flutter_docs_portion.txt

<topic_start>
flutter for android developers
this document is meant for android developers looking to apply their
existing android knowledge to build mobile apps with flutter.
if you understand the fundamentals of the android framework then you
can use this document as a jump start to flutter development.
info note
  to integrate flutter code into your android app, see
  add flutter to existing app.
your android knowledge and skill set are highly valuable when building with
flutter, because flutter relies on the mobile operating system for numerous
capabilities and configurations. flutter is a new way to build UIs for mobile,
but it has a plugin system to communicate with android (and iOS) for non-UI
tasks. if you’re an expert with android, you don’t have to relearn everything
to use flutter.
this document can be used as a cookbook by jumping around and
finding questions that are most relevant to your needs.
<topic_end>
<topic_start>
views
<topic_end>
<topic_start>
what is the equivalent of a view in flutter?
how is react-style, or declarative, programming different than the
  traditional imperative style?
  for a comparison, see introduction to declarative UI.
in android, the view is the foundation of everything that shows up on the
screen. buttons, toolbars, and inputs, everything is a view.
in flutter, the rough equivalent to a view is a widget.
widgets don’t map exactly to android 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 view. 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, an android view is drawn once
and does not redraw until invalidate is called.
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.
for example, on iOS, you can use the cupertino widgets
to produce an interface that looks like apple’s iOS design language.
<topic_end>
<topic_start>
how do i update widgets?
in android, you update your views by directly mutating them. however,
in flutter, widgets are immutable and are not updated directly,
instead you have to work with the widget’s state.
this is where the concept of stateful and stateless widgets comes from.
a StatelessWidget is just what it sounds like—a
widget with no state information.
StatelessWidgets are useful when the part of the user interface
you are describing does not depend on anything other than the configuration
information in the object.
for example, in android, this is similar to placing an ImageView
with your logo. the logo is not going to change during runtime,
so use a StatelessWidget in flutter.
if you want to dynamically change the UI based on data received
after making an HTTP call or user interaction then you have to work
with StatefulWidget and tell the flutter framework that the widget’s
state has been updated so it can update that widget.
the important thing to note here is at the core both stateless and stateful
widgets behave the same. they rebuild every frame, the difference is the
StatefulWidget has a state object that stores state data across frames
and restores it.
if you are in doubt, then always remember this rule: if a widget changes
(because of user interactions, for example) it’s stateful.
however, if a widget reacts to change, the containing parent widget can
still be stateless if it doesn’t itself react to change.
the following example shows how to use a StatelessWidget. a common
StatelessWidget is the text widget. if you look at the implementation of
the text widget you’ll find that it subclasses StatelessWidget.
<code_start>
text(
  'i like flutter!',
  style: TextStyle(fontWeight: FontWeight.bold),
);
<code_end>
as you can see, the text widget has no state information associated 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>
import 'package:flutter/material.dart';
void main() {
  runApp(const SampleApp());
}
class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  // this widget is the root of your application.
  @override
  widget build(BuildContext context) {
    return MaterialApp(
      title: 'sample app',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const 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>
how do i lay out my widgets? where is my XML layout file?
in android, you write layouts in XML, but in flutter you write your layouts
with 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: ElevatedButton(
        style: ElevatedButton.styleFrom(
          padding: const EdgeInsets.only(left: 20, right: 30),
        ),
        onPressed: () {},
        child: const Text('Hello'),
      ),
    ),
  );
}
<code_end>
you can view some of the layouts that flutter has to offer in the
widget catalog.
<topic_end>
<topic_start>
how do i add or remove a component from my layout?
in android, you call addChild() or removeChild()
on a parent to dynamically add or remove child views.
in flutter, because widgets are immutable there is
no direct equivalent to addChild().  instead,
you can pass a function to the parent that returns a widget,
and control that child’s creation with a boolean flag.
for example, here is how you can toggle between two
widgets when you click on a FloatingActionButton:
<code_start>
import 'package:flutter/material.dart';
void main() {
  runApp(const SampleApp());
}
class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  // this widget is the root of your application.
  @override
  widget build(BuildContext context) {
    return MaterialApp(
      title: 'sample app',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const SampleAppPage(),
    );
  }
}
class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});
  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}
class _SampleAppPageState extends State<SampleAppPage> {
  // default value for toggle.
  bool toggle = true;
  void _toggle() {
    setState(() {
      toggle = !toggle;
    });
  }
  widget _getToggleChild() {
    if (toggle) {
      return const Text('Toggle one');
    } else {
      return ElevatedButton(
        onPressed: () {},
        child: const Text('Toggle two'),
      );
    }
  }
  @override
  widget build(BuildContext context) {
    return scaffold(
      appBar: AppBar(
        title: const Text('Sample app'),
      ),
      body: center(
        child: _getToggleChild(),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _toggle,
        tooltip: 'update text',
        child: const Icon(Icons.update),
      ),
    );
  }
}
<code_end>
<topic_end>
<topic_start>
how do i animate a widget?
in android, you either create animations using XML, or call the animate()
method on a view. in flutter, animate widgets using the animation
library by wrapping widgets inside an animated widget.
in flutter, use an AnimationController which is an animation<double>
that can pause, seek, stop and reverse the animation. it requires a ticker
that signals when vsync happens, and produces a linear interpolation between
0 and 1 on each frame while it’s running. you then create one or more
animations and attach them to the controller.
for example, you might use CurvedAnimation to implement an animation
along an interpolated curve. in this sense, the controller
is the “master” source of the animation progress and the CurvedAnimation
computes the curve that replaces the controller’s default linear motion.
like widgets, animations in flutter work with composition.
when building the widget tree you assign the animation to an animated
property of a widget, such as the opacity of a FadeTransition, and tell the
controller to start the animation.
the following example shows how to write a FadeTransition that fades the
widget into a logo when you press the FloatingActionButton:
<code_start>
import 'package:flutter/material.dart';
void main() {
  runApp(const FadeAppTest());
}
class FadeAppTest extends StatelessWidget {
  const FadeAppTest({super.key});
  // this widget is the root of your application.
  @override
  widget build(BuildContext context) {
    return MaterialApp(
      title: 'fade demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const MyFadeTest(title: 'fade demo'),
    );
  }
}
class MyFadeTest extends StatefulWidget {
  const MyFadeTest({super.key, required this.title});
  final string title;
  @override
  State<MyFadeTest> createState() => _MyFadeTest();
}
class _MyFadeTest extends State<MyFadeTest> with TickerProviderStateMixin {
  late AnimationController controller;
  late CurvedAnimation curve;
  @override
  void initState() {
    super.initState();
    controller = AnimationController(
      duration: const duration(milliseconds: 2000),
      vsync: this,
    );
    curve = CurvedAnimation(
      parent: controller,
      curve: Curves.easeIn,
    );
  }
  @override
  widget build(BuildContext context) {
    return scaffold(
      appBar: AppBar(
        title: text(widget.title),
      ),
      body: center(
        child: FadeTransition(
          opacity: curve,
          child: const FlutterLogo(
            size: 100,
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        tooltip: 'fade',
        onPressed: () {
          controller.forward();
        },
        child: const Icon(Icons.brush),
      ),
    );
  }
}
<code_end>
for more information, see
animation & motion widgets,
the animations tutorial,
and the animations overview.
<topic_end>
<topic_start>
how do i use a canvas to draw/paint?
in android, you would use the canvas and drawable
to draw images and shapes to the screen.
flutter has a similar canvas API as well,
since it’s based on the same low-level rendering engine, skia.
as a result, painting to a canvas in flutter
is a very familiar task for android developers.
flutter has two classes that help you draw to the canvas: CustomPaint
and CustomPainter,
the latter of which implements your algorithm to draw to the canvas.
to learn how to implement a signature painter in flutter,
see collin’s answer on custom paint.
<code_start>
import 'package:flutter/material.dart';
void main() => runApp(const MaterialApp(home: DemoApp()));
class DemoApp extends StatelessWidget {
  const DemoApp({super.key});
  @override
  widget build(BuildContext context) => const scaffold(body: signature());
}
class signature extends StatefulWidget {
  const signature({super.key});
  @override
  SignatureState createState() => SignatureState();
}
class SignatureState extends State<Signature> {
  List<Offset?> _points = <offset>[];
  @override
  widget build(BuildContext context) {
    return GestureDetector(
      onPanUpdate: (details) {
        setState(() {
          RenderBox? referenceBox = context.findRenderObject() as RenderBox;
          offset localPosition =
              referenceBox.globalToLocal(details.globalPosition);
          _points = List.from(_points)..add(localPosition);
        });
      },
      onPanEnd: (details) => _points.add(null),
      child: CustomPaint(
        painter: SignaturePainter(_points),
        size: size.infinite,
      ),
    );
  }
}
class SignaturePainter extends CustomPainter {
  SignaturePainter(this.points);
  final List<Offset?> points;
  @override
  void paint(Canvas canvas, size size) {
    var paint = paint()
      ..color = colors.black
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 5;
    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null) {
        canvas.drawLine(points[i]!, points[i + 1]!, paint);
      }
    }
  }
  @override
  bool shouldRepaint(SignaturePainter oldDelegate) =>
      oldDelegate.points != points;
}
<code_end>
<topic_end>
<topic_start>
how do i build custom widgets?
in android, you typically subclass view, or use a pre-existing view,
to override and implement methods that achieve the desired behavior.
in flutter, build a custom widget by composing
smaller widgets (instead of extending them).
it is somewhat similar to implementing a custom ViewGroup
in android, where all the building blocks are already existing,
but you provide a different behavior—for example,
custom layout logic.
for example, how do you build a CustomButton that takes a label in
the constructor? create a CustomButton that composes a ElevatedButton with
a label, rather than by extending ElevatedButton:
<code_start>
class CustomButton extends StatelessWidget {
  final string label;
  const CustomButton(this.label, {super.key});
  @override
  widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: () {},
      child: text(label),
    );
  }
}
<code_end>
then use CustomButton, just as you’d use any other flutter widget:
<code_start>
@override
widget build(BuildContext context) {
  return const center(
    child: CustomButton('Hello'),
  );
}
<code_end>
<topic_end>
<topic_start>
intents
<topic_end>
<topic_start>
what is the equivalent of an intent in flutter?
in android, there are two main use cases for intents: navigating between
activities, and communicating with components. flutter, on the other hand,
does not have the concept of intents, although you can still start intents
through native integrations (using a plugin).
flutter doesn’t really have a direct equivalent to activities and fragments;
rather, in flutter you navigate between screens, using a navigator and
routes, all within the same activity.
a route is an abstraction for a “screen” or “page” of an app, and a
navigator is a widget that manages routes. a route roughly maps to an
activity, but it does not carry the same meaning. a navigator can push
and pop routes to move from screen to screen. navigators work like a stack
on which you can push() new routes you want to navigate to, and from
which you can pop() routes when you want to “go back”.
in android, you declare your activities inside the app’s AndroidManifest.xml.
in flutter, you have a couple options to navigate between pages:
the following example builds a map.
<code_start>
void main() {
  runApp(MaterialApp(
    home: const MyAppHome(), // becomes the route named '/'.
    routes: <string, WidgetBuilder>{
      '/a': (context) => const MyPage(title: 'page a'),
      '/b': (context) => const MyPage(title: 'page b'),
      '/c': (context) => const MyPage(title: 'page c'),
    },
  ));
}
<code_end>
navigate to a route by pushing its name to the navigator.
<code_start>
Navigator.of(context).pushNamed('/b');
<code_end>
the other popular use-case for intents is to call external components such
as a camera or file picker. for this, you would need to create a native platform
integration (or use an existing plugin).
to learn how to build a native platform integration,
see developing packages and plugins.
<topic_end>
<topic_start>
how do i handle incoming intents from external applications in flutter?
flutter can handle incoming intents from android by directly talking to the
android layer and requesting the data that was shared.
the following example registers a text share intent filter on the native
activity that runs our flutter code, so other apps can share text with
our flutter app.
the basic flow implies that we first handle the shared text data on the
android native side (in our activity), and then wait until flutter requests
for the data to provide it using a MethodChannel.
first, register the intent filter for all intents in AndroidManifest.xml:
then in MainActivity, handle the intent, extract the text that was
shared from the intent, and hold onto it. when flutter is ready to process,
it requests the data using a platform channel, and it’s sent
across from the native side:
finally, request the data from the flutter side
when the widget is rendered:
<code_start>
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
void main() {
  runApp(const SampleApp());
}
class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  // this widget is the root of your application.
  @override
  widget build(BuildContext context) {
    return MaterialApp(
      title: 'sample shared app handler',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const SampleAppPage(),
    );
  }
}
class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});
  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}
class _SampleAppPageState extends State<SampleAppPage> {
  static const platform = MethodChannel('app.channel.shared.data');
  string dataShared = 'no data';
  @override
  void initState() {
    super.initState();
    getSharedText();
  }
  @override
  widget build(BuildContext context) {
    return scaffold(body: center(child: Text(dataShared)));
  }
  future<void> getSharedText() async {
    var sharedData = await platform.invokeMethod('getSharedText');
    if (shareddata != null) {
      setState(() {
        dataShared = sharedData;
      });
    }
  }
}
<code_end>
<topic_end>
<topic_start>
what is the equivalent of startActivityForResult()?
the navigator class handles routing in flutter and is used to get
a result back from a route that you have pushed on the stack.
this is done by awaiting on the future returned by push().
for example, to start a location route that lets the user select
their location, you could do the following:
<code_start>
object? coordinates = await Navigator.of(context).pushNamed('/location');
<code_end>
and then, inside your location route, once the user has selected their location
you can pop the stack with the result:
<code_start>
navigator.of(context).pop({'lat': 43.821757, 'long': -79.226392});
<code_end>
<topic_end>
<topic_start>
async UI
<topic_end>
<topic_start>
what is the equivalent of runOnUiThread() in flutter?
dart has a single-threaded execution model, with support for isolates
(a way to run dart code on another thread), an event loop, and
asynchronous programming. unless you spawn an isolate, your dart code
runs in the main UI thread and is driven by an event loop. flutter’s event
loop is equivalent to android’s main looper—that is, the looper that
is attached to the main thread.
dart’s single-threaded model doesn’t mean you need to run everything as a
blocking operation that causes the UI to freeze. unlike android, which
requires you to keep the main thread free at all times, in flutter,
use the asynchronous facilities that the dart language provides, such as
async/await, to perform asynchronous work. you might be familiar with
the async/await paradigm if you’ve used it in c#, javascript, or if you
have used kotlin’s coroutines.
for example, you can run network code without causing the UI to hang by
using async/await and letting dart do the heavy lifting:
<code_start>
future<void> loadData() async {
  var dataURL = uri.parse('https://jsonplaceholder.typicode.com/posts');
  http.Response response = await http.get(dataURL);
  setState(() {
    widgets = jsonDecode(response.body);
  });
}
<code_end>
once the awaited network call is done, update the UI by calling setState(),
which triggers a rebuild of the widget subtree and updates the data.
the following example loads data asynchronously and displays it in a ListView:
<code_start>
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
void main() {
  runApp(const SampleApp());
}
class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  @override
  widget build(BuildContext context) {
    return MaterialApp(
      title: 'sample app',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const SampleAppPage(),
    );
  }
}
class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});
  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}
class _SampleAppPageState extends State<SampleAppPage> {
  list widgets = [];
  @override
  void initState() {
    super.initState();
    loadData();
  }
  @override
  widget build(BuildContext context) {
    return scaffold(
      appBar: AppBar(
        title: const Text('Sample app'),
      ),
      body: ListView.builder(
        itemCount: widgets.length,
        itemBuilder: (context, position) {
          return getRow(position);
        },
      ),
    );
  }
  widget getRow(int i) {
    return padding(
      padding: const EdgeInsets.all(10),
      child: Text("Row ${widgets[i]["title"]}"),
    );
  }
  future<void> loadData() async {
    var dataURL = uri.parse('https://jsonplaceholder.typicode.com/posts');
    http.Response response = await http.get(dataURL);
    setState(() {
      widgets = jsonDecode(response.body);
    });
  }
}
<code_end>
refer to the next section for more information on doing work in the
background, and how flutter differs from android.
<topic_end>
<topic_start>
how do you move work to a background thread?
in android, when you want to access a network resource you would typically
move to a background thread and do the work, as to not block the main thread,
and avoid ANRs. for example, you might be using an AsyncTask, a LiveData,
an IntentService, a JobScheduler job, or an RxJava pipeline with a
scheduler that works on background threads.
since flutter is single threaded and runs an event loop (like node.js), you
don’t have to worry about thread management or spawning background threads. if
you’re doing I/O-bound work, such as disk access or a network call, then
you can safely use async/await and you’re all set. if, on the other
hand, you need to do computationally intensive work that keeps the CPU busy,
you want to move it to an isolate to avoid blocking the event loop, like
you would keep any sort of work out of the main thread in android.
for I/O-bound work, declare the function as an async function,
and await on long-running tasks inside the function:
<code_start>
future<void> loadData() async {
  var dataURL = uri.parse('https://jsonplaceholder.typicode.com/posts');
  http.Response response = await http.get(dataURL);
  setState(() {
    widgets = jsonDecode(response.body);
  });
}
<code_end>
this is how you would typically do network or database calls, which are both
I/O operations.
on android, when you extend AsyncTask, you typically override 3 methods,
onPreExecute(), doInBackground() and onPostExecute(). there is no
equivalent in flutter, since you await on a long-running function, and
dart’s event loop takes care of the rest.
however, there are times when you might be processing a large amount of data and
your UI hangs. in flutter, use isolates to take advantage of
multiple CPU cores to do long-running or computationally intensive tasks.
isolates are separate execution threads that do not share any memory
with the main execution memory heap. this means you can’t access variables from
the main thread, or update your UI by calling setState().
unlike android threads,
isolates are true to their name, and cannot share memory
(in the form of static fields, for example).
the following example shows, in a simple isolate, how to share data back to
the main thread to update the UI.
<code_start>
future<void> loadData() async {
  ReceivePort receivePort = ReceivePort();
  await Isolate.spawn(dataLoader, receivePort.sendPort);
  // the 'echo' isolate sends its SendPort as the first message.
  SendPort sendPort = await receivePort.first;
  list msg = await sendReceive(
    sendPort,
    'https://jsonplaceholder.typicode.com/posts',
  );
  setState(() {
    widgets = msg;
  });
}
// the entry point for the isolate.
static future<void> dataLoader(SendPort sendPort) async {
  // open the ReceivePort for incoming messages.
  ReceivePort port = ReceivePort();
  // notify any other isolates what port this isolate listens to.
  sendPort.send(port.sendPort);
  await for (var msg in port) {
    string data = msg[0];
    SendPort replyTo = msg[1];
    string dataURL = data;
    http.Response response = await http.get(Uri.parse(dataURL));
    // lots of JSON to parse
    replyTo.send(jsonDecode(response.body));
  }
}
future sendReceive(SendPort port, msg) {
  ReceivePort response = ReceivePort();
  port.send([msg, response.sendPort]);
  return response.first;
}
<code_end>
here, dataLoader() is the isolate that runs in its own separate
execution thread.  in the isolate you can perform more CPU intensive
processing (parsing a big JSON, for example),
or perform computationally intensive math,
such as encryption or signal processing.
you can run the full example below:
<code_start>
import 'dart:async';
import 'dart:convert';
import 'dart:isolate';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
void main() {
  runApp(const SampleApp());
}
class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  @override
  widget build(BuildContext context) {
    return MaterialApp(
      title: 'sample app',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const SampleAppPage(),
    );
  }
}
class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});
  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}
class _SampleAppPageState extends State<SampleAppPage> {
  list widgets = [];
  @override
  void initState() {
    super.initState();
    loadData();
  }
  widget getBody() {
    bool showLoadingDialog = widgets.isEmpty;
    if (showloadingdialog) {
      return getProgressDialog();
    } else {
      return getListView();
    }
  }
  widget getProgressDialog() {
    return const center(child: CircularProgressIndicator());
  }
  @override
  widget build(BuildContext context) {
    return scaffold(
      appBar: AppBar(
        title: const Text('Sample app'),
      ),
      body: getBody(),
    );
  }
  ListView getListView() {
    return ListView.builder(
      itemCount: widgets.length,
      itemBuilder: (context, position) {
        return getRow(position);
      },
    );
  }
  widget getRow(int i) {
    return padding(
      padding: const EdgeInsets.all(10),
      child: Text("Row ${widgets[i]["title"]}"),
    );
  }
  future<void> loadData() async {
    ReceivePort receivePort = ReceivePort();
    await Isolate.spawn(dataLoader, receivePort.sendPort);
    // the 'echo' isolate sends its SendPort as the first message.
    SendPort sendPort = await receivePort.first;
    list msg = await sendReceive(
      sendPort,
      'https://jsonplaceholder.typicode.com/posts',
    );
    setState(() {
      widgets = msg;
    });
  }
  // the entry point for the isolate.
  static future<void> dataLoader(SendPort sendPort) async {
    // open the ReceivePort for incoming messages.
    ReceivePort port = ReceivePort();
    // notify any other isolates what port this isolate listens to.
    sendPort.send(port.sendPort);
    await for (var msg in port) {
      string data = msg[0];
      SendPort replyTo = msg[1];
      string dataURL = data;
      http.Response response = await http.get(Uri.parse(dataURL));
      // lots of JSON to parse
      replyTo.send(jsonDecode(response.body));
    }
  }
  future sendReceive(SendPort port, msg) {
    ReceivePort response = ReceivePort();
    port.send([msg, response.sendPort]);
    return response.first;
  }
}
<code_end>
<topic_end>
<topic_start>
what is the equivalent of OkHttp on flutter?
making a network call in flutter is easy when you use the
popular http package.
while the http package doesn’t have every feature found in OkHttp,
it abstracts away much of the networking that you would normally implement
yourself, making it a simple way to make network calls.
to add the http package as a dependency, run flutter pub add:
to make a network call, call await on the async function http.get():
<code_start>
import 'dart:developer' as developer;
import 'package:http/http.dart' as http;
future<void> loadData() async {
  var dataURL = uri.parse('https://jsonplaceholder.typicode.com/posts');
  http.Response response = await http.get(dataURL);
  developer.log(response.body);
}
<code_end>
<topic_end>
<topic_start>
how do i show the progress for a long-running task?
in android you would typically show a ProgressBar view in your UI while
executing a long-running task on a background thread.
in flutter, use a ProgressIndicator widget.
show the progress programmatically by controlling when it’s rendered
through a boolean flag. tell flutter to update its state before your
long-running task starts, and hide it after it ends.
in the following example, the build function is separated into three different
functions. if showLoadingDialog is true (when widgets.isEmpty),
then render the ProgressIndicator. otherwise, render the
ListView with the data returned from a network call.
<code_start>
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
void main() {
  runApp(const SampleApp());
}
class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  @override
  widget build(BuildContext context) {
    return MaterialApp(
      title: 'sample app',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const SampleAppPage(),
    );
  }
}
class SampleAppPage extends StatefulWidget {
  const SampleAppPage({super.key});
  @override
  State<SampleAppPage> createState() => _SampleAppPageState();
}
class _SampleAppPageState extends State<SampleAppPage> {
  list widgets = [];
  @override
  void initState() {
    super.initState();
    loadData();
  }
  widget getBody() {
    bool showLoadingDialog = widgets.isEmpty;
    if (showloadingdialog) {
      return getProgressDialog();
    } else {
      return getListView();
    }
  }
  widget getProgressDialog() {
    return const center(child: CircularProgressIndicator());
  }
  @override
  widget build(BuildContext context) {
    return scaffold(
      appBar: AppBar(
        title: const Text('Sample app'),
      ),
      body: getBody(),
    );
  }
  ListView getListView() {
    return ListView.builder(
      itemCount: widgets.length,
      itemBuilder: (context, position) {
        return getRow(position);
      },
    );
  }
  widget getRow(int i) {
    return padding(
      padding: const EdgeInsets.all(10),
      child: Text("Row ${widgets[i]["title"]}"),
    );
  }
  future<void> loadData() async {
    var dataURL = uri.parse('https://jsonplaceholder.typicode.com/posts');
    http.Response response = await http.get(dataURL);
    setState(() {
      widgets = jsonDecode(response.body);
    });
  }
}
<code_end>
<topic_end>