Flutter for UIKit developers

Contents

Overview
Navigation
ViewControllers
- Equivalent of ViewController in Flutter
- Listening to lifecycle events
Layouts
Gesture detection and touch event handling
- Adding a click listener
- Handling other gestures
Themes, styles, and media
Form input
Threading & asynchronicity

iOS developers with experience using UIKit who want to write mobile apps using Flutter should review this guide. It explains how to apply existing UIKit knowledge to Flutter.

Flutter is a framework for building cross-platform applications that uses the Dart programming language. To understand some differences between programming with Dart and programming with Swift, see Learning Dart as a Swift Developer and Flutter concurrency for Swift developers.

Your iOS and UIKit knowledge and experience are highly valuable when building with Flutter. Flutter also makes a number of adaptations to app behavior when running on iOS. To learn how, see Platform adaptations.

This document can be used as a cookbook by jumping around and finding questions that are most relevant to your needs.

Overview

Views vs. Widgets

In UIKit, most of what you create in the UI is done using view objects, which are instances of the UIView class. These can act as containers for other UIView classes, which form your layout.

In Flutter, the rough equivalent to a UIView is a Widget. Widgets don’t map exactly to iOS views, but while you’re getting acquainted with how Flutter works you can think of them as “the way you declare and construct UI”.

However, these have a few differences to a UIView. To start, widgets have a different lifespan: they are immutable and only exist until they need to be changed. Whenever widgets or their state change, Flutter’s framework creates a new tree of widget instances. In comparison, a UIKit view is not recreated when it changes, but rather it’s a mutable entity that is drawn once and doesn’t redraw until it is invalidated using setNeedsDisplay().

Furthermore, unlike UIView, Flutter’s widgets are lightweight, in part due to their immutability. Because they aren’t views themselves, and aren’t directly drawing anything, but rather are a description of the UI and its semantics that get “inflated” into actual view objects under the hood.

Flutter includes the Material Components library. These are widgets that implement the Material Design guidelines. Material Design is a flexible design system optimized for all platforms, including iOS.

But Flutter is flexible and expressive enough to implement any design language. On iOS, you can use the Cupertino widgets to produce an interface that looks like Apple’s iOS design language.

Updating Widgets

To update your views in UIKit, you directly mutate them. In Flutter, widgets are immutable and not updated directly. Instead, you have to manipulate the widget’s state.

This is where the concept of Stateful vs Stateless widgets comes in. A StatelessWidget is just what it sounds like—a widget with no state attached.

StatelessWidgets are useful when the part of the user interface you are describing does not depend on anything other than the initial configuration information in the widget.

For example, with UIKit, this is similar to placing a UIImageView with your logo as the image. If the logo is not changing during runtime, use a StatelessWidget in Flutter.

If you want to dynamically change the UI based on data received after making an HTTP call, use a StatefulWidget. After the HTTP call has completed, tell the Flutter framework that the widget’s State is updated, so it can update the UI.

The important difference between stateless and stateful widgets is that StatefulWidgets have a State object that stores state data and carries it over across tree rebuilds, so it’s not lost.

If you are in doubt, remember this rule: if a widget changes outside of the build method (because of runtime user interactions, for example), it’s stateful. If the widget never changes, once built, it’s stateless. However, even if a widget is stateful, the containing parent widget can still be stateless if it isn’t itself reacting to those changes (or other inputs).

The following example shows how to use a StatelessWidget. A commonStatelessWidget is the Text widget. If you look at the implementation of the Text widget, you’ll find it subclasses StatelessWidget.

Text(
  'I like Flutter!',
  style: TextStyle(fontWeight: FontWeight.bold),
);

If you look at the code above, you might notice that the Text widget carries no explicit state with it. It renders what is passed in its constructors and nothing more.

But, what if you want to make “I Like Flutter” change dynamically, for example when clicking a FloatingActionButton?

To achieve this, wrap the Text widget in a StatefulWidget and update it when the user clicks the button.

For example:


class SampleApp extends StatelessWidget {
  // This widget is the root of your application.
  const SampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  // Default placeholder text
  String textToShow = 'I Like Flutter';

  void _updateText() {
    setState(() {
      // Update the text
      textToShow = 'Flutter is Awesome!';
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Sample App')),
      body: Center(child: Text(textToShow)),
      floatingActionButton: FloatingActionButton(
        onPressed: _updateText,
        tooltip: 'Update Text',
        child: const Icon(Icons.update),
      ),
    );
  }
}

In UIKit, you might use a Storyboard file to organize your views and set constraints, or you might set your constraints programmatically in your view controllers. In Flutter, declare your layout in code by composing a widget tree.

The following example shows how to display a simple widget with padding:

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(title: const Text('Sample App')),
    body: Center(
      child: CupertinoButton(
        onPressed: () {},
        padding: const EdgeInsets.only(left: 10.0, right: 10.0),
        child: const Text('Hello'),
      ),
    ),
  );
}

You can add padding to any widget, which mimics the functionality of constraints in iOS.

You can view the layouts that Flutter has to offer in the widget catalog.

Removing Widgets

In UIKit, you call addSubview() on the parent, or removeFromSuperview() on a child view to dynamically add or remove child views. In Flutter, because widgets are immutable, there is no direct equivalent to addSubview(). Instead, you can pass a function to the parent that returns a widget, and control that child’s creation with a boolean flag.

The following example shows how to toggle between two widgets when the user clicks the FloatingActionButton:

class SampleApp extends StatelessWidget {
  // This widget is the root of your application.
  const SampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  // Default value for toggle.
  bool toggle = true;

  void _toggle() {
    setState(() {
      toggle = !toggle;
    });
  }

  Widget _getToggleChild() {
    if (toggle) {
      return const Text('Toggle One');
    }

    return CupertinoButton(
      onPressed: () {},
      child: const Text('Toggle Two'),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: Center(
        child: _getToggleChild(),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _toggle,
        tooltip: 'Update Text',
        child: const Icon(Icons.update),
      ),
    );
  }
}

Animations

In UIKit, you create an animation by calling the animate(withDuration:animations:) method on a view. In Flutter, use the animation library to wrap widgets inside an animated widget.

In Flutter, use an AnimationController, which is an Animation<double> that can pause, seek, stop, and reverse the animation. It requires a Ticker that signals when vsync happens and produces a linear interpolation between 0 and 1 on each frame while it’s running. You then create one or more Animations and attach them to the controller.

For example, you might use CurvedAnimation to implement an animation along an interpolated curve. In this sense, the controller is the “master” source of the animation progress and the CurvedAnimation computes the curve that replaces the controller’s default linear motion. Like widgets, animations in Flutter work with composition.

When building the widget tree you assign the Animation to an animated property of a widget, such as the opacity of a FadeTransition, and tell the controller to start the animation.

The following example shows how to write a FadeTransition that fades the widget into a logo when you press the FloatingActionButton:

import 'package:flutter/material.dart';

class SampleApp extends StatelessWidget {
  // This widget is the root of your application.
  const SampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Fade Demo',
      home: MyFadeTest(title: 'Fade Demo'),
    );
  }
}

class MyFadeTest extends StatefulWidget {
  const MyFadeTest({super.key, required this.title});

  final String title;

  @override
  State<MyFadeTest> createState() => _MyFadeTest();
}

class _MyFadeTest extends State<MyFadeTest>
    with SingleTickerProviderStateMixin {
  late AnimationController controller;
  late CurvedAnimation curve;

  @override
  void initState() {
    super.initState();
    controller = AnimationController(
      duration: const Duration(milliseconds: 2000),
      vsync: this,
    );
    curve = CurvedAnimation(
      parent: controller,
      curve: Curves.easeIn,
    );
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      body: Center(
        child: FadeTransition(
          opacity: curve,
          child: const FlutterLogo(size: 100.0),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          controller.forward();
        },
        tooltip: 'Fade',
        child: const Icon(Icons.brush),
      ),
    );
  }
}

For more information, see Animation & Motion widgets, the Animations tutorial, and the Animations overview.

Drawing on the screen

In UIKit, you use CoreGraphics to draw lines and shapes to the screen. Flutter has a different API based on the Canvas class, with two other classes that help you draw: CustomPaint and CustomPainter, the latter of which implements your algorithm to draw to the canvas.

To learn how to implement a signature painter in Flutter, see Collin’s answer on StackOverflow.

import 'package:flutter/material.dart';

void main() => runApp(const MaterialApp(home: DemoApp()));

class DemoApp extends StatelessWidget {
  const DemoApp({super.key});

  @override
  Widget build(BuildContext context) => const Scaffold(body: Signature());
}

class Signature extends StatefulWidget {
  const Signature({super.key});

  @override
  State<Signature> createState() => SignatureState();
}

class SignatureState extends State<Signature> {
  List<Offset?> _points = <Offset?>[];
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onPanUpdate: (details) {
        setState(() {
          RenderBox? referenceBox = context.findRenderObject() as RenderBox;
          Offset localPosition =
              referenceBox.globalToLocal(details.globalPosition);
          _points = List.from(_points)..add(localPosition);
        });
      },
      onPanEnd: (details) => _points.add(null),
      child:
          CustomPaint(
        painter: SignaturePainter(_points),
        size: Size.infinite,
      ),
    );
  }
}

class SignaturePainter extends CustomPainter {
  SignaturePainter(this.points);

  final List<Offset?> points;

  @override
  void paint(Canvas canvas, Size size) {
    final Paint paint = Paint()
      ..color = Colors.black
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 5.0;
    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null) {
        canvas.drawLine(points[i]!, points[i + 1]!, paint);
      }
    }
  }

  @override
  bool shouldRepaint(SignaturePainter oldDelegate) =>
      oldDelegate.points != points;
}

In UIKit, everything has .opacity or .alpha. In Flutter, most of the time you need to wrap a widget in an Opacity widget to accomplish this.

Custom Widgets

In UIKit, you typically subclass UIView, or use a pre-existing view, to override and implement methods that achieve the desired behavior. In Flutter, build a custom widget by composing smaller widgets (instead of extending them).

For example, how do you build a CustomButton that takes a label in the constructor? Create a CustomButton that composes a ElevatedButton with a label, rather than by extending ElevatedButton:

class CustomButton extends StatelessWidget {
  const CustomButton(this.label, {super.key});

  final String label;

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: () {},
      child: Text(label),
    );
  }
}

Then use CustomButton, just as you’d use any other Flutter widget:

@override
Widget build(BuildContext context) {
  return const Center(
    child: CustomButton('Hello'),
  );
}

This section of the document discusses navigation between pages of an app, the push and pop mechanism, and more.

Navigating between pages

In UIKit, to travel between view controllers, you can use a UINavigationController that manages the stack of view controllers to display.

Flutter has a similar implementation, using a Navigator and Routes. A Route is an abstraction for a “screen” or “page” of an app, and a Navigator is a widget that manages routes. A route roughly maps to a UIViewController. The navigator works in a similar way to the iOS UINavigationController, in that it can push() and pop() routes depending on whether you want to navigate to, or back from, a view.

To navigate between pages, you have a couple options:

Specify a Map of route names.
Directly navigate to a route.

The following example builds a Map.

void main() {
  runApp(
    CupertinoApp(
      home: const MyAppHome(), // becomes the route named '/'
      routes: <String, WidgetBuilder>{
        '/a': (context) => const MyPage(title: 'page A'),
        '/b': (context) => const MyPage(title: 'page B'),
        '/c': (context) => const MyPage(title: 'page C'),
      },
    ),
  );
}

Navigate to a route by pushing its name to the Navigator.

Navigator.of(context).pushNamed('/b');

The Navigator class handles routing in Flutter and is used to get a result back from a route that you have pushed on the stack. This is done by awaiting on the Future returned by push().

For example, to start a location route that lets the user select their location, you might do the following:

Object? coordinates = await Navigator.of(context).pushNamed('/location');

And then, inside your location route, once the user has selected their location, pop() the stack with the result:

Navigator.of(context).pop({'lat': 43.821757, 'long': -79.226392});

Navigating to another app

In UIKit, to send the user to another application, you use a specific URL scheme. For the system level apps, the scheme depends on the app. To implement this functionality in Flutter, create a native platform integration, or use an existing plugin, such as url_launcher.

Manually pop back

Calling SystemNavigator.pop() from your Dart code invokes the following iOS code:

UIViewController* viewController = [UIApplication sharedApplication].keyWindow.rootViewController;
if ([viewController isKindOfClass:[UINavigationController class]]) {
  [((UINavigationController*)viewController) popViewControllerAnimated:NO];
}

If that doesn’t do what you want, you can create your own platform channel to invoke arbitrary iOS code.

Handling localization

Unlike iOS, which has the Localizable.strings file, Flutter doesn’t currently have a dedicated system for handling strings. At the moment, the best practice is to declare your copy text in a class as static fields and access them from there. For example:

class Strings {
  static const String welcomeMessage = 'Welcome To Flutter';
}

You can access your strings as such:

Text(Strings.welcomeMessage);

By default, Flutter only supports US English for its strings. If you need to add support for other languages, include the flutter_localizations package. You might also need to add Dart’s intl package to use i10n machinery, such as date/time formatting.

dependencies:
  flutter_localizations:
    sdk: flutter
  intl: '^0.17.0'

To use the flutter_localizations package, specify the localizationsDelegates and supportedLocales on the app widget:

import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

class MyWidget extends StatelessWidget {
  const MyWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      localizationsDelegates: <LocalizationsDelegate<dynamic>>[
        // Add app-specific localization delegate[s] here
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: <Locale>[
        Locale('en', 'US'), // English
        Locale('he', 'IL'), // Hebrew
        // ... other locales the app supports
      ],
    );
  }
}

The delegates contain the actual localized values, while the supportedLocales defines which locales the app supports. The above example uses a MaterialApp, so it has both a GlobalWidgetsLocalizations for the base widgets localized values, and a MaterialWidgetsLocalizations for the Material widgets localizations. If you use WidgetsApp for your app, you don’t need the latter. Note that these two delegates contain “default” values, but you’ll need to provide one or more delegates for your own app’s localizable copy, if you want those to be localized too.

When initialized, the WidgetsApp (or MaterialApp) creates a Localizations widget for you, with the delegates you specify. The current locale for the device is always accessible from the Localizations widget from the current context (in the form of a Locale object), or using the Window.locale.

To access localized resources, use the Localizations.of() method to access a specific localizations class that is provided by a given delegate. Use the intl_translation package to extract translatable copy to arb files for translating, and importing them back into the app for using them with intl.

For further details on internationalization and localization in Flutter, see the internationalization guide, which has sample code with and without the intl package.

Managing dependencies

In iOS, you add dependencies with CocoaPods by adding to your Podfile. Flutter uses Dart’s build system and the Pub package manager to handle dependencies. The tools delegate the building of the native Android and iOS wrapper apps to the respective build systems.

While there is a Podfile in the iOS folder in your Flutter project, only use this if you are adding native dependencies needed for per-platform integration. In general, use pubspec.yaml to declare external dependencies in Flutter. A good place to find great packages for Flutter is on pub.dev.

ViewControllers

This section of the document discusses the equivalent of ViewController in Flutter and how to listen to lifecycle events.

Equivalent of ViewController in Flutter

In UIKit, a ViewController represents a portion of user interface, most commonly used for a screen or section. These are composed together to build complex user interfaces, and help scale your application’s UI. In Flutter, this job falls to Widgets. As mentioned in the Navigation section, screens in Flutter are represented by Widgets since “everything is a widget!” Use a Navigator to move between different Routes that represent different screens or pages, or maybe different states or renderings of the same data.

Listening to lifecycle events

In UIKit, you can override methods to the ViewController to capture lifecycle methods for the view itself, or register lifecycle callbacks in the AppDelegate. In Flutter, you have neither concept, but you can instead listen to lifecycle events by hooking into the WidgetsBinding observer and listening to the didChangeAppLifecycleState() change event.

The observable lifecycle events are:

inactive: The application is in an inactive state and is not receiving user input. This event only works on iOS, as there is no equivalent event on Android.
paused: The application is not currently visible to the user, is not responding to user input, but is running in the background.
resumed: The application is visible and responding to user input.
suspending: The application is suspended momentarily. The iOS platform has no equivalent event.

For more details on the meaning of these states, see AppLifecycleState documentation.

Layouts

This section discusses different layouts in Flutter and how they compare with UIKit.

Displaying a list view

In UIKit, you might show a list in either a UITableView or a UICollectionView. In Flutter, you have a similar implementation using a ListView. In UIKit, these views have delegate methods for deciding the number of rows, the cell for each index path, and the size of the cells.

Due to Flutter’s immutable widget pattern, you pass a list of widgets to your ListView, and Flutter takes care of making sure that scrolling is fast and smooth.

import 'package:flutter/material.dart';

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  List<Widget> _getListData() {
    final List<Widget> widgets = [];
    for (int i = 0; i < 100; i++) {
      widgets.add(Padding(
        padding: const EdgeInsets.all(10.0),
        child: Text('Row $i'),
      ));
    }
    return widgets;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: ListView(children: _getListData()),
    );
  }
}

Detecting what was clicked

In UIKit, you implement the delegate method, tableView:didSelectRowAtIndexPath:. In Flutter, use the touch handling provided by the passed-in widgets.

import 'dart:developer' as developer;
import 'package:flutter/material.dart';

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  List<Widget> _getListData() {
    List<Widget> widgets = [];
    for (int i = 0; i < 100; i++) {
      widgets.add(
        GestureDetector(
          onTap: () {
            developer.log('row tapped');
          },
          child: Padding(
            padding: const EdgeInsets.all(10.0),
            child: Text('Row $i'),
          ),
        ),
      );
    }
    return widgets;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: ListView(children: _getListData()),
    );
  }
}

Dynamically updating ListView

In UIKit, you update the data for the list view, and notify the table or collection view using the reloadData method.

In Flutter, if you update the list of widgets inside a setState(), you quickly see that your data doesn’t change visually. This is because when setState() is called, the Flutter rendering engine looks at the widget tree to see if anything has changed. When it gets to your ListView, it performs an == check, and determines that the two ListViews are the same. Nothing has changed, so no update is required.

For a simple way to update your ListView, create a new List inside of setState(), and copy the data from the old list to the new list. While this approach is simple, it is not recommended for large data sets, as shown in the next example.

import 'dart:developer' as developer;

import 'package:flutter/material.dart';

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  List<Widget> widgets = <Widget>[];

  @override
  void initState() {
    super.initState();
    for (int i = 0; i < 100; i++) {
      widgets.add(getRow(i));
    }
  }

  Widget getRow(int i) {
    return GestureDetector(
      onTap: () {
        setState(() {
          widgets = List.from(widgets);
          widgets.add(getRow(widgets.length));
          developer.log('row $i');
        });
      },
      child: Padding(
        padding: const EdgeInsets.all(10.0),
        child: Text('Row $i'),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: ListView(children: widgets),
    );
  }
}

The recommended, efficient, and effective way to build a list uses a ListView.Builder. This method is great when you have a dynamic list or a list with very large amounts of data.

import 'dart:developer' as developer;

import 'package:flutter/material.dart';

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  List<Widget> widgets = [];

  @override
  void initState() {
    super.initState();
    for (int i = 0; i < 100; i++) {
      widgets.add(getRow(i));
    }
  }

  Widget getRow(int i) {
    return GestureDetector(
      onTap: () {
        setState(() {
          widgets.add(getRow(widgets.length));
          developer.log('row $i');
        });
      },
      child: Padding(
        padding: const EdgeInsets.all(10.0),
        child: Text('Row $i'),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: ListView.builder(
        itemCount: widgets.length,
        itemBuilder: (context, position) {
          return getRow(position);
        },
      ),
    );
  }
}

Instead of creating a ListView, create a ListView.builder that takes two key parameters: the initial length of the list, and an ItemBuilder function.

The ItemBuilder function is similar to the cellForItemAt delegate method in an iOS table or collection view, as it takes a position, and returns the cell you want rendered at that position.

Finally, but most importantly, notice that the onTap() function doesn’t recreate the list anymore, but instead .adds to it.

Creating a scroll view

In UIKit, you wrap your views in a ScrollView that allows a user to scroll your content if needed.

In Flutter the easiest way to do this is using the ListView widget. This acts as both a ScrollView and an iOS TableView, as you can layout widgets in a vertical format.

@override
Widget build(BuildContext context) {
  return ListView(
    children: const <Widget>[
      Text('Row One'),
      Text('Row Two'),
      Text('Row Three'),
      Text('Row Four'),
    ],
  );
}

For more detailed docs on how to lay out widgets in Flutter, see the layout tutorial.

Gesture detection and touch event handling

This section discusses how to detect gestures and handle different events in Flutter, and how they compare with UIKit.

Adding a click listener

In UIKit, you attach a GestureRecognizer to a view to handle click events. In Flutter, there are two ways of adding touch listeners:

If the widget supports event detection, pass a function to it, and handle the event in the function. For example, the ElevatedButton widget has an onPressed parameter:

@override
Widget build(BuildContext context) {
  return ElevatedButton(
    onPressed: () {
      developer.log('click');
    },
    child: const Text('Button'),
  );
}

If the Widget doesn’t support event detection, wrap the widget in a GestureDetector and pass a function to the onTap parameter.

class SampleTapApp extends StatelessWidget {
  const SampleTapApp({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: GestureDetector(
          onTap: () {
            developer.log('tap');
          },
          child: const FlutterLogo(
            size: 200.0,
          ),
        ),
      ),
    );
  }
}

Handling other gestures

Using GestureDetector you can listen to a wide range of gestures such as:

Tapping

onTapDown

A pointer that might cause a tap has contacted the screen at a particular location.

onTapUp

A pointer that triggers a tap has stopped contacting the screen at a particular location.

onTap

A tap has occurred.

onTapCancel

The pointer that previously triggered the onTapDown won’t cause a tap.
Double tapping

onDoubleTap

The user tapped the screen at the same location twice in quick succession.
Long pressing

onLongPress

A pointer has remained in contact with the screen at the same location for a long period of time.
Vertical dragging

onVerticalDragStart

A pointer has contacted the screen and might begin to move vertically.

onVerticalDragUpdate

A pointer in contact with the screen has moved further in the vertical direction.

onVerticalDragEnd

A pointer that was previously in contact with the screen and moving vertically is no longer in contact with the screen and was moving at a specific velocity when it stopped contacting the screen.
Horizontal dragging

onHorizontalDragStart

A pointer has contacted the screen and might begin to move horizontally.

onHorizontalDragUpdate

A pointer in contact with the screen has moved further in the horizontal direction.

onHorizontalDragEnd

A pointer that was previously in contact with the screen and moving horizontally is no longer in contact with the screen.

The following example shows a GestureDetector that rotates the Flutter logo on a double tap:

class SampleApp extends StatefulWidget {
  const SampleApp({super.key});

  @override
  State<SampleApp> createState() => _SampleAppState();
}

class _SampleAppState extends State<SampleApp>
    with SingleTickerProviderStateMixin {
  late AnimationController controller;
  late CurvedAnimation curve;

  @override
  void initState() {
    super.initState();
    controller = AnimationController(
      vsync: this,
      duration: const Duration(milliseconds: 2000),
    );
    curve = CurvedAnimation(
      parent: controller,
      curve: Curves.easeIn,
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: GestureDetector(
          onDoubleTap: () {
            if (controller.isCompleted) {
              controller.reverse();
            } else {
              controller.forward();
            }
          },
          child: RotationTransition(
            turns: curve,
            child: const FlutterLogo(
              size: 200.0,
            ),
          ),
        ),
      ),
    );
  }
}

Themes, styles, and media

Flutter applications are easy to style; you can switch between light and dark themes, change the style of your text and UI components, and more. This section covers aspects of styling your Flutter apps and compares how you might do the same in UIKit.

Using a theme

Out of the box, Flutter comes with a beautiful implementation of Material Design, which takes care of a lot of styling and theming needs that you would typically do.

To take full advantage of Material Components in your app, declare a top-level widget, MaterialApp, as the entry point to your application. MaterialApp is a convenience widget that wraps a number of widgets that are commonly required for applications implementing Material Design. It builds upon a WidgetsApp by adding Material specific functionality.

But Flutter is flexible and expressive enough to implement any design language. On iOS, you can use the Cupertino library to produce an interface that adheres to the Human Interface Guidelines. For the full set of these widgets, see the Cupertino widgets gallery.

You can also use a WidgetsApp as your app widget, which provides some of the same functionality, but is not as rich as MaterialApp.

To customize the colors and styles of any child components, pass a ThemeData object to the MaterialApp widget. For example, in the code below, the primary swatch is set to blue and divider color is grey.

import 'package:flutter/material.dart';

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sample App',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        dividerColor: Colors.grey,
      ),
      home: const SampleAppPage(),
    );
  }
}

Using custom fonts

In UIKit, you import any ttf font files into your project and create a reference in the info.plist file. In Flutter, place the font file in a folder and reference it in the pubspec.yaml file, similar to how you import images.

fonts:
  - family: MyCustomFont
    fonts:
      - asset: fonts/MyCustomFont.ttf
      - style: italic

Then assign the font to your Text widget:

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: const Text('Sample App'),
    ),
    body: const Center(
      child: Text(
        'This is a custom font text',
        style: TextStyle(fontFamily: 'MyCustomFont'),
      ),
    ),
  );
}

Styling text

Along with fonts, you can customize other styling elements on a Text widget. The style parameter of a Text widget takes a TextStyle object, where you can customize many parameters, such as:

color
decoration
decorationColor
decorationStyle
fontFamily
fontSize
fontStyle
fontWeight
hashCode
height
inherit
letterSpacing
textBaseline
wordSpacing

Bundling images in apps

While iOS treats images and assets as distinct items, Flutter apps have only assets. Resources that are placed in the Images.xcasset folder on iOS, are placed in an assets’ folder for Flutter. As with iOS, assets are any type of file, not just images. For example, you might have a JSON file located in the my-assets folder:

my-assets/data.json

Declare the asset in the pubspec.yaml file:

assets:
 - my-assets/data.json

And then access it from code using an AssetBundle:

import 'dart:async' show Future;
import 'package:flutter/services.dart' show rootBundle;

Future<String> loadAsset() async {
  return await rootBundle.loadString('my-assets/data.json');
}

For images, Flutter follows a simple density-based format like iOS. Image assets might be 1.0x, 2.0x, 3.0x, or any other multiplier. Flutter’s devicePixelRatio expresses the ratio of physical pixels in a single logical pixel.

Assets are located in any arbitrary folder— Flutter has no predefined folder structure. You declare the assets (with location) in the pubspec.yaml file, and Flutter picks them up.

For example, to add an image called my_icon.png to your Flutter project, you might decide to store it in a folder arbitrarily called images. Place the base image (1.0x) in the images folder, and the other variants in sub-folders named after the appropriate ratio multiplier:

images/my_icon.png       // Base: 1.0x image
images/2.0x/my_icon.png  // 2.0x image
images/3.0x/my_icon.png  // 3.0x image

Next, declare these images in the pubspec.yaml file:

assets:
 - images/my_icon.png

You can now access your images using AssetImage:

AssetImage('images/a_dot_burr.jpeg')

or directly in an Image widget:

@override
Widget build(BuildContext context) {
  return Image.asset('images/my_image.png');
}

For more details, see Adding Assets and Images in Flutter.

Form input

This section discusses how to use forms in Flutter and how they compare with UIKit.

Retrieving user input

Given how Flutter uses immutable widgets with a separate state, you might be wondering how user input fits into the picture. In UIKit, you usually query the widgets for their current values when it’s time to submit the user input, or action on it. How does that work in Flutter?

In practice forms are handled, like everything in Flutter, by specialized widgets. If you have a TextField or a TextFormField, you can supply a TextEditingController to retrieve user input:

class _MyFormState extends State<MyForm> {
  // Create a text controller and use it to retrieve the current value.
  // of the TextField!
  final myController = TextEditingController();

  @override
  void dispose() {
    // Clean up the controller when disposing of the Widget.
    myController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Retrieve Text Input')),
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: TextField(controller: myController),
      ),
      floatingActionButton: FloatingActionButton(
        // When the user presses the button, show an alert dialog with the
        // text the user has typed into our text field.
        onPressed: () {
          showDialog(
            context: context,
            builder: (context) {
              return AlertDialog(
                // Retrieve the text the user has typed in using our
                // TextEditingController.
                content: Text(myController.text),
              );
            },
          );
        },
        tooltip: 'Show me the value!',
        child: const Icon(Icons.text_fields),
      ),
    );
  }
}

You can find more information and the full code listing in Retrieve the value of a text field, from the Flutter cookbook.

Placeholder in a text field

In Flutter, you can easily show a “hint” or a placeholder text for your field by adding an InputDecoration object to the decoration constructor parameter for the Text widget:

Center(
  child: TextField(
    decoration: InputDecoration(hintText: 'This is a hint'),
  ),
)

Showing validation errors

Just as you would with a “hint”, pass an InputDecoration object to the decoration constructor for the Text widget.

However, you don’t want to start off by showing an error. Instead, when the user has entered invalid data, update the state, and pass a new InputDecoration object.

import 'package:flutter/material.dart';

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  String? _errorText;

  bool isEmail(String em) {
    String emailRegexp =
        r'^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|'
        r'(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|'
        r'(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$';

    RegExp regExp = RegExp(emailRegexp);

    return regExp.hasMatch(em);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: Center(
        child: TextField(
          onSubmitted: (text) {
            setState(() {
              if (!isEmail(text)) {
                _errorText = 'Error: This is not an email';
              } else {
                _errorText = null;
              }
            });
          },
          decoration: InputDecoration(
            hintText: 'This is a hint',
            errorText: _errorText,
          ),
        ),
      ),
    );
  }
}

Threading & asynchronicity

This section discusses concurrency in Flutter and how it compares with UIKit.

Writing asynchronous code

Dart has a single-threaded execution model, with support for Isolates (a way to run Dart code on another thread), an event loop, and asynchronous programming. Unless you spawn an Isolate, your Dart code runs in the main UI thread and is driven by an event loop. Flutter’s event loop is equivalent to the iOS main loop—that is, the Looper that is attached to the main thread.

Dart’s single-threaded model doesn’t mean you are required to run everything as a blocking operation that causes the UI to freeze. Instead, use the asynchronous facilities that the Dart language provides, such as async/await, to perform asynchronous work.

For example, you can run network code without causing the UI to hang by using async/await and letting Dart do the heavy lifting:

Future<void> loadData() async {
  final Uri dataURL = Uri.parse('https://jsonplaceholder.typicode.com/posts');
  final http.Response response = await http.get(dataURL);
  setState(() {
    data = jsonDecode(response.body);
  });
}

Once the awaited network call is done, update the UI by calling setState(), which triggers a rebuild of the widget sub-tree and updates the data.

The following example loads data asynchronously and displays it in a ListView:

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  List<Map<String, dynamic>> data = <Map<String, dynamic>>[];

  @override
  void initState() {
    super.initState();
    loadData();
  }

  Future<void> loadData() async {
    final Uri dataURL = Uri.parse('https://jsonplaceholder.typicode.com/posts');
    final http.Response response = await http.get(dataURL);
    setState(() {
      data = jsonDecode(response.body);
    });
  }

  Widget getRow(int index) {
    return Padding(
      padding: const EdgeInsets.all(10.0),
      child: Text('Row ${data[index]['title']}'),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: ListView.builder(
        itemCount: data.length,
        itemBuilder: (context, index) {
          return getRow(index);
        },
      ),
    );
  }
}

Refer to the next section for more information on doing work in the background, and how Flutter differs from iOS.

Moving to the background thread

Since Flutter is single threaded and runs an event loop (like Node.js), you don’t have to worry about thread management or spawning background threads. If you’re doing I/O-bound work, such as disk access or a network call, then you can safely use async/await and you’re done. If, on the other hand, you need to do computationally intensive work that keeps the CPU busy, you want to move it to an Isolate to avoid blocking the event loop.

For I/O-bound work, declare the function as an async function, and await on long-running tasks inside the function:

Future<void> loadData() async {
  final Uri dataURL = Uri.parse('https://jsonplaceholder.typicode.com/posts');
  final http.Response response = await http.get(dataURL);
  setState(() {
    data = jsonDecode(response.body);
  });
}

This is how you typically do network or database calls, which are both I/O operations.

However, there are times when you might be processing a large amount of data and your UI hangs. In Flutter, use Isolates to take advantage of multiple CPU cores to do long-running or computationally intensive tasks.

Isolates are separate execution threads that do not share any memory with the main execution memory heap. This means you can’t access variables from the main thread, or update your UI by calling setState(). Isolates are true to their name, and cannot share memory (in the form of static fields, for example).

The following example shows, in a simple isolate, how to share data back to the main thread to update the UI.

Future<void> loadData() async {
  final ReceivePort receivePort = ReceivePort();
  await Isolate.spawn(dataLoader, receivePort.sendPort);

  // The 'echo' isolate sends its SendPort as the first message.
  final SendPort sendPort = await receivePort.first as SendPort;

  final List<Map<String, dynamic>> msg = await sendReceive(
    sendPort,
    'https://jsonplaceholder.typicode.com/posts',
  );

  setState(() {
    data = msg;
  });
}

// The entry point for the isolate.
static Future<void> dataLoader(SendPort sendPort) async {
  // Open the ReceivePort for incoming messages.
  final ReceivePort port = ReceivePort();

  // Notify any other isolates what port this isolate listens to.
  sendPort.send(port.sendPort);

  await for (final dynamic msg in port) {
    final String url = msg[0] as String;
    final SendPort replyTo = msg[1] as SendPort;

    final Uri dataURL = Uri.parse(url);
    final http.Response response = await http.get(dataURL);
    // Lots of JSON to parse
    replyTo.send(jsonDecode(response.body) as List<Map<String, dynamic>>);
  }
}

Future<List<Map<String, dynamic>>> sendReceive(SendPort port, String msg) {
  final ReceivePort response = ReceivePort();
  port.send(<dynamic>[msg, response.sendPort]);
  return response.first as Future<List<Map<String, dynamic>>>;
}

Here, dataLoader() is the Isolate that runs in its own separate execution thread. In the isolate, you can perform more CPU intensive processing (parsing a big JSON, for example), or perform computationally intensive math, such as encryption or signal processing.

You can run the full example below:

import 'dart:async';
import 'dart:convert';
import 'dart:isolate';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  List<Map<String, dynamic>> data = <Map<String, dynamic>>[];

  @override
  void initState() {
    super.initState();
    loadData();
  }

  bool get showLoadingDialog => data.isEmpty;

  Future<void> loadData() async {
    final ReceivePort receivePort = ReceivePort();
    await Isolate.spawn(dataLoader, receivePort.sendPort);

    // The 'echo' isolate sends its SendPort as the first message.
    final SendPort sendPort = await receivePort.first as SendPort;

    final List<Map<String, dynamic>> msg = await sendReceive(
      sendPort,
      'https://jsonplaceholder.typicode.com/posts',
    );

    setState(() {
      data = msg;
    });
  }

  // The entry point for the isolate.
  static Future<void> dataLoader(SendPort sendPort) async {
    // Open the ReceivePort for incoming messages.
    final ReceivePort port = ReceivePort();

    // Notify any other isolates what port this isolate listens to.
    sendPort.send(port.sendPort);

    await for (final dynamic msg in port) {
      final String url = msg[0] as String;
      final SendPort replyTo = msg[1] as SendPort;

      final Uri dataURL = Uri.parse(url);
      final http.Response response = await http.get(dataURL);
      // Lots of JSON to parse
      replyTo.send(jsonDecode(response.body) as List<Map<String, dynamic>>);
    }
  }

  Future<List<Map<String, dynamic>>> sendReceive(SendPort port, String msg) {
    final ReceivePort response = ReceivePort();
    port.send(<dynamic>[msg, response.sendPort]);
    return response.first as Future<List<Map<String, dynamic>>>;
  }

  Widget getBody() {
    bool showLoadingDialog = data.isEmpty;

    if (showLoadingDialog) {
      return getProgressDialog();
    } else {
      return getListView();
    }
  }

  Widget getProgressDialog() {
    return const Center(child: CircularProgressIndicator());
  }

  ListView getListView() {
    return ListView.builder(
      itemCount: data.length,
      itemBuilder: (context, position) {
        return getRow(position);
      },
    );
  }

  Widget getRow(int i) {
    return Padding(
      padding: const EdgeInsets.all(10.0),
      child: Text("Row ${data[i]["title"]}"),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: getBody(),
    );
  }
}

Making network requests

Making a network call in Flutter is easy when you use the popular http package. This abstracts away a lot of the networking that you might normally implement yourself, making it simple to make network calls.

To use the http package, add it to your dependencies in pubspec.yaml:

dependencies:
  http: ^0.13.4

To make a network call, call await on the async function http.get():

Future<void> loadData() async {
  final Uri dataURL = Uri.parse('https://jsonplaceholder.typicode.com/posts');
  final http.Response response = await http.get(dataURL);
  setState(() {
    data = jsonDecode(response.body);
  });
}

Showing the progress on long running tasks

In UIKit, you typically use a UIProgressView while executing a long-running task in the background.

In Flutter, use a ProgressIndicator widget. Show the progress programmatically by controlling when it’s rendered through a boolean flag. Tell Flutter to update its state before your long-running task starts, and hide it after it ends.

In the example below, the build function is separated into three different functions. If showLoadingDialog is true (when widgets.length == 0), then render the ProgressIndicator. Otherwise, render the ListView with the data returned from a network call.

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Sample App',
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});

  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  List<Map<String, dynamic>> data = <Map<String, dynamic>>[];

  @override
  void initState() {
    super.initState();
    loadData();
  }

  bool get showLoadingDialog => data.isEmpty;

  Future<void> loadData() async {
    final Uri dataURL = Uri.parse('https://jsonplaceholder.typicode.com/posts');
    final http.Response response = await http.get(dataURL);
    setState(() {
      data = jsonDecode(response.body);
    });
  }

  Widget getBody() {
    if (showLoadingDialog) {
      return getProgressDialog();
    }

    return getListView();
  }

  Widget getProgressDialog() {
    return const Center(child: CircularProgressIndicator());
  }

  ListView getListView() {
    return ListView.builder(
      itemCount: data.length,
      itemBuilder: (context, index) {
        return getRow(index);
      },
    );
  }

  Widget getRow(int i) {
    return Padding(
      padding: const EdgeInsets.all(10.0),
      child: Text("Row ${data[i]["title"]}"),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample App'),
      ),
      body: getBody(),
    );
  }
}