如何让 ScrollView 的 Scrolled 事件只响应 Xamarin.Forms 中的用户操作？答案

【问题标题】：How to make ScrollView's Scrolled event only respond to user actions in Xamarin.Forms？如何让 ScrollView 的 Scrolled 事件只响应 Xamarin.Forms 中的用户操作？
【发布时间】：2021-09-16 05:02:53
【问题描述】：

我有一个 ScrollView，它使用代码来控制自动滚动。我需要在用户输入时停止自动滚动，但是 ScrollView 的ScrollToAsync 方法也会触发Scrolled 事件。

   <StackLayout>
            <ScrollView Scrolled="ScrollView_Scrolled">
                <StackLayout></StackLayout>
            </ScrollView>
   </StackLayout>

【问题讨论】：

标签： c# xamarin xamarin.forms scrollview

【解决方案1】：

简短的回答

您可以使用一个标志来告诉事件是谁触发了滚动事件，如下所示

Boolean scrolledByUser = true;

private async void MyScrollView_Scrolled(object sender, ScrolledEventArgs e)
{
  if (scrolledByUser)
  {
    // do something
  }
  else // scrolled automagically
  {
    // do something else
  }
}

private async void ScrollAutomagically(object sender, EventArgs e)
{
  scrolledByUser = false;
  await MyScrollView.ScrollToAsync(ScrollView.X, 800, true);
  scrolledByUser = true;
}

长答案

接下来你可以找到一个完整的示例来说明我的建议。

您从一个新的空白项目开始，然后修改 App.xaml.cs 使其看起来像

App.xaml.cs

using Xamarin.Forms;

namespace scrollviewPrompt
{
    public partial class App : Application
    {
        public App()
        {
            InitializeComponent();

            MainPage = new NavigationPage(new MainPage());
        }
    }
}

然后继续修改 MainPage.xaml 以包含一个 ScrollView，其中包含许多垂直的 BoxView，一个在另一个之上： p>

MainPage.xaml

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="scrollviewPrompt.MainPage">

    <ContentPage.ToolbarItems>
        <ToolbarItem Text="ScrollTo"
                     Clicked="ScrollAutomagically"/>
    </ContentPage.ToolbarItems>

    <ScrollView x:Name="MyScrollView" 
                    Scrolled="MyScrollView_Scrolled">
        <StackLayout>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
            <BoxView BackgroundColor="Red"         HeightRequest="128"/>
            <BoxView BackgroundColor="Orange"      HeightRequest="128"/>
            <BoxView BackgroundColor="Blue"      HeightRequest="128"/>
            <BoxView BackgroundColor="Green"      HeightRequest="128"/>
            <BoxView BackgroundColor="Black"      HeightRequest="128"/>
            <BoxView BackgroundColor="AliceBlue"         HeightRequest="128"/>
            <BoxView BackgroundColor="Brown"      HeightRequest="128"/>
            <BoxView BackgroundColor="Pink"      HeightRequest="128"/>
            <BoxView BackgroundColor="White"      HeightRequest="128"/>
            <BoxView BackgroundColor="Yellow"      HeightRequest="128"/>
        </StackLayout>
    </ScrollView>
</ContentPage>

最后添加事件处理程序

using System;
using Xamarin.Forms;

namespace scrollviewPrompt
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }

        Boolean handled = false;

        Boolean scrolledByUser = true;

        private async void MyScrollView_Scrolled(object sender, ScrolledEventArgs e)
        {
            if (!handled)
            {
                handled = true;

                if (scrolledByUser)
                {
                    await DisplayAlert("Alert", "ScrollView Scrolled by user!", "Ok");
                }
                else
                {
                    await DisplayAlert("Alert", "ScrollView Scrolled automagically!", "Ok");
                }
                handled = false;
            }
        }

        private async void ScrollAutomagically(object sender, EventArgs e)
        {
            scrolledByUser = false;
            await MyScrollView.ScrollToAsync(MyScrollView.X, 800, true);
            scrolledByUser = true;
        }
    }
}

希望这会有所帮助！

【讨论】：