Flutter After Layout

Brings functionality to execute code after the first layout of a widget has been performed, i.e. after the first frame has been displayed.

Quick Usage

Add with AfterLayoutMixin<MyWidget> mixin to your State<MyWidget> class and then implement the void afterFirstLayout(BuildContext context) abstract method. Code in this method will be called the first time this widget is laid out on the screen.

Motivation

If you want to display a widget that depends on the layout, such as a Dialog or BottomSheet, you can not use that widget in initState.

You might have tried this.

BAD CODE

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'After Layout - Bad Example',
      home: new HomeScreen(),
    );
  }
}

class HomeScreen extends StatefulWidget {
  @override
  HomeScreenState createState() => new HomeScreenState();
}

class HomeScreenState extends State<HomeScreen> {
  @override
  void initState() {
    super.initState();
    // NOTE: Calling this function here would crash the app.
    showHelloWorld();
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(body: new Container(color: Colors.red));
  }

  void showHelloWorld() {
    showDialog(
      context: context,
      builder: (context) => new AlertDialog(
            content: new Text('Hello World'),
            actions: <Widget>[
              new FlatButton(
                child: new Text('DISMISS'),
                onPressed: () => Navigator.of(context).pop(),
              )
            ],
          ),
    );
  }
}

Usage

This demo showcases how this package resolves the shortcomings shown above:

GOOD CODE

import 'package:flutter/material.dart';
import 'package:after_layout/after_layout.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'After Layout - Good Example',
      home: new HomeScreen(),
    );
  }
}

class HomeScreen extends StatefulWidget {
  @override
  HomeScreenState createState() => new HomeScreenState();
}

class HomeScreenState extends State<HomeScreen> with AfterLayoutMixin<HomeScreen> {

  @override
  Widget build(BuildContext context) {
    return new Scaffold(body: new Container(color: Colors.red));
  }

  @override
  void afterFirstLayout(BuildContext context) {
    // Calling the same function "after layout" to resolve the issue.
    showHelloWorld();
  }

  void showHelloWorld() {
    showDialog(
      context: context,
      builder: (context) => new AlertDialog(
            content: new Text('Hello World'),
            actions: <Widget>[
              new FlatButton(
                child: new Text('DISMISS'),
                onPressed: () => Navigator.of(context).pop(),
              )
            ],
          ),
    );
  }
}