dart - 如何使SingleChildScrollView从底部开始？

Question

我正在开发一个应用程序，遇到一个问题，其中SingleChildScrollView从顶部开始:



我如何使它像这样从底部开始:



滚动 View 是在屏幕顶部的topContent中实现的:

    final topContent = Stack(
      children: <Widget>[
        Container(
            padding: EdgeInsets.only(left: 10.0),
            height: MediaQuery.of(context).size.height * 0.5,
            decoration: new BoxDecoration(
              image: new DecorationImage(
                image: new AssetImage("assets/images/drones1.jpg"),
                fit: BoxFit.cover,
              ),
            )),
        Container(
          height: MediaQuery.of(context).size.height * 0.5,
          padding: EdgeInsets.all(40.0),
          width: MediaQuery.of(context).size.width,
          decoration: BoxDecoration(color: Color.fromRGBO(58, 66, 86, .9)),
          child: SingleChildScrollView(      //scroll view implemented here 
            child: Center(
              child: topContentText,
            ),
          ),
        ),
        Positioned(
          left: 8.0,
          top: 60.0,
          child: InkWell(
            onTap: () {
              Navigator.pop(context);
            },
            child: Icon(Icons.arrow_back, color: Colors.white),
          ),
        )
      ],
    );

更新:我分配了一个 Controller ，并按照此link's示例尝试使用SchedulerBinding.instance.addPostFrameCallback调用滚动 Controller 的jumpTo函数，但它给了我一个异常(exception)，其结果与之前没有向下滚动的结果相同:

import 'package:garuda_academy_app/Constants.dart';
import 'Lesson.dart';
import 'package:flutter/material.dart';
import 'package:flutter/scheduler.dart';

class DetailPage extends StatefulWidget {
  final Lesson lesson;
  DetailPage({Key key, this.lesson}) : super(key: key);

  @override
  _DetailPageState createState() => _DetailPageState(lesson: lesson);
}
class _DetailPageState extends State<DetailPage> {
  final Lesson lesson;
  ScrollController _scrollController;
  _DetailPageState({this.lesson});

  @override                             // I use the SchedularBinding here
  void initState() {
    super.initState();
    SchedulerBinding.instance.addPostFrameCallback((_) =>
      _scrollController.jumpTo(_scrollController.position.maxScrollExtent));
  }

  @override
  Widget build(BuildContext context) {
    final levelIndicator = Container(
      child: Container(
        child: LinearProgressIndicator(
            backgroundColor: Color.fromRGBO(209, 224, 224, 0.2),
            value: lesson.indicatorValue,
            valueColor: AlwaysStoppedAnimation(Colors.green)),
      ),
    );

    final course = Container(
      padding: const EdgeInsets.all(7.0),
      decoration: new BoxDecoration(
          border: new Border.all(color: Colors.white),
          borderRadius: BorderRadius.circular(5.0)),
      child: new Text(
        lesson.course,
        style: TextStyle(color: Colors.white),
      ),
    );

    final topContentText = Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: <Widget>[
        SizedBox(height: 120.0),
        Icon(
          Icons.flight,
          color: Colors.white,
          size: 40.0,
        ),
        Container(
          width: 90.0,
          child: new Divider(color: Colors.green),
        ),
        SizedBox(height: 10.0),
        Text(
          lesson.title,
          style: TextStyle(color: Colors.white, fontSize: 45.0),
        ),
        SizedBox(height: 30.0),
        Row(
          mainAxisAlignment: MainAxisAlignment.start,
          children: <Widget>[
            Expanded(flex: 1, child: levelIndicator),
            Expanded(
                flex: 6,
                child: Padding(
                    padding: EdgeInsets.only(left: 10.0),
                    child: Text(
                      lesson.level,
                      style: TextStyle(color: Colors.white),
                    ))),
            Expanded(flex: 1, child: course)
          ],
        ),
      ],
    );

    final topContent = Stack(
      children: <Widget>[
        Container(
            padding: EdgeInsets.only(left: 10.0),
            height: MediaQuery.of(context).size.height * 0.5,
            decoration: new BoxDecoration(
              image: new DecorationImage(
                image: new AssetImage(lesson.imagePath),
                fit: BoxFit.cover,
              ),
            )),
        Container(
          height: MediaQuery.of(context).size.height * 0.5,
          padding: EdgeInsets.all(40.0),
          width: MediaQuery.of(context).size.width,
          decoration: BoxDecoration(color: Color.fromRGBO(58, 66, 86, .9)),
          child: SingleChildScrollView(
            controller: _scrollController,      // Where I pin the ScrollController
            child: Center(
              child: topContentText,
            ),
          ),
        ),
        Positioned(
          left: 8.0,
          top: 60.0,
          child: InkWell(
            onTap: () {
              Navigator.pop(context);
            },
            child: Icon(Icons.arrow_back, color: Colors.white),
          ),
        )
      ],
    );

    final bottomContentText = Text(
      lesson.content,
      style: TextStyle(fontSize: 18.0),
    );
    final downloadButton = Container(
        padding: EdgeInsets.symmetric(vertical: 16.0),
        width: MediaQuery.of(context).size.width,
        child: RaisedButton(
          onPressed: () => {},
          color: Color.fromRGBO(58, 66, 86, 1.0),
          child:
              Text(DETAIL_PAGE_DOWNLOAD, style: TextStyle(color: Colors.white)),
        ));
    final bottomContent = Container(
      width: MediaQuery.of(context).size.width,
      padding: EdgeInsets.all(40.0),
      child: Center(
        child: Column(
          children: <Widget>[bottomContentText, downloadButton],
        ),
      ),
    );

    return Scaffold(
      body: Column(                  
        children: <Widget>[topContent, bottomContent],
      ),
    );
  }  
}

Exception: I/flutter ( 1567): Another exception was thrown: NoSuchMethodError: The getter 'position' was called on null.

Answer 1

我通过定义ScrollController时进一步添加(通过更新)来解决此问题:

  ScrollController _scrollController = new ScrollController(
    initialScrollOffset: 0.0,
    keepScrollOffset: true,
  );