BindableProperty：类型不匹配

Question

I want to extend the Xamarin.Forms Picker so I can bind a collection to it. After a quick search I found this page: Picker Example with two great examples. Refusing to just copy&paste the code (for learning purposes) I went on and make my own based on the two examples.

It's almost identical except mine does not work.

When I do not provide a collection to the ItemsSource everything works fine. Whenever I do assign a collection I get the following error:

Xamarin.Forms.Xaml.XamlParseException: Position 9:32. Cannot assign property "ItemsSource": type mismatch between "Xamarin.Forms.Binding" and "System.Collections.IEnumerable"

Picker:

public class BindablePicker : Picker
{
    private static readonly BindableProperty ItemsSourceProperty =
        BindableProperty.Create("ItemsSource", typeof(IEnumerable), typeof(BindablePicker), null, propertyChanged: OnItemsSourceChanged);

    private static readonly BindableProperty SelectedItemProperty =
        BindableProperty.Create("SelectedItem", typeof(object), typeof(BindablePicker));

    public IEnumerable ItemsSource
    {
        get { return (IEnumerable)GetValue(ItemsSourceProperty); }
        set { SetValue(ItemsSourceProperty, value); }
    }
    public object SelectedItem
    {
        get { return GetValue(SelectedItemProperty); }
        set { SetValue(SelectedItemProperty, value); }
    }
    public string DisplayMember { get; set; }


    private static void OnItemsSourceChanged(BindableObject bindable, Object oldValue, Object newValue)
    {
        var newval = newValue as IEnumerable; //Had to implement this because of the non-generic .Create() method expects Object as param.
        var picker = bindable as BindablePicker;

        if (picker != null)
        {
            picker.Items.Clear();
            if (newval == null) return;

            foreach (var item in newval)
            {
                if (string.IsNullOrEmpty(picker.DisplayMember))
                {
                    picker.Items.Add(item.ToString());
                }
                else
                {
                    var prop = item.GetType()
                        .GetRuntimeProperties()
                        .FirstOrDefault(p => string.Equals(p.Name, picker.DisplayMember, StringComparison.OrdinalIgnoreCase));

                    picker.Items.Add(prop.GetValue(item).ToString());
                }
            }
        }
    }

    private void OnSelectedIndexChanged(object sender, EventArgs args)
    {
        if (SelectedIndex < 0 || SelectedIndex > Items.Count - 1)
        {
            SelectedItem = null;
        }
        else
        {
            SelectedItem = ItemsSource.ItemOf(SelectedIndex); //ItemOf is an extension method I made for IEnumerable (has to be tested).
        }
    }
}

ViewModel (parts):

public class HomePageViewModel : ViewModelBase
{
    //In the app this is populated with a List<Person>.
    public IEnumerable<Person> People { get; set; }
}

XAML:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:controls="clr-namespace:TestApp.Controls;assembly=TestApp"
         x:Class="TestApp.Views.HomePage">
  <ContentPage.Content>
     <StackLayout>

         <controls:BindablePicker ItemsSource="{Binding People}"
                                  HorizontalOptions="Center" />
     </StackLayout>
  </ContentPage.Content>
</ContentPage>

Note that the first example picker from the linked page works with the provided VM/View setup.

I'm also not finished with the picker, I still want to provide TwoWay binding to the SelectedItem property and support for ObservableCollection.

Yours,

Answer 1

At first I thought I was turning mad.

Then I thought something was seriously broken.

But finally I figured it out...

private static readonly BindableProperty ItemsSourceProperty =
    BindableProperty.Create("ItemsSource", typeof(IEnumerable),  
    typeof(BindablePicker), null, propertyChanged: OnItemsSourceChanged);

In order for the Xaml parser to see a BindableProperty as such, it has to be public (and static , but you got that part right).

In your case, the Xaml parser doesn't see the BindableProperty , so it fallback to the property, but it doesn't have any way to set the Binding , and as the types doesn't match, you get the exception.

Change your code to

public static readonly BindableProperty ItemsSourceProperty =
    BindableProperty.Create("ItemsSource", typeof(IEnumerable),  
    typeof(BindablePicker), null, propertyChanged: OnItemsSourceChanged);