Xamarin Forms - 检测用户何时开始在滚动视图中向下滚动答案

【问题标题】：Xamarin Forms - detect when user starts to scroll down in a scrollviewXamarin Forms - 检测用户何时开始在滚动视图中向下滚动
【发布时间】：2019-02-19 11:18:53
【问题描述】：

我有一个滚动视图，一旦用户开始向上滚动，我就会运行一些代码 (OnSwipeUp())。我使用

检测到这一点

textScroll.Scrolled += (sender, e) => { onScrolled(); };

private void onScrolled()
{
      if(textScroll.ScrollY > 0)
      {
          OnSwipeUp(null, null);
      }
}

我还需要检测用户何时开始向下滚动，所以我想获取滚动视图的最大 Y 位置然后说类似

if(textScroll.ScrollY < maxY)

我能做到吗？

【问题讨论】：

标签： c# xamarin.forms scrollview

【解决方案1】：

试试这个：

private double previousScrollPosition = 0;
void Handle_Scrolled(object sender, Xamarin.Forms.ScrolledEventArgs e) 
{

  if (previousScrollPosition < e.ScrollY) 
  {
    //scrolled down
    previousScrollPosition = e.ScrollY;
  } 
  else 
  {
      //scrolled up

    if (Convert.ToInt16(e.ScrollY) == 0)
    previousScrollPosition = 0;

  }
}

【讨论】：

感谢您的发帖，但我认为这将在滚动完成后开始？如果可能的话，我需要在滚动开始时进行锻炼
不，只需将断点放在 if 和 else 块中，一旦滚动开始就会命中。

【解决方案2】：

您不需要获取最大 Y 位置，因为在向下滚动之前，Y 位置始终为 0。因此您只需要获取滚动方向即可。我写了一个演示来证明这一点：

MainPage.xaml.cs

private double previousScrollPosition = 0;

        public MainPage()
        {
            InitializeComponent();
        }

        void Handle_Scrolled(object sender, Xamarin.Forms.ScrolledEventArgs e)
        {
            if (previousScrollPosition < e.ScrollY)
            {
                //scrolled down
                Console.WriteLine("!!!!!!!!!scrolled down   ScrollY=>" + textScroll.ScrollY);
            }
            else
            {
                //scrolled up
                Console.WriteLine("!!!!!!!!!scrolled up   ScrollY=>" + textScroll.ScrollY);
            }
            previousScrollPosition = e.ScrollY;
        }

MainPage.xaml

<ContentPage.Content>
        <StackLayout>
            <BoxView BackgroundColor="Red" HeightRequest="200" WidthRequest="150" />
            <ScrollView x:Name="textScroll" Scrolled="Handle_Scrolled">
                <StackLayout>
                    <Entry Text="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"/>
                    <Entry Text="bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"/>
                    <Entry Text="ccccccccccccccccccccccccccccccccccccccccccccccccccc"/>
                    <Entry Text="ddddddddddddddddddddddddddddddddddddddddddddddddddddddddd"/>
                    <Entry Text="eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee"/>
                    <Entry Text="fffffffffffffffffffffffffffffffffffffffffff"/>
                    <Entry Text="ggggggggggggggggggggggggggggggggggggggggggggg"/>
                    <Entry Text="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"/>
                    <Entry Text="bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"/>
                    <Entry Text="ccccccccccccccccccccccccccccccccccccccccccccc"/>
                    <Entry Text="dddddddddddddddddddddddd"/>
                    <Entry Text="eeeeeeeeeeeeeeeeeeeeeeeeee"/>
                    <Entry Text="fffffffffffffffffffffffffffffffffffffffffff"/>
                    <Entry Text="ggggggggggggggggggggggggggggggggggggggggggggg"/>
                    <Entry Text="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"/>
                    <Entry Text="bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"/>
                    <Entry Text="ccccccccccccccccccccccccccccccccccccccccccccccccccc"/>
                    <Entry Text="ddddddddddddddddddddddddddddddddddddddddddddddddddddddddd"/>
                    <Entry Text="eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee"/>
                    <Entry Text="fffffffffffffffffffffffffffffffffffffffffff"/>
                    <Entry Text="ggggggggggggggggggggggggggggggggggggggggggggg"/>
                    <Entry Text="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"/>
                    <Entry Text="bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"/>
                    <Entry Text="ccccccccccccccccccccccccccccccccccccccccccccc"/>
                    <Entry Text="dddddddddddddddddddddddd"/>
                    <Entry Text="eeeeeeeeeeeeeeeeeeeeeeeeee"/>
                    <Entry Text="fffffffffffffffffffffffffffffffffffffffffff"/>
                    <Entry Text="ggggggggggggggggggggggggggggggggggggggggggggg"/>
                </StackLayout>
            </ScrollView>

        </StackLayout>

    </ContentPage.Content>

【讨论】：