How to limit scroll distance in a scroll view in Flutter?

Question

I\'ve made a page that contains several textfields and buttons in a column which is contained in a container that has a background image. And this container is itself the child

Answer 1

For auto-scrolling the fields into view, it sounds like you are wrestling with issue 10826. I posted a workaround on that issue. I adapted the workaround to your sample code; see below. (You may want to tweak it a little.)

If you want to prevent users from scrolling, you might want to just ensure that all the fields are visible using the same techniques below and then use a NeverScrollableScrollPhysics as the physics of the ListView. Or if you're feeling ambitious you could implement a custom scroll physics as shown in the Gallery example. If I were you I'd hold out for #10826 to be fixed, though.

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

void main() {
  runApp(new MaterialApp(home: new LoginPage()));
}

/// A widget that ensures it is always visible when focused.
class EnsureVisibleWhenFocused extends StatefulWidget {
  const EnsureVisibleWhenFocused({
    Key key,
    @required this.child,
    @required this.focusNode,
    this.curve: Curves.ease,
    this.duration: const Duration(milliseconds: 100),
  }) : super(key: key);

  /// The node we will monitor to determine if the child is focused
  final FocusNode focusNode;

  /// The child widget that we are wrapping
  final Widget child;

  /// The curve we will use to scroll ourselves into view.
  ///
  /// Defaults to Curves.ease.
  final Curve curve;

  /// The duration we will use to scroll ourselves into view
  ///
  /// Defaults to 100 milliseconds.
  final Duration duration;

  EnsureVisibleWhenFocusedState createState() => new EnsureVisibleWhenFocusedState();
}

class EnsureVisibleWhenFocusedState extends State {
  @override
  void initState() {
    super.initState();
    widget.focusNode.addListener(_ensureVisible);
  }

  @override
  void dispose() {
    super.dispose();
    widget.focusNode.removeListener(_ensureVisible);
  }

  Future _ensureVisible() async {
    // Wait for the keyboard to come into view
    // TODO: position doesn't seem to notify listeners when metrics change,
    // perhaps a NotificationListener around the scrollable could avoid
    // the need insert a delay here.
    await new Future.delayed(const Duration(milliseconds: 600));

    if (!widget.focusNode.hasFocus)
      return;

    final RenderObject object = context.findRenderObject();
    final RenderAbstractViewport viewport = RenderAbstractViewport.of(object);
    assert(viewport != null);

    ScrollableState scrollableState = Scrollable.of(context);
    assert(scrollableState != null);

    ScrollPosition position = scrollableState.position;
    double alignment;
    if (position.pixels > viewport.getOffsetToReveal(object, 0.0)) {
      // Move down to the top of the viewport
      alignment = 0.0;
    } else if (position.pixels < viewport.getOffsetToReveal(object, 1.0)) {
      // Move up to the bottom of the viewport
      alignment = 1.0;
    } else {
      // No scrolling is necessary to reveal the child
      return;
    }
    position.ensureVisible(
      object,
      alignment: alignment,
      duration: widget.duration,
      curve: widget.curve,
    );
  }

  Widget build(BuildContext context) => widget.child;
}

class LoginPage extends StatefulWidget {
  LoginPageState createState() => new LoginPageState();
}

class LoginPageState extends State {
  FocusNode _usernameFocusNode = new FocusNode();
  FocusNode _passwordFocusNode = new FocusNode();

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Example App'),
      ),
      body: new Container(
        child: new ListView(
          physics: new NeverScrollableScrollPhysics(),
          key: new PageStorageKey("Divider 1"),
          children: [
            new Container(
              constraints: new BoxConstraints.expand(height: 640.0),
              decoration: new BoxDecoration(
                image: new DecorationImage(
                  image: new NetworkImage(
                    'https://flutter.io/images/flutter-mark-square-100.png',
                  ),
                  fit: BoxFit.cover,
                ),
              ),
              child: new Column(
                children: [
                  new Container(
                    height: 300.0,
                  ),
                  new Center(
                    child: new EnsureVisibleWhenFocused(
                      focusNode: _usernameFocusNode,
                      child: new TextFormField(
                        focusNode: _usernameFocusNode,
                        decoration: new InputDecoration(
                          labelText: 'Username',
                        ),
                      ),
                    ),
                  ),
                  new Container(height: 8.0),
                  new Center(
                    child: new EnsureVisibleWhenFocused(
                      focusNode: _passwordFocusNode,
                      child: new TextFormField(
                        focusNode: _passwordFocusNode,
                        obscureText: true,
                        decoration: new InputDecoration(
                          labelText: 'Password',
                        ),
                      ),
                    ),
                  ),
                  new Container(),
                  new RaisedButton(
                    onPressed: () {},
                    child: new Text('Log in'),
                  ),
                  new Divider(),
                  new RaisedButton(
                    onPressed: () {},
                    child: new Text('Sign up'),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}