Run method on Widget build complete

Question

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?

Answer 1

You can use the WidgetsBindingObserver to be notified when the application's widget tree has been built, laid out, and painted. You can then use this callback to run your function after the widget has finished building.

Here is an example of how you can use WidgetsBindingObserver:

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    if (state == AppLifecycleState.resumed) {
      // Run your function here after the widget has finished building
      checkAuthentication();
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Text('My Widget'),
    );
  }

  void checkAuthentication() {
    // Check if user is authenticated and redirect if not
  }
}
``

In the example above, we are implementing the WidgetsBindingObserver and adding it to our widget's state using WidgetsBinding.instance.addObserver(this). We are also removing it using WidgetsBinding.instance.removeObserver(this) in the dispose() method to avoid memory leaks.

The didChangeAppLifecycleState() method is called whenever the application's lifecycle state changes, and we are checking if the state is resumed which means that the widget tree has been built, laid out, and painted.

In the didChangeAppLifecycleState() method, we are calling checkAuthentication() method to perform the authentication check after the widget has finished building.

Note that WidgetsBindingObserver is a mixin that requires you to override its methods, which includes didChangeAppLifecycleState(). If you need to override other methods in your widget's state, make sure to include them in the mixin as well.