Flutter: Run method on Widget build complete

You could use

https://github.com/slightfoot/flutter_after_layout

which executes a function only one time after the layout is completed. Or just look at its implementation and add it to your code :-)

Which is basically

  void initState() {
    super.initState();
    WidgetsBinding.instance
        .addPostFrameCallback((_) => yourFunction(context));
  }

---

UPDATE: Flutter v1.8.4

Both mentioned codes are working now:

Working:

WidgetsBinding.instance
        .addPostFrameCallback((_) => yourFunction(context));

Working

import 'package:flutter/scheduler.dart';

SchedulerBinding.instance.addPostFrameCallback((_) => yourFunction(context));

---

Best ways of doing this,

1. WidgetsBinding

WidgetsBinding.instance.addPostFrameCallback((_) {
      print("WidgetsBinding");
    });

2. SchedulerBinding

SchedulerBinding.instance.addPostFrameCallback((_) {
  print("SchedulerBinding");
});

It can be called inside initState, both will be called only once after Build widgets done with rendering.

@override
  void initState() {
    // TODO: implement initState
    super.initState();
    print("initState");
    WidgetsBinding.instance.addPostFrameCallback((_) {
      print("WidgetsBinding");
    });
    SchedulerBinding.instance.addPostFrameCallback((_) {
      print("SchedulerBinding");
    });
  }

both above codes will work the same as both use the similar binding framework. For the difference find the below link.

https://medium.com/flutterworld/flutter-schedulerbinding-vs-widgetsbinding-149c71cb607f

---

There are 3 possible ways:

1) WidgetsBinding.instance.addPostFrameCallback((_) => yourFunc(context));

2) Future.delayed(Duration.zero, () => yourFunc(context));

3) Timer.run(() => yourFunc(context));

As for context, I needed it for use in Scaffold.of(context) after all my widgets were rendered.

But in my humble opinion, the best way to do it is this:

void main() async {
  WidgetsFlutterBinding.ensureInitialized(); //all widgets are rendered here
  await yourFunc();
  runApp( MyApp() );
}

---

Flutter 1.2 - dart 2.2

According with the official guidelines and sources if you want to be certain that also the last frame of your layout was drawned you can write for example:

import 'package:flutter/scheduler.dart';

void initState() {
   super.initState();
   if (SchedulerBinding.instance.schedulerPhase == SchedulerPhase.persistentCallbacks) {
        SchedulerBinding.instance.addPostFrameCallback((_) => yourFunction(context));
   }
}

---

If you are looking for ReactNative's componentDidMount equivalent, Flutter has it. It's not that simple but it's working just the same way. In Flutter, Widgets do not handle their events directly. Instead they use their State object to do that.

class MyWidget extends StatefulWidget{

  @override
  State<StatefulWidget> createState() => MyState(this);

  Widget build(BuildContext context){...} //build layout here

  void onLoad(BuildContext context){...} //callback when layout build done
}

class MyState extends State<MyWidget>{

  MyWidget widget;

  MyState(this.widget);

  @override
  Widget build(BuildContext context) => widget.build(context);

  @override
  void initState() => widget.onLoad(context);
}

State.initState immediately will be called once upon screen has finishes rendering the layout. And will never again be called even on hot reload if you're in debug mode, until explicitly reaches time to do so.

---

In flutter version 1.14.6, Dart version 28.

Below is what worked for me, You simply just need to bundle everything you want to happen after the build method into a separate method or function.

@override
void initState() {
super.initState();
print('hello girl');

WidgetsBinding.instance
    .addPostFrameCallback((_) => afterLayoutWidgetBuild());

}

---

Try SchedulerBinding,

 SchedulerBinding.instance
                .addPostFrameCallback((_) => setState(() {
              isDataFetched = true;
            }));

---

The PostFrameCallback fires before the screen has fully painted. Therefore Devv's answer above was helpful with the added delay to allow the screen to paint.

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
       Future.delayed(Duration(seconds: 3), () => yourFunction());
    });
  }

---

if you having issue with new SDK and old answer you can try my solution.I have tested it on v3.0.4

WidgetsBinding.instance.endOfFrame.then(
  (_) {
    if (mounted) {
          // do some suff 
          // you can get width height of specific widget based on GlobalKey
       };
  },
);

---

my english is poor forgive me

import 'package:flutter/material.dart';

class TestBox extends StatefulWidget {
  final Color color;
  final Duration delay;

  const TestBox({
    Key? key,
    this.color = Colors.red,
    this.delay = const Duration(seconds: 5),
  }) : super(key: key);

  @override
  _TestBoxState createState() => _TestBoxState();
}

class _TestBoxState extends State<TestBox> {
  String? label;

  @override
  void initState() {
    initialMembers();
    super.initState();
  }

  void initialMembers() async {
    label = await fetchLabel();

    if (mounted) setState(() {});

    /// don't worry
    /// if `(!mounted)`, means wen `build` calld
    /// the label already has the newest value
  }

  Future<String> fetchLabel() async {
    await Future.delayed(widget.delay);
    print('fetchLabel call');
    return 'from fetchLabel()';
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedContainer(
      margin: EdgeInsets.symmetric(vertical: 12),
      duration: Duration(milliseconds: 500),
      width: 220,
      height: 120,
      color: label == null ? Colors.white : widget.color,
      child: Center(
        child: Text(label ?? 'fetching...'),
      ),
    );
  }
}

Column(
  children: [
    TestBox(
      delay: Duration(seconds: 1),
      color: Colors.green,
    ),
    TestBox(
      delay: Duration(seconds: 3),
      color: Colors.yellow,
    ),
    TestBox(
      delay: Duration(seconds: 5),
      color: Colors.red,
    ),
  ],
),

---

I have a Stateful widget where I use html_editor_enhanced plugin widget. This is the only way to set initial message in it.

class _SendChatMessageState extends State<SendChatMessage> {
  final _htmlController = HtmlEditorController();

      @override
      void initState() {
        super.initState();
    
          Future.delayed(const Duration(seconds: 3), () {
            _htmlController.setText(widget.chatMessage.message ?? '');
          });
      }

I tried addPostFrameCallback but it didn't work because a JavaScript generates exception "HTML editor is still loading, please wait before evaluating this JS ..."

---

another solution that worked pretty well for me is wrapping the function you want to call by Future.delayed() as showen below:

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
       Future.delayed(Duration(seconds: 3), () => yourFunction());
    });
  }

---