SliverAppBar这个控件有个问题就是设置了标题之后一直显示，参考这篇文章

import 'package:flutter/material.dart';class SABT extends StatefulWidget {
  final Widget child;  const SABT({
    Key key,
    @required this.child,
  }) : super(key: key);  @override
  _SABTState createState() {
    return new _SABTState();
  }
}class _SABTState extends State<SABT> {
  ScrollPosition _position;
  bool _visible;  @override
  void dispose() {
    _removeListener();
    super.dispose();
  }  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    _removeListener();
    _addListener();
  }  void _addListener() {
    _position = Scrollable.of(context)?.position;
    _position?.addListener(_positionListener);
    _positionListener();
  }  void _removeListener() {
    _position?.removeListener(_positionListener);
  }  void _positionListener() {
    final FlexibleSpaceBarSettings settings =
      context.inheritFromWidgetOfExactType(FlexibleSpaceBarSettings);
    bool visible = settings == null || settings.currentExtent <= settings.minExtent;
    if (_visible != visible) {
      setState(() {
        _visible = visible;
      });
    }
  }  @override
  Widget build(BuildContext context) {
    return Visibility(
      visible: _visible,
      child: widget.child,
    );
  }
}

使用方式

 SliverAppBar(
            primary: true,
            pinned: true,
            expandedHeight: 500,
            title: SABT(child: Text("The title")),
            flexibleSpace: FlexibleSpaceBar(
              background: Container(
                color: Colors.purple,
              ),
            ),
          ),