Xamarin.Forms ListView size to content?

问题:

I have a pretty large form (adapted mainly for tablets), that has a TabbedPage nesting a ScrollView and a vertical StackPanel containing many controls.

I have few occurrences where I have a ListView that contains a few single-line items, and I need it to size to content.
I'd like to get rid of its scroll-bars, but anyway I don't want it to take up more space than what's required for its items.
Is there a way (even an ugly one) to achieve that without have to write a renderer x3 platforms?

Here's a pseudo describing my tree:

<ContentPage>
  <MasterDetailPage>
    <MasterDetailPage.Detail>
      <TabbedPage>
        <ContentPage>
          <ScrollView>
            <StackPanel>
              <!-- many controls-->
              <ListView>

When rendered, there is a huge gap coming after the ListView. How can I avoid that?

I tried messing around with the VerticalOptions and HeightRequest, non of which worked.

I'm looking for a dynamic way (preferably without inheritance) to achieve that without involving custom renderers.

回答1:

Based on Lutaaya's answer, I made a behavior that automates this, determining and setting the row-height (Gist).

Behavior:

namespace Xamarin.Forms
{
  using System;
  using System.Linq;
  public class AutoSizeBehavior : Behavior<ListView>
  {
    ListView _ListView;
    ITemplatedItemsView<Cell> Cells => _ListView;

    protected override void OnAttachedTo(ListView bindable)
    {
      bindable.ItemAppearing += AppearanceChanged;
      bindable.ItemDisappearing += AppearanceChanged;
      _ListView = bindable;
    }

    protected override void OnDetachingFrom(ListView bindable)
    {
      bindable.ItemAppearing -= AppearanceChanged;
      bindable.ItemDisappearing -= AppearanceChanged;
      _ListView = null;
    }

    void AppearanceChanged(object sender, ItemVisibilityEventArgs e) =>
      UpdateHeight(e.Item);

    void UpdateHeight(object item)
    {
      if (_ListView.HasUnevenRows)
      {
        double height;
        if ((height = _ListView.HeightRequest) == 
            (double)VisualElement.HeightRequestProperty.DefaultValue)
          height = 0;

        height += MeasureRowHeight(item);
        SetHeight(height);
      }
      else if (_ListView.RowHeight == (int)ListView.RowHeightProperty.DefaultValue)
      {
        var height = MeasureRowHeight(item);
        _ListView.RowHeight = height;
        SetHeight(height);
      }
    }

    int MeasureRowHeight(object item)
    {
      var template = _ListView.ItemTemplate;
      var cell = (Cell)template.CreateContent();
      cell.BindingContext = item;
      var height = cell.RenderHeight;
      var mod = height % 1;
      if (mod > 0)
        height = height - mod + 1;
      return (int)height;
    }

    void SetHeight(double height)
    {
      //TODO if header or footer is string etc.
      if (_ListView.Header is VisualElement header)
        height += header.Height;
      if (_ListView.Footer is VisualElement footer)
        height += footer.Height;
      _ListView.HeightRequest = height;
    }
  }
}

Usage:

<ContentPage xmlns:xf="clr-namespace:Xamarin.Forms">
  <ListView>
    <ListView.Behaviors>
      <xf:AutoSizeBehavior />

回答2:

Ok Assume your ListView is Populated with NewsFeeds, lets use an ObservableCollection to contain our data to populate a ListView as Below :

XAML Code :

<ListView x:Name="newslist"/>

C# Code

ObservableCollection <News> trends = new ObservableCollection<News>();

Then you assign the trends List to the ListView :

newslist.ItemSource = trends;

Then , we have make some Logic on the ListView and the data , So that the ListView Wraps the data , as the data increases the ListView also increases and viceversa :

int i = trends.Count;
int heightRowList = 90;
i = (i * heightRowList);
newslist.HeightRequest = i;

Therefore the complete code is :

ObservableCollection <News> trends = new ObservableCollection<News>();
newslist.ItemSource = trends;
int i = trends.Count;
int heightRowList = 90;
i = (i * heightRowList);
newslist.HeightRequest = i;

Hope it Helps .

回答3:

I may be grossly oversimplifying things here, but just adding HasUnevenRows="True" to the ListView worked for me.

回答4:

I could make a event handler that takes into account the on the changing size of the ListView cells. Here's it:

    <ListView ItemsSource="{Binding Items}"
         VerticalOptions="Start"
         HasUnevenRows="true"
         CachingStrategy="RecycleElement"
         SelectionMode="None" 
         SizeChanged="ListView_OnSizeChanged">
               <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell >
                           <Frame Padding="10,0" SizeChanged="VisualElement_OnSizeChanged">

Frame can be changed by Grid, StackLayout, etc. xaml.cs:

        static readonly Dictionary<ListView, Dictionary<VisualElement, int>> _listViewHeightDictionary = new Dictionary<ListView, Dictionary<VisualElement, int>>();

    private void VisualElement_OnSizeChanged(object sender, EventArgs e)
    {
        var frame = (VisualElement) sender;
        var listView = (ListView)frame.Parent.Parent;
        var height = (int) frame.Measure(1000, 1000, MeasureFlags.IncludeMargins).Minimum.Height;
        if (!_listViewHeightDictionary.ContainsKey(listView))
        {
            _listViewHeightDictionary[listView] = new Dictionary<VisualElement, int>();
        }
        if (!_listViewHeightDictionary[listView].TryGetValue(frame, out var oldHeight) || oldHeight != height)
        {
            _listViewHeightDictionary[listView][frame] = height;
            var fullHeight = _listViewHeightDictionary[listView].Values.Sum();
            if ((int) listView.HeightRequest != fullHeight 
                && listView.ItemsSource.Cast<object>().Count() == _listViewHeightDictionary[listView].Count
                )
            {
                listView.HeightRequest = fullHeight;
                listView.Layout(new Rectangle(listView.X, listView.Y, listView.Width, fullHeight));
            }
        }
    }

    private void ListView_OnSizeChanged(object sender, EventArgs e)
    {
        var listView = (ListView)sender;
        if (listView.ItemsSource == null || listView.ItemsSource.Cast<object>().Count() == 0)
        {
            listView.HeightRequest = 0;
        }
    }

When Frame is displaying (ListView.ItemTemplate is applying), size of frame changing. We take it's actual height via Measure() method and put it into Dictionary, which knows about current ListView and holds Frame's height. When last Frame is shown, we sum all heights. If there's no items, ListView_OnSizeChanged() sets listView.HeightRequest to 0.