Question

I have an example here that replicates what I am trying to accomplish.
As the following code will show – I have ViewModel that updates a ObservableCollection Property bound to a View. Normally I would update the collection from a result retrieved from a Model, but hopefully this example will suffice.

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading;
using System.Windows.Controls;

namespace MVVMWpf.ViewModel
{
    public class ListViewModel
    {

        public ObservableCollection<int> SomeObjectCollection { get; set; }

        public ListViewModel()
        {

            SomeObjectCollection = new ObservableCollection<int>();

        }

        public void Do()
        {
             for (int i = 1; i < 1000000; i++)
             {
                 int i1 = i;
                 SomeObjectCollection.Add(i1);
             }
        }

    }
}

Unfortunately, this blocks this UI. It will only update the View when loop runs to completion. The way I solved it breaks MVVM concepts. That’s why I need your help. I did it this way.

public class ListViewModel
{
    private delegate void LongRunningProcess();
    public ObservableCollection<int> SomeObjectCollection { get; set; }
    private ListBox listBox;
    public ListViewModel(ListBox listBox)
    {
        this.listBox = listBox;
        SomeObjectCollection = new ObservableCollection<int>();

    }

    public void Do()
    {
        Thread thread = new Thread(() =>
        {
           for (int i = 1; i < int.MaxValue; i++)
           {
               int i1 = i;
               listBox.Dispatcher.Invoke(
                   new LongRunningProcess(() =>
                   SomeObjectCollection.Add(i1);
                 }});

        thread.SetApartmentState(ApartmentState.STA);
        thread.Start();
    }

}

As you can see the ViewModel now knows about the listBox element from the UI. And looking at the MVVM diagram only the View should have a reference to the ViewModel through binding. How does one overcome this issue?
Thanks.

Answer 1

You need to let your loop unleash updates to the screen – some kind of DoEvents() will do:

public static void DoEvents() 
{ 
    Application.Current.Dispatcher.Invoke(
    DispatcherPriority.Background,new Action(delegate { })); 
}

Add it and call that from within your loop.

---

Using timer as another option, your code should look somthing like that:

private System.Timers.Timer operationsTimer = new System.Timers.Timer();
private int x;

in your ctor:

operationsTimer.Elapsed += new System.Timers.ElapsedEventHandler 
(operationsTimer_Elapsed);
operationsTimer.Enabled = true;

in your timer elpased:

operationsTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
{ 
    operationsTimer.Enabled = false;
    //add item to collection code
    x++;
    if(x<100)
        operationsTimer.Enabled = true;
}