MVVM和异步属性

Question

I have a ViewModel with the following property

public Employee SelectedEmployee
        {
            get { return _selectedEmployee; }
            set
            {
                if (value == _selectedEmployee) return;
                _selectedEmployee = value;

                if (_selectedEmployee != null)
                {
                    StaffHolidaysViewModel.HolidayAllowance = _staffDataService.EmployeeHolidayAllowance(_selectedEmployee.Id);
                    FireEmployeeSelectedMessage(SelectedEmployee.Id);
                }

                RaisePropertyChanged();
                RaisePropertyChanged(nameof(Allowance));
                RaisePropertyChanged(nameof(Taken));
                RaisePropertyChanged(nameof(Remaining));
                RaisePropertyChanged(nameof(TotalAbsences));
                RaisePropertyChanged(nameof(TotalSick));
                RaisePropertyChanged(nameof(TotalNonSickAbsences));
                RaisePropertyChanged(nameof(SelectedEmployeeLeavingDate));
                UpdateCanExecuteChanged();
            }
        }

The line

StaffHolidaysViewModel.HolidayAllowance = _staffDataService.EmployeeHolidayAllowance(_selectedEmployee.Id);

Contains a call to a method that I'd like to make asynchronous. I don't know how I can do this as SelectedEmployee is a bound property.

<ComboBox Name="StaffMembers" ItemsSource="{Binding FilteredEmployees}" SelectedItem="{Binding SelectedEmployee}" Width="200" BorderThickness="1" BorderBrush="DimGray">
                    <ComboBox.ItemTemplate>
                        <DataTemplate>
                            <TextBlock Text="{Binding Name}"/>
                        </DataTemplate>
                    </ComboBox.ItemTemplate>
                </ComboBox>

Since it's a property, I can't make its type Task<Employee>

What's the recommended approach to async bound MVVM properties? Or is it just not possible?

Answer 1

it's technically not the idea approach to have a property perform Asynchronous Tasks as they need to be returning a value.

Why? let's look at it from another point of view:

Asynchronous means that the UI thread will not stop (UI won't hang while processing) while the main processing thread would wait for the operation to finish and get the results (considering you're wana be using an await ).

When you define a binding, the UI is directly dependant on the property for data which is using await that means the UI will have to wait (which cannot happen with async operations) and hence properties can't be made async and because of which you can't use await .

What all work arounds I have used uptil now:

1. There is a property of an async Task returning method called as .Result . I have tried it, it doesn't really provide with the best outputs.
2. You can make your method return an IAsyncOperation<> instead of Task<> . This will give you the advantage of calling the the .GetResults() method which will get you the results and will not freeze your UI thread. More information on it here . Use this if you don't have any post result fetching operation to perform.
3. Finally, this option is my most used one, I create an async method returning Task<> . I perform all my operations there and verify results. Once I have the results I need, I simply update the property and call RaisePropertyChanged . This ways I can handle other view processing or intermediate loading screens as well without having to be limited with not being able to stop the processing thread.

---

Bottom Line

Use either of the three methods I defined on top. they're all good. below are the summary points for use cases considering as an example getting a score form server of a game:

1. Use IAsyncOperation<> when you don't want to stop any threads and just define the source of the data for the property. The Code processing will not stop or wait for the process to get the results. eg: you just wana fetch the score and display it.
2. Use Task<> returning method if you want the code to wait until the task has been completed to perform other operations on a result. eg: you want to convert the integer value to a percentage once the score is calculated.
3. Use .Result but it hasn't worked for me ever.

If there is anything else feel free to mention in the comments

Answer 2

Contains a call to a method that I'd like to make asynchronous.

I have an article on async MVVM data binding . Essentially, what I usually do is create a data-bindable Task - what I call NotifyTaskCompletion in the article, shortened to NotifyTask in my updated library .

You can use it by changing the type of StaffHolidaysViewModel.HolidayAllowance from whatever it is (I'll call it THolidayAllowance ) to a NotifyTask<T> wrapper (eg, NotifyTask<THolidayAllowance> ).

Then, you can set it synchronously:

StaffHolidaysViewModel.HolidayAllowance = NotifyTask.Create(
    _staffDataService.EmployeeHolidayAllowanceAsync(_selectedEmployee.Id));

What you're doing here is starting EmployeeHolidayAllowanceAsync and then wrapping its Task with a NotifyTask . This is done synchronously, so it's possible to do this in a property setter.

Then, your data binding also needs to be updated. NotifyTask<T> has several properties that you can use . The most obvious is Result , which will eventually hold the result of EmployeeHolidayAllowanceAsync (it returns a default value until EmployeeHolidayAllowanceAsync completes). There's also IsNotCompleted and IsFaulted , which you'll want to use to indicate to the user that the data hasn't arrived yet or that there was some error.