Flutter widgets are built using a modern framework that takes inspiration from React. The central idea is that you build your UI out of widgets. Widgets describe what their view should look like given their current configuration and state. When a widget’s state changes, the widget rebuilds its description, which the framework diffs against the previous description in order to determine the minimal changes needed in the underlying render tree to transition from one state to the next.
Contents
The minimal Flutter app simply calls the runApp()
function with a widget:
import 'package:flutter/material.dart';
void main() {
runApp(
Center(
child: Text(
'Hello, world!',
textDirection: TextDirection.ltr,
),
),
);
}
The runApp()
function takes the given Widget
and makes it the root of the widget tree. In this example, the widget tree consists of two widgets, the Center
widget and its child, the Text
widget. The framework forces the root widget to cover the screen, which means the text “Hello, world” ends up centered on screen. The text direction needs to be specified in this instance; when the MaterialApp
widget is used, this is taken care of for you, as demonstrated later.
When writing an app, you’ll commonly author new widgets that are subclasses of either StatelessWidget
or StatefulWidget
, depending on whether your widget manages any state. A widget’s main job is to implement a build()
function, which describes the widget in terms of other, lower-level widgets. The framework builds those widgets in turn until the process bottoms out in widgets that represent the underlying RenderObject
, which computes and describes the geometry of the widget.
Flutter comes with a suite of powerful basic widgets, of which the following are commonly used:
The Text
widget lets you create a run of styled text within your application.
These flex widgets let you create flexible layouts in both the horizontal (Row
) and vertical (Column
) directions. The design of these objects is based on the web’s flexbox layout model.
Instead of being linearly oriented (either horizontally or vertically), a Stack
widget lets you place widgets on top of each other in paint order. You can then use the Positioned
widget on children of a Stack
to position them relative to the top, right, bottom, or left edge of the stack. Stacks are based on the web’s absolute positioning layout model.
The Container
widget lets you create a rectangular visual element. A container can be decorated with a BoxDecoration
, such as a background, a border, or a shadow. A Container
can also have margins, padding, and constraints applied to its size. In addition, a Container
can be transformed in three dimensional space using a matrix.
Below are some simple widgets that combine these and other widgets:
import 'package:flutter/material.dart';
class MyAppBar extends StatelessWidget {
MyAppBar({this.title});
// Fields in a Widget subclass are always marked "final".
final Widget title;
@override
Widget build(BuildContext context) {
return Container(
height: 56.0, // in logical pixels
padding: const EdgeInsets.symmetric(horizontal: 8.0),
decoration: BoxDecoration(color: Colors.blue[500]),
// Row is a horizontal, linear layout.
child: Row(
// <Widget> is the type of items in the list.
children: <Widget>[
IconButton(
icon: Icon(Icons.menu),
tooltip: 'Navigation menu',
onPressed: null, // null disables the button
),
// Expanded expands its child to fill the available space.
Expanded(
child: title,
),
IconButton(
icon: Icon(Icons.search),
tooltip: 'Search',
onPressed: null,
),
],
),
);
}
}
class MyScaffold extends StatelessWidget {
@override
Widget build(BuildContext context) {
// Material is a conceptual piece of paper on which the UI appears.
return Material(
// Column is a vertical, linear layout.
child: Column(
children: <Widget>[
MyAppBar(
title: Text(
'Example title',
style: Theme.of(context).primaryTextTheme.title,
),
),
Expanded(
child: Center(
child: Text('Hello, world!'),
),
),
],
),
);
}
}
void main() {
runApp(MaterialApp(
title: 'My app', // used by the OS task switcher
home: MyScaffold(),
));
}
Be sure to have a uses-material-design: true
entry in the flutter
section of your pubspec.yaml
file. It allows you to use the predefined set of Material icons.
name: my_app
flutter:
uses-material-design: true
Many Material Design widgets need to be inside of a MaterialApp
to display properly, in order to inherit theme data. Therefore, run the application with a MaterialApp
.
The MyAppBar
widget creates a Container
with a height of 56 device-independent pixels with an internal padding of 8 pixels, both on the left and the right. Inside the container, MyAppBar
uses a Row
layout to organize its children. The middle child, the title
widget, is marked as Expanded
, which means it expands to fill any remaining available space that hasn’t been consumed by the other children. You can have multiple Expanded
children and determine the ratio in which they consume the available space using the flex
argument to Expanded
.
The MyScaffold
widget organizes its children in a vertical column. At the top of the column it places an instance of MyAppBar
, passing the app bar a Text
widget to use as its title. Passing widgets as arguments to other widgets is a powerful technique that lets you create generic widgets that can be reused in a wide variety of ways. Finally, MyScaffold
uses an Expanded
to fill the remaining space with its body, which consists of a centered message.
For more information, see Layouts.
Flutter provides a number of widgets that help you build apps that follow Material Design. A Material app starts with the MaterialApp
widget, which builds a number of useful widgets at the root of your app, including a Navigator
, which manages a stack of widgets identified by strings, also known as “routes”. The Navigator
lets you transition smoothly between screens of your application. Using the MaterialApp
widget is entirely optional but a good practice.
import 'package:flutter/material.dart';
void main() {
runApp(MaterialApp(
title: 'Flutter Tutorial',
home: TutorialHome(),
));
}
class TutorialHome extends StatelessWidget {
@override
Widget build(BuildContext context) {
// Scaffold is a layout for the major Material Components.
return Scaffold(
appBar: AppBar(
leading: IconButton(
icon: Icon(Icons.menu),
tooltip: 'Navigation menu',
onPressed: null,
),
title: Text('Example title'),
actions: <Widget>[
IconButton(
icon: Icon(Icons.search),
tooltip: 'Search',
onPressed: null,
),
],
),
// body is the majority of the screen.
body: Center(
child: Text('Hello, world!'),
),
floatingActionButton: FloatingActionButton(
tooltip: 'Add', // used by assistive technologies
child: Icon(Icons.add),
onPressed: null,
),
);
}
}
Now that the code has switched from MyAppBar
and MyScaffold
to the AppBar
and Scaffold
widgets, and from material.dart
, the app is starting to look at bit more Material. For example, the app bar has a shadow and the title text inherits the correct styling automatically. A floating action button is also added.
Notice that widgets are passed as arguments to other widgets. The Scaffold
widget takes a number of different widgets as named arguments, each of which are placed in the Scaffold
layout in the appropriate place. Similarly, the AppBar
widget lets you pass in widgets for the leading
widget, and the actions
of the title
widget. This pattern recurs throughout the framework and is something you might consider when designing your own widgets.
For more information, see Material Components widgets.
Most applications include some form of user interaction with the system. The first step in building an interactive application is to detect input gestures. See how that works by creating a simple button:
class MyButton extends StatelessWidget {
@override
Widget build(BuildContext context) {
return GestureDetector(
onTap: () {
print('MyButton was tapped!');
},
child: Container(
height: 36.0,
padding: const EdgeInsets.all(8.0),
margin: const EdgeInsets.symmetric(horizontal: 8.0),
decoration: BoxDecoration(
borderRadius: BorderRadius.circular(5.0),
color: Colors.lightGreen[500],
),
child: Center(
child: Text('Engage'),
),
),
);
}
}
The GestureDetector
widget doesn’t have a visual representation but instead detects gestures made by the user. When the user taps the Container
, the GestureDetector
calls its onTap()
callback, in this case printing a message to the console. You can use GestureDetector
to detect a variety of input gestures, including taps, drags, and scales.
Many widgets use a GestureDetector
to provide optional callbacks for other widgets. For example, the IconButton
, RaisedButton
, and FloatingActionButton
widgets have onPressed()
callbacks that are triggered when the user taps the widget.
For more information, see Gestures in Flutter.
So far, this page has used only stateless widgets. Stateless widgets receive arguments from their parent widget, which they store in final
member variables. When a widget is asked to build()
, it uses these stored values to derive new arguments for the widgets it creates.
In order to build more complex experiences—for example, to react in more interesting ways to user input—applications typically carry some state. Flutter uses StatefulWidgets
to capture this idea. StatefulWidgets
are special widgets that know how to generate State
objects, which are then used to hold state. Consider this basic example, using the RaisedButton
mentioned earlier:
class Counter extends StatefulWidget {
// This class is the configuration for the state. It holds the
// values (in this case nothing) provided by the parent and used by the build
// method of the State. Fields in a Widget subclass are always marked "final".
@override
_CounterState createState() => _CounterState();
}
class _CounterState extends State<Counter> {
int _counter = 0;
void _increment() {
setState(() {
// This call to setState tells the Flutter framework that
// something has changed in this State, which causes it to rerun
// the build method below so that the display can reflect the
// updated values. If you change _counter without calling
// setState(), then the build method won't be called again,
// and so nothing would appear to happen.
_counter++;
});
}
@override
Widget build(BuildContext context) {
// This method is rerun every time setState is called,
// for instance, as done by the _increment method above.
// The Flutter framework has been optimized to make rerunning
// build methods fast, so that you can just rebuild anything that
// needs updating rather than having to individually change
// instances of widgets.
return Row(
children: <Widget>[
RaisedButton(
onPressed: _increment,
child: Text('Increment'),
),
Text('Count: $_counter'),
],
);
}
}
You might wonder why StatefulWidget
and State
are separate objects. In Flutter, these two types of objects have different life cycles. Widgets
are temporary objects, used to construct a presentation of the application in its current state. State
objects, on the other hand, are persistent between calls to build()
, allowing them to remember information.
The example above accepts user input and directly uses the result in its build()
method. In more complex applications, different parts of the widget hierarchy might be responsible for different concerns; for example, one widget might present a complex user interface with the goal of gathering specific information, such as a date or location, while another widget might use that information to change the overall presentation.
In Flutter, change notifications flow “up” the widget 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:
class CounterDisplay extends StatelessWidget {
CounterDisplay({this.count});
final int count;
@override
Widget build(BuildContext context) {
return Text('Count: $count');
}
}
class CounterIncrementor extends StatelessWidget {
CounterIncrementor({this.onPressed});
final VoidCallback onPressed;
@override
Widget build(BuildContext context) {
return RaisedButton(
onPressed: onPressed,
child: Text('Increment'),
);
}
}
class Counter extends StatefulWidget {
@override
_CounterState createState() => _CounterState();
}
class _CounterState extends State<Counter> {
int _counter = 0;
void _increment() {
setState(() {
++_counter;
});
}
@override
Widget build(BuildContext context) {
return Row(children: <Widget>[
CounterIncrementor(onPressed: _increment),
CounterDisplay(count: _counter),
]);
}
}
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, see:
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
:
class Product {
const Product({this.name});
final String name;
}
typedef void CartChangedCallback(Product product, bool inCart);
class ShoppingListItem extends StatelessWidget {
ShoppingListItem({Product product, this.inCart, this.onCartChanged})
: product = product,
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 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)),
);
}
}
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 runApp()
persists for the lifetime of the application.
When the parent receives the onCartChanged
callback, the parent updates its internal state, which triggers the parent to rebuild and create a new instance of ShoppingListItem
with the new inCart
value. Although the parent creates a new instance of ShoppingListItem
when it rebuilds, that operation is cheap because the framework compares the newly built widgets with the previously built widgets and applies only the differences to the underlying RenderObject
.
Here’s an example parent widget that stores mutable state:
class ShoppingList extends StatefulWidget {
ShoppingList({Key key, this.products}) : super(key: key);
final List<Product> products;
// The framework calls createState the first time a widget appears at a given
// location in the tree. If the parent rebuilds and uses the same type of
// widget (with the same key), the framework re-uses the State object
// instead of creating a new State object.
@override
_ShoppingListState createState() => _ShoppingListState();
}
class _ShoppingListState extends State<ShoppingList> {
Set<Product> _shoppingCart = Set<Product>();
void _handleCartChanged(Product product, bool inCart) {
setState(() {
// When a user changes what's in the cart, you need to change
// _shoppingCart inside a setState call to trigger a rebuild.
// The framework then calls build, below,
// which updates the visual appearance of the app.
if (!inCart)
_shoppingCart.add(product);
else
_shoppingCart.remove(product);
});
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Shopping List'),
),
body: ListView(
padding: EdgeInsets.symmetric(vertical: 8.0),
children: widget.products.map((Product product) {
return ShoppingListItem(
product: product,
inCart: _shoppingCart.contains(product),
onCartChanged: _handleCartChanged,
);
}).toList(),
),
);
}
}
void main() {
runApp(MaterialApp(
title: 'Shopping App',
home: ShoppingList(
products: <Product>[
Product(name: 'Eggs'),
Product(name: 'Flour'),
Product(name: 'Chocolate chips'),
],
),
));
}
The ShoppingList
class extends StatefulWidget
, which means this widget stores mutable state. When the ShoppingList
widget is first inserted into the tree, the framework calls the createState()
function to create a fresh instance of _ShoppingListState
to associate with that location in the tree. (Notice that subclasses of State
are typically named with leading underscores to indicate that they are private implementation details.) When this widget’s parent rebuilds, the parent creates a new instance of ShoppingList
, but the framework reuses the _ShoppingListState
instance that is already in the tree rather than calling createState
again.
To access properties of the current ShoppingList
, the _ShoppingListState
can use its widget
property. If the parent rebuilds and creates a new ShoppingList
, the _ShoppingListState
rebuilds with the new widget value. If you wish to be notified when the widget
property changes, override the didUpdateWidget()
function, which is passed as oldWidget
to let you compare the old widget with the current widget.
When handling the onCartChanged
callback, the _ShoppingListState
mutates its internal state by either adding or removing a product from _shoppingCart
. To signal to the framework that it changed its internal state, it wraps those calls in a setState()
call. Calling setState
marks this widget as dirty and schedules it to be rebuilt the next time your app needs to update the screen. If you forget to call setState
when modifying the internal state of a widget, the framework won’t know your widget is dirty and might not call the widget’s build()
function, which means the user interface might not update to reflect the changed state.
By managing state in this way, you don’t need to write separate code for creating and updating child widgets. Instead, you simply implement the build
function, which handles both situations.
After calling createState()
on the StatefulWidget
, the framework inserts the new state object into the tree and then calls initState()
on the state object. A subclass of State
can override initState
to do work that needs to happen just once. For example, override initState
to configure animations or to subscribe to platform services. Implementations of initState
are required to start by calling super.initState
.
When a state object is no longer needed, the framework calls dispose()
on the state object. Override the dispose
function to do cleanup work. For example, override dispose
to cancel timers or to unsubscribe from platform services. Implementations of dispose
typically end by calling super.dispose
.
For more information, see State
.
Use keys to control which widgets the framework matches up with other widgets when a widget rebuilds. By default, the framework matches widgets in the current and previous build according to their runtimeType
and the order in which they appear. With keys, the framework requires that the two widgets have the same key
as well as the same runtimeType
.
Keys are most useful in widgets that build many instances of the same type of widget. For example, the ShoppingList
widget, which builds just enough ShoppingListItem
instances to fill its visible region:
Without keys, the first entry in the current build would always sync with the first entry in the previous build, even if, semantically, the first entry in the list just scrolled off screen and is no longer visible in the viewport.
By assigning each entry in the list a “semantic” key, the infinite list can be more efficient because the framework syncs entries with matching semantic keys and therefore similar (or identical) visual appearances. Moreover, syncing the entries semantically means that state retained in stateful child widgets remains attached to the same semantic entry rather than the entry in the same numerical position in the viewport.
For more information, see the Key
API.
Use global keys to uniquely identify child widgets. Global keys must be globally unique across the entire widget hierarchy, unlike local keys which need only be unique among siblings. Because they are globally unique, a global key can be used to retrieve the state associated with a widget.
For more information, see the GlobalKey
API.
#flutter #mobile-apps #ios