MVP: Updating View’s Binding Asynchronously with the SynchronizationContext Object

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Introduction

I was tasked with a small WinForms application a while back which contained a long running calculation. The user would initiate the calculation by pressing a button and when the calculation was complete, the result would then be displayed in a textbox. I also wanted to build the application using different layers so that each layer could vary independently.

Details

I decided to implement this application using the Model-View-Presenter (MVP) pattern. I would set the long running process to run on a different thread (of course) and when the calculation was complete, I would have the presenter update the view. Sounds easy, right? Let’s see.

I used basic data binding to bind a textbox on the view which would display the results:

txtTotal.DataBindings.Add("Text", this, "Total");

The Windows Form itself is the view (implements the IView interface), so this is actually a self binding.

The Problem

Now here's the issue. If the ‘Total’ variable was updated on the UI thread, the data binding worked fine and the result was displayed in the textbox. However because I wanted the long running task to be executed on a different thread, when I updated the view from the presenter in a different thread, I received the following error:

'Cross-thread operation not valid: Control 'txtTotal' accessed from a thread other than the thread it was created on'

Every developer has seen this error before and hates it with every 'fiber' of his/her being. No pun intended. Data binding in .NET does not allow updating of data from a different thread. So how can we solve this?

The Solution

There are quite a few solutions to this issue like the BackgroundWorker component. However the solution I chose involved the SynchronizationContext object (which a BackgroundWorker uses behind the scenes. Each thread has SynchronizationContext associated with it. You can use this context to execute code using a specified thread either using the Post (asynchronous) or Send (synchronous) methods.

To get a reference to the SynchronizationContext, you can use the following code:

SynchronizationContext.Current;

I simply use the view’s SynchronizationContext object whenever I update any databinded property on the view from the presenter. Here is my code for the presenter:

 internal class Presenter : IPresenter
  {
    public Presenter(IView view)
    {
        this.View = view;

        //Keep a reference to the view's SynchronizationContext
        this.Context = this.View.Context;

        //Subscribe to the view's events
        this.View.OnLongRunningCalculationRequested += 
		new EventHandler(View_OnLongRunningCalculationRequested);
    }

    private IView View
    {
        get;
        set;
    }

    private SynchronizationContext Context
    {
        get;
        set;
    }

    private void View_OnLongRunningCalculationRequested(object sender, EventArgs e)
    {
        Thread thread = new Thread(RunLongRunningProcess);
        thread.Start();
    }

    private void RunLongRunningProcess()
    {
        //Simulate a long running process
        if (Context != null)
        {
            ///Perform intense or long running calculations here
            decimal data = 1000 + 10;


            //But update the view using the SynchronizationContext here 
            Context.Post(delegate
                         {
                             //The Post method sends this message asynchronously
                             View.Total = data;

                         }, null);    
        }
    }
  }

Notice I update the Total property of the view using its own SynchronizationContext. That means the actual delegate gets called from the UI thread. And now, the view's binding to the 'Total' property works as expected.

Have fun and send any comments my way.

History

• 15^th March, 2010: Initial post