How to invoke async-operation on two-way bound Combobox (WPF) - wpf

Question

What's the appropriate way to handle running an async operation when an item is selected from a two-way bound control such as a combobox (wpf data binding)?
When I have a two-way binding property (e.g. SelectedValue on ComboBox) I don't think I can use Stephen Cleary's NotifyTaskCompletion because when a user selects a value from the dropdown, the ComboBox itself would need to modify the bound Result property, which is the Task's result.
The only viable solution I've come up with is calling an async Task -method from the databound setter without awaiting the result. This should be fine as long as the async-method triggers a property changed event for whatever ui-related stuff is being done, and that any exceptions are picked up and propagated to the ui accordingly, right?
I assume this would be a common case in async WPF applications. How do you guys approach this?
My solution so far:
<ComboBox
ItemsSource="{Binding PossibleItems}"
DisplayMemberPath="Name"
SelectedValue="{Binding SelectedItem}"/>
...
public Item SelectedItem
{
get { return m_selectedItem; }
set
{
m_selectedItem = value;
OnPropertyChanged();
InitializeAsyncAndFirePropertyChanged(); // async Task method not awaited - gives compiler warning CS4014
}
}
public async Task InitializeAsyncAndFirePropertyChanged(ObservableCollection<RFEnvironment> possibleRfEnvironments)
{
//should check this method for exceptions and propagate them to the UI via databinding
OtherDataBoundProperty = await GetSomeStringFromWebAsync();
}
public string OtherDataBoundProperty
{
get { return m_otherDataBoundProperty; }
set
{
m_otherDataBoundProperty = value;
OnPropertyChanged();
}
}
Note: I have found similar questions asked, but none that addresses two-way bindings on controls such as a Combobox.

Answer 1

If you are using a functional reactive MVVM framework such as ReactiveUI, you would simply observe the SelectedItem property and kick off any operation you want when the property is set. e.g.:
this.WhenAnyValue(x => x.SelectedItem)
.Subscribe(async _ => await InitializeAsyncAndFirePropertyChanged());
A property itself should ne be kicking off background operations, but a view model may do this when a property is set.
Please refer to the docs for more information: https://reactiveui.net/docs/concepts/.

Answer 2

I have a similar issue with async call in property setter when using WCF databinding.
My solution is slightly better, because in your case when an exception occurs in InitializeAsyncAndFirePropertyChanged, no exception is thrown nor caught. The modified code is below. It uses task continuation to throw an exception and raising OnPropertyChanged. OnPropertyChanged call can stay in an original place, it depends on your needs.
public class MyViewModel: INotifyPropertyChanged
{
private readonly TaskScheduler _uiScheduler;
public MyViewModel()
{
_uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();
}
public Item SelectedItem
{
get { return m_selectedItem; }
set
{
m_selectedItem = value;
InitializeAsyncAndFirePropertyChanged()
.ContinueWith(t =>
{
if (t.Exception != null)
{
throw t.Exception;
}
OnPropertyChanged();
}, _uiScheduler);
}
}
public async Task InitializeAsyncAndFirePropertyChanged(ObservableCollection<RFEnvironment> possibleRfEnvironments)
{
//should check this method for exceptions and propagate them to the UI via databinding
OtherDataBoundProperty = await GetSomeStringFromWebAsync();
}
public string OtherDataBoundProperty
{
get { return m_otherDataBoundProperty; }
set
{
m_otherDataBoundProperty = value;
OnPropertyChanged();
}
}
.... other code to support INotifyPropertyChanged
}