如何将ListView设置为开始在Xamarin Forms中显示最后一个项目？

Question

I have a list of Items being handled by the ListView. By default, the ListView shows the from start to bottom (scrolling).

How do I set the ListView to start at the bottom instead?

Usage: Chat Messages View - Wherein I need to show the last message of the chat and scroll to that.

Answer 1

You can use ScrollTo in a ListView to scroll to a any position you set. You need to overwrite the OnAppearing method. This is an example for scrolling to the end of the ListView ViewModel.Messages:

protected override void OnAppearing()
    {
        base.OnAppearing();

        ViewModel.RefreshScrollDown = () => {
            if (ViewModel.Messages.Count > 0) {
                Device.BeginInvokeOnMainThread (() => {

                    ListViewMessages.ScrollTo (ViewModel.Messages [ViewModel.Messages.Count - 1], ScrollToPosition.End, true);
                });
            }
        };
    }

Then just call RefreshScrollDown (which is System.Action ) every time you need to scroll down, eg when you receive a new message or when you load the chats.

RefreshScrollDown in ViewModel:

public System.Action RefreshScrollDown;

You can get your ViewModel in code behind like this:

private MessagePhonePageViewModel ViewModel {
    get { return BindingContext as MessagePhonePageViewModel;}
}

---

NOTE: There is a bug when using a fixed ListView height. When changing the HeightRequest , ScrollTo still uses the original height of the list to calculate where it scrolls to. The original height is not updated when you change the value in HeightRequest . To fix this issue:

 protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);
            if (e.PropertyName == Xamarin.Forms.ListView.HeightRequestProperty.PropertyName)
            {
                Control.LayoutParameters.Height =(int)(sender as Xamarin.Forms.ListView).HeightRequest;
                Control.RequestLayout();

            }
        }