Xamarin.Forms：如何获取引用绑定上的属性通知

Question

Consider the following pseudo code.

ViewModel:

public class ViewModel
{
    IEnumerable<string> _AvailableNames
    public IEnumerable<string> AvailableNames
    {
        get => _AvailableNames;
        set => SetProperty(ref _AvailableNames, value); //Notifies
    }
}

View:
ContentPage 's ViewModel is a ViewModel instance set by dependency injection.

<ContentPage x:Name="page" prism:ViewModelLocator.AutowireViewModel="True">
    <Element BindingContext="{Binding SelectedElement}">
        <ListView ItemsSource="{Binding BindingContext.AvailabeNames, Source={Reference page}}" />
    </Element>
</ContentPage>

The AvailableNames property gets updated on demand, but doesn't get reflected in the UI. I suspect it's because the bound property is BindingContext.AvailableNames , is that so? I mean it binds to the AvailableNames property, but doesn't get updated when the property changes.

Note, I tried making AvailableNames a get-only ObservableCollection but adding values to it doesn't reflect to the page either.

I understand that this is because the ItemsSource property is bound to the BindingContext property of the usersPage , is there a way to overcome this (preferably a clean one...)?

Answer 1

For my scenario, it was enough to set back the BindingContext to the upper property (ie the ViewModel), this will be a limitation in some other scenarios, for instance if I'm binding from within a ListView etc.

So the solution is to set it as follows:

<ListView
  BindingContext="{Binding BindingContext, Source={Reference usersPage}}"
  ItemsSource="{Binding AvailabeNames}"/>

In some scenarios, another option would be a better solution, wrap it with a dummy control like so:

<ContentView BindingContext="{Binding BindingContext, Source={Reference usersPage}}">
  <ListView ItemsSource="{Binding AvailableItems}"/>
</ContentView>

I don't know why, but for some controls expecting some value other than string (ie Switch ) I can't seem to bind this even with the converter as the value doesn't update. The solution to that is use a dummy control getting the string representation of the value.
In my case I'm using the MultiBinding class, that even its converter always returns bool (otherwise will internally throw a NullReferenceException and will display nothing), still doesn't update itself properly. With a 3rd party property it works well:

<Label x:Name="dummyLabel" Text="{MultiBinding}"/>
<Switch IsToggled="{Binding Text, Source={Reference dummyLabel}}"/>

Obviously I'm not a fan of this hacky and ugly code, but that's the only way I found that works.