MVVM - long-running operation without async-await

Question

I want to start long-running operation like requesting a web page from ViewModel, and perform some progress-update operations on my View. Before, I easily achieved this by awaiting my Model's async methods, but in current project I'm restricted with .NET 4.0, so I can't use C#5 features.

What is the recommended way of doing this?

so you have to deal with Threading traditionally. In simple cases, it's fairly easy to use. — King King, Nov 17 '14 at 14:29
You don't have to change your code http://stackoverflow.com/questions/19421878/how-can-i-use-the-async-keywords-in-a-project-targeting-net-4-0/19421907#19421907 — Moti Azu, Nov 17 '14 at 14:29
http://pmichaels.net/2014/08/03/updating-a-progress-bar-using-mvvm-cross/ — Paul Michaels, Nov 17 '14 at 14:30
@MotiAzu Model code is not written yet, so it's another case — astef, Nov 17 '14 at 14:32
I'd recommend [Microsoft.Bcl.Async](http://www.nuget.org/packages/Microsoft.Bcl.Async/). — Stephen Cleary, Nov 17 '14 at 14:34
@astef Doesn't matter, just write it the way you know and you can use `async/await`. Stephen's link goes to the same nuget. — Moti Azu, Nov 17 '14 at 14:35

Mayur Dhingra · Answer 1 · 2014-11-18T07:54:33.013

1

Use this -

Task.Factory.StartNew(() => 
{
   // Code to load web page or any code you want to run asynchronously
}).ContinueWith(task => 
{
   // Code you want to execute on completion of the above synchronous task,
}, UIHelper.GetUITaskScheduler());

wherein the UIHelper class has the following static method -

public class UIHelper
{
   /* Some other methods */

   public static TaskScheduler GetUITaskScheduler()
   {
      TaskScheduler scheduler = null;
      Application.Current.Dispatcher.Invoke(() =>
      {
         scheduler = TaskScheduler.FromCurrentSynchronizationContext();
      });
      return scheduler;
   }    
}

edited Nov 18 '14 at 07:54

answered Nov 17 '14 at 14:43

Mayur Dhingra

1,527
10
27

In this way I can't update my View's controls. They should be updated from UI's thread and ViewModel must not depend on UI classes – astef Nov 17 '14 at 14:45
1

You can update the controls if you invoke the update methods via the ui dispatcher – stijn Nov 17 '14 at 14:54
I can, but it is dirty way. ViewModel becomes untestable and you have no reason to docouple it from View – astef Nov 17 '14 at 15:00
1

You can pass in [TaskScheduler.FromCurrentSynchronizationContext()](http://msdn.microsoft.com/en-us/library/system.threading.tasks.taskscheduler.fromcurrentsynchronizationcontext%28v=vs.110%29.aspx) to cause the continuation to dispatch to the UI thread (assuming you get the TaskScheduler on the UI thread of course.) – Dan Bryant Nov 17 '14 at 15:43
@astef INPC bindings automatically marshall event calls onto the UI thread. Don't believe me? Try it. Alternatively, you can provide a Dispatcher to the ViewModel on which it executes its code. You can provide this in tests. It isn't hard to do. – Nov 17 '14 at 15:44
@astef I agree with you that it should be updated from the UI thread, and as stijn said, you can provide UI thread dispatcher to it for the continouation task. I have updated my answer for it. – Mayur Dhingra Nov 18 '14 at 07:25

score 0 · Answer 2 · answered Nov 17 '14 at 15:40

0

use RX https://rx.codeplex.com/

You can also observe on the SyncronizationContext.Current

Check them out...

answered Nov 17 '14 at 15:40

fattikus

542
1
4
15

MVVM - long-running operation without async-await

2 Answers2