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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.<topic_end>
<topic_start>
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.
<code_start>Text(
'I like Flutter!',
style: TextStyle(fontWeight: FontWeight.bold),
);<code_end>
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:
<code_start>
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),
),
);
}
}<code_end>