How do you tell if scrolling is not needed in a SingleChildScrollView when the screen is first built?

Question

I have a SingleChildScrollView. Sometimes its child is longer than the screen, in which case the SingleChildScrollView lets you scroll. But also sometimes its child is shorter than the screen, in which case no scrolling is needed.

I am trying to add an arrow to the bottom of the screen which hints to the user that they can/should scroll down to see the rest of the contents. I successfully implemented this except in the case where the child of the SingleChildScrollView is shorter than the screen. In this case no scrolling is needed, so I would like to not show the arrow at all.

I've tried making a listener to do this, but the listener isn't activated until you start scrolling, and in this case you can't scroll.

I've also tried accessing the properties of the _scrollController in the ternary operator which shows the arrow, but an exception is thrown: ScrollController not attached to any scroll views.

Here is a complete sample app showing what I'm doing, so you can just copy and paste it if you want to see it run. I replaced all of the contents with a Column of Text widgets for simplicity:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) =>
      MaterialApp(home: Scaffold(body: MyScreen()));
}

class MyScreen extends StatefulWidget {
  @override
  _MyScreenState createState() => _MyScreenState();
}

class _MyScreenState extends State<MyScreen> {
  ScrollController _scrollController = ScrollController();
  bool atBottom = false;

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

    // Activated when you get to the bottom:
    _scrollController.addListener(() {
      if (_scrollController.position.extentAfter == 0) {
        setState(() {
          atBottom = true;
        });
      }
    });

    // Activated as soon as you start scrolling back up after getting to the bottom:
    _scrollController.addListener(() {
      if (_scrollController.position.extentAfter > 0 && atBottom) {
        setState(() {
          atBottom = false;
        });
      }
    });

    // I want this to activate if you are at the top of the screen and there is
    // no scrolling to do, i.e. the widget being displayed fits in the screen:
    _scrollController.addListener(() {
      if (_scrollController.offset == 0 &&
          _scrollController.position.extentAfter == 0) {
        setState(() {
          atBottom = false;
        });
      }
    });
  }

  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        SingleChildScrollView(
          controller: _scrollController,
          scrollDirection: Axis.vertical,
          child: Container(
            width: MediaQuery.of(context).size.width,
            child: Column(
              children: [
                for (int i = 0; i < 100; i++)
                  Text(
                    i.toString(),
                  ),
              ],
            ),
          ),
        ),
        atBottom
            ? Container()
            : Positioned(
                bottom: 10,
                right: 10,
                child: Container(
                  child: Icon(
                    Icons.arrow_circle_down,
                  ),
                ),
              ),
      ],
    );
  }
}

Answer 1

1. Import the scheduler Flutter library:

import 'package:flutter/scheduler.dart';

2. Create a boolean flag inside the state object but outside of the build method to track whether build has been called yet:

bool buildCalledYet = false;

3. Add the following in the beginning of the build method:

if (!firstBuild) {
      firstBuild = true;
      SchedulerBinding.instance.addPostFrameCallback((_) {
        setState(() {
          atBottom = !(_scrollController.position.maxScrollExtent > 0);
        });
      });
    }

(The boolean flag prevents this code from causing build to be called over and over again.)

Here's the complete code of the sample app implementing this solution:

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) =>
      MaterialApp(home: Scaffold(body: MyScreen()));
}

class MyScreen extends StatefulWidget {
  @override
  _MyScreenState createState() => _MyScreenState();
}

class _MyScreenState extends State<MyScreen> {
  ScrollController _scrollController = ScrollController();
  bool atBottom = false;
  // ======= new code =======
  bool buildCalledYet = false;
  // ========================

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

    _scrollController.addListener(() {
      if (_scrollController.position.extentAfter == 0) {
        setState(() {
          atBottom = true;
        });
      }
    });

    _scrollController.addListener(() {
      if (_scrollController.position.extentAfter > 0 && atBottom) {
        setState(() {
          atBottom = false;
        });
      }
    });

    // ======= The third listener is not needed. =======
  }

  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    // =========================== new code ===========================
    if (!buildCalledYet) {
      buildCalledYet = true;
      SchedulerBinding.instance.addPostFrameCallback((_) {
        setState(() {
          atBottom = !(_scrollController.position.maxScrollExtent > 0);
        });
      });
    }
    // ================================================================

    return Stack(
      children: [
        SingleChildScrollView(
          controller: _scrollController,
          scrollDirection: Axis.vertical,
          child: Container(
            width: MediaQuery.of(context).size.width,
            child: Column(
              children: [
                for (int i = 0; i < 100; i++)
                  Text(
                    i.toString(),
                  ),
              ],
            ),
          ),
        ),
        atBottom
            ? Container()
            : Positioned(
                bottom: 10,
                right: 10,
                child: Container(
                  child: Icon(
                    Icons.arrow_circle_down,
                  ),
                ),
              ),
      ],
    );
  }
}

I found this solution on another Stack Overflow question: Determine Scroll Widget height