区分加载到列表项与滚动事件Xamarin表单ListView的项目

Question

This is my code that can successfully detect scroll up or down:

 MyListView.ItemAppearing += async (object sender, ItemVisibilityEventArgs e) =>
        {
            var currentIdx = CurrentList.IndexOf((MyClass)e.Item);

            if (currentIdx > _lastItemAppearedIdx)
                ShowChopped();
            else
                ShowFull();

            _lastItemAppearedIdx = CurrentList.IndexOf((MyClass)e.Item);
        };

What is working is the following: Items get added to the list, then once i start scrolling it works fine where ShowChopped and ShowFull are methods with animations that just makes a simple animation to either half the size of an object or make it full. This works fine, but if i however click a new category that changes the content in the list, ItemAppearing gets triggered of course and ShowChopped and ShowFull are called even though i only want it called during a scrollevent.

How would i be able to differentiate a scroll to a item collection change? I have only tried this on iOS.

Updated code:

public class ListView_iOS : ListViewRenderer
{
    private IDisposable _offsetObserver;
    private double _prevYOffset;
    private IListView _myListView;

    protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.ListView> e)
    {
        base.OnElementChanged(e);

        if (e.NewElement is IListView)
            _offsetObserver = Control.AddObserver("contentOffset",
                         Foundation.NSKeyValueObservingOptions.New, HandleAction);
    }

    private static bool CloseTo(double x, double y)
    {
        return Math.Abs(x - y) < 0.1;
    }

    private void HandleAction(Foundation.NSObservedChange obj)
    {
        var effectiveY = Math.Max(Control.ContentOffset.Y, 0);
        if (!CloseTo(effectiveY, _prevYOffset) && Element is IListView)
        {
            var myList = Element as IListView;
            myList.IsScrolling = true;
        }
    }
}

Answer 1

You can differentiate items loading from list scrolling by

1 adding the code if (EmployeeView.IsScrolling) within ItemAppearing method.

2 adding the code EmployeeView.IsScrolling = false; within any function you write to change the appearing of items without scrolling action, for example, when you add items or change category.

And the EmployeeView.IsScrolling value is set from listview renderer.

So the code is like:

NativeListView.cs

public class NativeListView : ListView
    {
        public static readonly BindableProperty
            IsScrollingProperty =
                BindableProperty.Create(nameof(IsScrolling),
                typeof(bool), typeof(NativeListView), false);

        public bool IsScrolling
        {
            get { return (bool)GetValue(IsScrollingProperty); }
            set { SetValue(IsScrollingProperty, value); }
        }
    }

NativeAndroidListViewRenderer.cs

[assembly: ExportRenderer(typeof(NativeListView), typeof(NativeAndroidListViewRenderer))]
namespace App2.Droid
{
    public class NativeAndroidListViewRenderer : ListViewRenderer
    {
        public NativeAndroidListViewRenderer(Context context) : base(context)
        {
        }

        protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.ListView> e)
        {
            base.OnElementChanged(e);

            if (e.NewElement is NativeListView)
                Control.Scroll += Control_Scroll;
        }
        private void Control_Scroll(object sender, AbsListView.ScrollEventArgs e)
        {
            var myList = Element as NativeListView;
            myList.IsScrolling = true;
        }

    }
}

NativeiOSListViewRenderer.cs

    private IDisposable _offsetObserver;
    private double _prevYOffset;
    protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.ListView> e)
    {
        base.OnElementChanged(e);

        if (e.NewElement is NativeListView)
            _offsetObserver = Control.AddObserver("contentOffset",
                         Foundation.NSKeyValueObservingOptions.New, HandleAction);
    }

    private void HandleAction(Foundation.NSObservedChange obj)
    {
        var effectiveY = Math.Max(Control.ContentOffset.Y, 0);
        if (!CloseTo(effectiveY, _prevYOffset) && Element is NativeListView)
        {
            var myList = Element as NativeListView;

            myList.IsScrolling = true;
            _prevYOffset = effectiveY;
        }
    }
    private static bool CloseTo(double x, double y)
    {
        return Math.Abs(x - y) < 0.1;
    }
    protected override void Dispose(bool disposing)
    {
        base.Dispose(disposing);
        if (disposing && _offsetObserver != null)
        {
            _offsetObserver.Dispose();
            _offsetObserver = null;
        }
    }

MainPage.xaml.cs

namespace App2
{
    public partial class MainPage : ContentPage
    {
        ObservableCollection<String> employeeList = new ObservableCollection<String>();
        int count = 0;

        public MainPage()
        {
            InitializeComponent();
            AddButtion.Clicked += AddButtion_Clicked;
            DelButtion.Clicked += DelButtion_Clicked;
            EmployeeView.ItemsSource = employeeList;

            EmployeeView.ItemAppearing += async (object sender, ItemVisibilityEventArgs e) =>
            {
                if (EmployeeView.IsScrolling) {
                    await DisplayAlert("ItemAppearing", e.Item + " row is appearing", "OK");
                    Console.WriteLine("ItemAppearing!!!!!!!!!!");
                }
            };

        }

        private void AddButtion_Clicked(object sender, EventArgs e)
        {
            employeeList.Add("Mr. Mono"+ count++);

            EmployeeView.IsScrolling = false;
        }

        private void DelButtion_Clicked(object sender, EventArgs e)
        {
            if (employeeList.Count > 0) {
                employeeList.RemoveAt(0);
            }
            EmployeeView.IsScrolling = false;

        }
    }
}