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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>
<topic_end>
<topic_start>
widget layout
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:
<code_start>
@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, right: 10),
child: const Text('Hello'),
),
),
);