如何检测用户滚动的最后方向答案

【问题标题】：How to detect the last direction user scrolled如何检测用户滚动的最后方向
【发布时间】：2020-08-04 18:54:13
【问题描述】：

我有下面的代码，可以在用户向下或向上滚动时更改工厂的不透明度。但为此，我多次调用setState() 方法。有什么方法可以检测用户滚动的最后一个方向，以便我当时只能Once调用setState()？或者，如果您可以为此定义一个新的增强功能，我将不胜感激。

@override
void initState() {
super.initState();
_controller = ScrollController()
  ..addListener(() {
    upDirection = _controller.position.userScrollDirection == ScrollDirection.forward;
    upDirection = _controller.position.userScrollDirection == ScrollDirection.reverse;


      if (upDirection)
        setState(() {
          _scrolled = 0.5;
          flag = false;
        });
      else if (!upDirection) {
        setState(() {
          _scrolled = 1.0;
        });
        flag = false;
      }

  });
}

这是列表

    return Scaffold(
    body:
    SingleChildScrollView(
      controller: _controller,
      child: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Container(),
            Container(),
            Container(),
            Container(),
            Container(),
            Container(),
          ],
        ),
      ),
    ),

  floatingActionButton: AnimatedOpacity(
    opacity: _scrolled,
    duration: Duration(milliseconds: 100),
    child: FloatingActionButton(
      onPressed: (){},
      child: Icon(Icons.settings),
          ),
        ),
  floatingActionButtonLocation: FloatingActionButtonLocation.endFloat,
);

【问题讨论】：

标签： flutter flutter-layout flutter-animation

【解决方案1】：

问题是每次控制器上有新事件时都会调用您添加的侦听器。这就是为什么您的setState 被反复调用的原因。

您可以使用标志来防止重复调用setState。

工作代码：

void initState() {
    ScrollDirection _lastScrollDirection; // <---- Notice the new variable here.
    _controller = ScrollController();

    _controller.addListener(() {
      if (_lastScrollDirection != _controller.position.userScrollDirection) {
        _lastScrollDirection = _controller.position.userScrollDirection;

        setState(() {
          //You can set whatever you want here. 
          //It will only get called once when user changes direction.
        });

        print('Scroll direction changed --> $_lastScrollDirection');
      }
    });
    super.initState();
  }

演示：

【讨论】：

【解决方案2】：

感谢 Omer Celik，完整代码如下所示，您可以对其进行自定义：

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

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

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MainPage(),
    );
  }
}

class MainPage extends StatefulWidget {

  @override
  _MainPageState createState() => _MainPageState();
}

class _MainPageState extends State<MainPage> {

  ScrollController _controller;
  double _scrolled = 1.0;


  @override
  void initState() {
    ScrollDirection _lastScrollDirection;
    _controller = ScrollController();

    _controller.addListener(() {
      if (_lastScrollDirection != _controller.position.userScrollDirection) {
        _lastScrollDirection = _controller.position.userScrollDirection;


          if (_lastScrollDirection == ScrollDirection.forward) {
            setState(() {
            _scrolled = 1.0;
            });
          }
          else if (_lastScrollDirection == ScrollDirection.reverse) {
            setState(() {
              _scrolled = 0.5;
            });
          }
      }
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {

    return Scaffold(
        body:

        ListView.builder(
          controller: _controller,
          physics: BouncingScrollPhysics(),
          padding: const EdgeInsets.all(16.0),
          itemCount: 50,
          itemBuilder: _buildItem,
        ),


      floatingActionButton: AnimatedOpacity(
        opacity: _scrolled,
        duration: Duration(milliseconds: 700),
        child: FloatingActionButton(
          onPressed: (){},
          child: Icon(Icons.settings),
        ),
      ),
      floatingActionButtonLocation: FloatingActionButtonLocation.endFloat,
    );
  }
  Widget _buildItem(BuildContext context, int index) {
    return Container(
      child: Text(index.toString()),
    );
  }
}

【讨论】：