How to limit scroll distance in a scroll view in F

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 of a scrollview widget.

So when a person clicks on one of the fields, their keyboard will pop up (taking a portion of the screen), which means some buttons/fields are offscreen, which is where the scrollview widget serves its purpose.

The problem here is that I want to limit how far the scroll view allows a user to scroll.

There are some blank space under the lowest button, and I don't want the user to be able to scroll all the way there. This is too keep the experience simple and not have the user "overscroll" past the fields he should be typing in.

But since the background image is part of the scroll view the view will allow a user to scroll as far down as the bottom of the image. I want to limit this.

As a follow-up I'm trying to figure out how to set an initial scroll position. (So that when clicking on a field the scroll view scrolls down to very first text field, so all fields are in view. without the user needing to scroll down to them. However I don't want this scroll position to be re-applied every time the user clicks on a field, of course.)

Here is the relevant (if any of my code looks really bad please say so, I'm new to programming in general and accept any advice to improve):

class LoginPageConstructor extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    AssetImage loginBackgroundAsset =
        new AssetImage("assets/loginscreen/backgroundrock.png");
//    var _scrollController = new ScrollController(
//        initialScrollOffset: 200.0,
//        keepScrollOffset: true);
    return new Scaffold(
        body: new Container(
      child: new ListView(key: new PageStorageKey("Divider 1"),
//        controller: _scrollController,
        children: <Widget>[
          new Stack(children: <Widget>[
            new Container(
            constraints: new BoxConstraints.expand(height: 640.0),
              decoration: new BoxDecoration(
                  image: new DecorationImage(
                      image: loginBackgroundAsset, fit: BoxFit.cover)),
            child: new Column(
              children: <Widget>[
                new Divider(height: 300.0,),
                new Center(child: new UsernameText(),),
                new Divider(height: 8.0,),
                new Center(child: new PasswordText(),),
                new Divider(),
                new LoginButton(),
                new Divider(),
                new SignUpButton(),
              ],
            ))
          ])
        ],
      ),
    ));
  }
}

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<EnsureVisibleWhenFocused> {
  @override
  void initState() {
    super.initState();
    widget.focusNode.addListener(_ensureVisible);
  }

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

  Future<Null> _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<LoginPage> {
  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: <Widget>[
            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: <Widget>[
                  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'),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}