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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;
return const TextStyle(
color: colors.black54,
decoration: TextDecoration.lineThrough,
);
}
@override
widget build(BuildContext context) {
return ListTile(
onTap: () {
onCartChanged(product, inCart);
},
leading: CircleAvatar(
backgroundColor: _getColor(context),
child: text(product.name[0]),
),
title: text(product.name, style: _getTextStyle(context)),
);
}
}
void main() {
runApp(
MaterialApp(
home: scaffold(
body: center(
child: ShoppingListItem(
product: const product(name: 'chips'),
inCart: true,
onCartChanged: (product, inCart) {},
),
),
),
),
);
}
<code_end>
the ShoppingListItem widget follows a common pattern
for stateless widgets. it stores the values it receives
in its constructor in final member variables,
which it then uses during its build() function.
for example, the inCart boolean toggles between two visual
appearances: one that uses the primary color from the current
theme, and another that uses gray.
when the user taps the list item, the widget doesn’t modify
its inCart value directly. instead, the widget calls the
onCartChanged function it received from its parent widget.
this pattern lets you store state higher in the widget
hierarchy, which causes the state to persist for longer periods of time.
in the extreme, the state stored on the widget passed to