How to show SnackBar after page transition

Question

I want to show SnackBar only once when the page is displayed. But we cannot call the showSnackBar method in the build method.

Is there a handler called after assembly?

Answer 1

You can use StatefulWidgetand call showSnackBarin initStateyours State. You will need to add a little delay before starting showSnackBar. Here is a sample code.

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

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      floatingActionButton: new FloatingActionButton(
        child: new Icon(Icons.developer_board),
        onPressed: () {
          Navigator.of(context).push(
            new MaterialPageRoute(builder: (_) => new MySecondPage()),
          );
        },
      ),
    );
  }
}

class MySecondPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Developer Mode'),
      ),
      body: new MySecondPageBody(),
    );
  }
}

class MySecondPageBody extends StatefulWidget {
  @override
  State createState() => new MySecondPageBodyState();
}

class MySecondPageBodyState extends State<MySecondPageBody> {
  @override
  void initState() {
    new Future<Null>.delayed(Duration.ZERO, () {
      Scaffold.of(context).showSnackBar(
        new SnackBar(content: new Text("You made it! Congrats.")),
      );
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return new Center(
      child: new Text('You are now a developer.'),
    );
  }
}

Answer 2

With StatelessWidget and scheduleMicrotask, you can also achieve this.

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

class App extends StatelessWidget {
  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
  App() {
    scheduleMicrotask(() => _scaffoldKey.currentState.showSnackBar(SnackBar(
          content: Text('Hey!'),
        )));
  }
  @override
  Widget build(BuildContext ctx) {
    return Scaffold(
        key: _scaffoldKey,
        appBar: AppBar(
          title: Text('Look mum!'),
        ),
        body: Container());
  }
}

Answer 3

You can also try attaching to addPostFrameCallback ( https://api.flutter.dev/flutter/scheduler/SchedulerBinding/addPostFrameCallback.html ).

SchedulerBinding.instance.addPostFrameCallback((_) {
    //show snackbar here
  });