Xamarin iOS自定义ListView

Question

I'm having trouble with the size of a ListView within a StackedLayout . I want the ListView to take only as much space as necessary by the contained items. However, my ListView almost always takes up all the space there is, even if it only contains like 3 items or so.

Is there a way to create a custom DynamicListView and override certain methods calculating a custom size whenever items are added or removed? For example, I want the list to disappear whenever it's empty. Can anyone tell me, which methods to override and/or how to react on items being added/removed to the underlying model?

My gut feeling tells me something like this:

namespace Foo.Controls
{
    public class SpottedView : ListView
    {
        public SpottedView()
        {
        }

        protected override void OnChildAdded(Element child)
        {
            Console.WriteLine($"{Util.methodName()}");
            base.OnChildAdded(child);
            InvalidateMeasure();
        }

        protected override void OnChildRemoved(Element child)
        {
            Console.WriteLine($"{Util.methodName()}");
            base.OnChildRemoved(child);
            InvalidateMeasure();
        }
    }
}

Answer 1

I found a solution to this. I changed from StackLayout to Grid as follows:

<ContentPage.Content>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>

        <ctrl:CameraView Grid.Row="0" />
        <ctrl:SpottedView Grid.Row="1" RowHeight="40" />

    </Grid>
</ContentPage.Content>

And then I added a custom ListView , a SpottedView as follows:

namespace Foo.Controls
{
    public class SpottedView : ListView
    {
        ObservableCollection<string> items;

        public SpottedView() : base()
        {
            items = new ObservableCollection<string>();
            ItemsSource = items;
            items.CollectionChanged += onItemsChanged;

            // TODO: test purposes, remove later
            var autoEvent = new AutoResetEvent(false);
            var stateTimer = new Timer(onTimer, autoEvent, 1000, 2000);
        }

        void onItemsChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            MainThread.BeginInvokeOnMainThread(() =>
            {
                HeightRequest = RowHeight * items.Count;
                InvalidateMeasure();
            });
        }

        // TODO test purposes, remove later.
        void onTimer(object state)
        {
            MainThread.BeginInvokeOnMainThread(() =>
            {
                if (items.Count < 2)
                {
                    items.Add("Blah");
                }
                else
                {
                    items.Clear();
                }
            });
        }
    }
}

Works like a charm.