[dart] Flutter: Run method on Widget build complete

I would like to be able to run functions once a Widget has finished building/loading but I am unsure how. My current use case is to check if a user is authenticated and if not, redirect to a login view. I do not want to check before and push either the login view or the main view, it needs to happen after the main view has loaded. Is there anything I can use to do this?

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));
   }
}

Try SchedulerBinding,

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

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

If you want to do this only once, then do it because The framework will call initState() method exactly once for each State object it creates.

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

If you want to do this again and again like on back or navigate to a next screen and etc..., then do it because didChangeDependencies() Called when a dependency of this State object changes.

For example, if the previous call to build referenced an InheritedWidget that later changed, the framework would call this method to notify this object about the change.

This method is also called immediately after initState. It is safe to call BuildContext.dependOnInheritedWidgetOfExactType from this method.

 @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    WidgetsBinding.instance
        .addPostFrameCallback((_) => executeAfterBuildComplete(context));
  }

This is the your Callback function

executeAfterBuildComplete([BuildContext context]){
    print("Build Process Complete");
  }

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));

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));
  }

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());

}

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.