget Function(BuildContext context, StateSetter setState);
  • This is takes two parameters context and setState.
  • context is the BuildContext or the elements.
  • setState

Use : You have a medium/large widget tree and state needs.

import 'package:flutter/material.dart';
void main() => runApp(MaterialApp(home: MyApp()));
class MyApp extends StatelessWidget {
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.blue,
      body: SafeArea(
        child: Center(
          child: StatefulBuilderFlutter(),
        ),
      ),
    );
  }
}

class StatefulBuilderFlutter extends StatelessWidget {
  int counter = 0;
  @override
  Widget build(BuildContext contex) {
    return Card(
      child: SizedBox(
        width: 300.0,
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceEvenly,
              children: <Widget>[
                Padding(
                  padding: const EdgeInsets.all(18.0),
                  child: Text(
                    "Lakhani Kamlesh",
                    style:
                    TextStyle(fontSize: 16.0, fontWeight: FontWeight.w700),
                  ),
                ),
                Padding(
                  padding: const EdgeInsets.all(18.0),
                  child: Text(
                    "Age:  27",
                    style:
                    TextStyle(fontSize: 16.0, fontWeight: FontWeight.w700),
                  ),
                ),
              ],
            ),
            SizedBox(height: 12.0),
            StatefulBuilder(
              builder: (BuildContext context, StateSetter setState) {
                return Row(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    IconButton(
                      icon: Icon(Icons.minimize),
                      onPressed: () {
                        setState(() {
                          counter--;
                        });
                      },
                    ),
                    Text(
                      "$counter",
                      style: Theme.of(context).textTheme.body2,
                    ),
                    IconButton(
                      icon: Icon(Icons.add),
                      onPressed: () {
                        setState(() {
                          counter++;
                        });
                      },
                    ),
                  ],
                );
              },
            ),
          ],
        ),
      ),
    );
  }
}