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hierarchy by way of callbacks, while current state flows
“down” to the stateless widgets that do presentation.
The common parent that redirects this flow is the State.
The following slightly more complex example shows how
this works in practice:
<code_start>import 'package:flutter/material.dart';
class CounterDisplay extends StatelessWidget {
const CounterDisplay({required this.count, super.key});
final int count;
@override
Widget build(BuildContext context) {
return Text('Count: $count');
}
}
class CounterIncrementor extends StatelessWidget {
const CounterIncrementor({required this.onPressed, super.key});
final VoidCallback onPressed;
@override
Widget build(BuildContext context) {
return ElevatedButton(
onPressed: onPressed,
child: const Text('Increment'),
);
}
}
class Counter extends StatefulWidget {
const Counter({super.key});
@override
State<Counter> createState() => _CounterState();
}
class _CounterState extends State<Counter> {
int _counter = 0;
void _increment() {
setState(() {
++_counter;
});
}
@override
Widget build(BuildContext context) {
return Row(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
CounterIncrementor(onPressed: _increment),
const SizedBox(width: 16),
CounterDisplay(count: _counter),
],
);
}
}
void main() {
runApp(
const MaterialApp(
home: Scaffold(
body: Center(
child: Counter(),
),
),
),
);
}<code_end>
Notice the creation of two new stateless widgets,
cleanly separating the concerns of displaying the counter
(CounterDisplay) and changing the counter (CounterIncrementor).
Although the net result is the same as the previous example,
the separation of responsibility allows greater complexity to
be encapsulated in the individual widgets,
while maintaining simplicity in the parent.For more information, check out:<topic_end>
<topic_start>
Bringing it all together
What follows is a more complete example that brings together
these concepts: A hypothetical shopping application displays various
products offered for sale, and maintains a shopping cart for
intended purchases. Start by defining the presentation class,
ShoppingListItem:
<code_start>import 'package:flutter/material.dart';
class Product {
const Product({required this.name});
final String name;
}
typedef CartChangedCallback = Function(Product product, bool inCart);
class ShoppingListItem extends StatelessWidget {
ShoppingListItem({
required this.product,
required this.inCart,
required this.onCartChanged,
}) : super(key: ObjectKey(product));
final Product product;
final bool inCart;
final CartChangedCallback onCartChanged;
Color _getColor(BuildContext context) {
// The theme depends on the BuildContext because different
// parts of the tree can have different themes.
// The BuildContext indicates where the build is
// taking place and therefore which theme to use.
return inCart //
? Colors.black54
: Theme.of(context).primaryColor;
}
TextStyle? _getTextStyle(BuildContext context) {
if (!inCart) return null;