如何将 ListView 设置为开始在 Xamarin Forms 中显示最后一个项目? [英] How to set ListView to Start showing the last Item instead in Xamarin Forms?

问题描述

我有一个 ListView 正在处理的项目列表.默认情况下，ListView 显示从开始到底部(滚动).

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

如何将 ListView 设置为从底部开始?

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

用法:聊天消息视图 - 其中我需要显示聊天的最后一条消息并滚动到该消息.

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

推荐答案

您可以在 ListView 中使用 ScrollTo 滚动到您设置的任何位置.您需要覆盖 OnAppearing 方法.这是滚动到 ListView ViewModel.Messages 末尾的示例:

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);
                });
            }
        };
    }

然后在每次需要向下滚动时调用 RefreshScrollDown(即 System.Action)，例如当您收到新消息或加载聊天时.

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

ViewModel 中的RefreshScrollDown:

public System.Action RefreshScrollDown;

您可以像这样在代码中隐藏您的 ViewModel:

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

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注意:使用固定的 ListView 高度时存在一个错误.在更改HeightRequest 时，ScrollTo 仍然使用列表的原始高度来计算它滚动到的位置.当您更改 HeightRequest 中的值时，原始高度不会更新.要解决此问题:

---

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();

            }
        }  

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