Dispatcher：中止调度程序操作

Question

I want adding items to DataGrid in thread.

I have view model for my user control:

public class Contact
{
    public string Name { get; set; }
    public string Phone { get; set; }
}

public class ContactGridViewModel
{
    public ContactGridViewModel()
    {
        Items = new ObservableCollection<Contact>();
    }

    public ObservableCollection<Contact> Items { get; private set; }

    public Dispatcher Dispatcher
    {
        get { return Dispatcher.CurrentDispatcher); }
    }
    private DispatcherOperation LastOperation { get; set; }

    public void Update(IEnumerable<Contact> contacts)
    {
        if (LastOperation != null) LastOperation.Abort();

        Items.Clear();
        foreach (var item in contacts)
            LastOperation = Dispatcher.BeginInvoke(
                    DispatcherPriority.Background,
                    new ParameterizedThreadStart(o => Items.Add(o))),
                    item);
    }
}

There is the markup example:

<!--DataContext="{Binding ContactGridViewModel}-->

<DataGrid ItemsSource="{Binding Items, Mode=OneWay}">
  <DataGrid.Columns>
    <DataGridTextColumn Header="Name" Binding="{Binding Path=Name}" />
    <DataGridTextColumn Header="Phone" Binding="{Binding Path=Phone}" />
  </DataGrid.Columns>
</DataGrid>

After Update method is executed second time, and if first collection is not completely added, DataGrid will contains part of first collection and full second collection.

// contactList1 and contactList2 are IEnumerable<Contact> objects
Model.Update(contactList1);
Model.Update(contactList2);

I try to abort DispatcherOperation, but it's not stop adding contacts into Items property.

Could you tell me the correct method of adding objects to DataGrid ItemsSource?

Answer 1

You should collect all DispatcherOpretations in list and then abort them all.

public class ContactGridViewModel
{
    public ContactGridViewModel()
    {
        Items = new ObservableCollection<Contact>();
        LastOperations = new List<DispatcherOperation>();
    }

    public ObservableCollection<Contact> Items { get; private set; }
    private ICollection<DispatcherOperation> LastOperations { get; set; }

    public Dispatcher Dispatcher
    {
        get { return Dispatcher.CurrentDispatcher); }
    }

    private void StopUpdating()
    {
        foreach (var operation in LastOperations)
        {
            operation.Abort();
        }
        LastOperations.Clear();
    }

    public void Update(IEnumerable<Contact> contacts)
    {
        StopUpdating();

        Items.Clear();
        foreach (var item in contacts)
            LastOperations.Add(Dispatcher.BeginInvoke(
                    DispatcherPriority.Background,
                    new ParameterizedThreadStart(o => Items.Add(item))),
                    item));
    }
}

Answer 2

In your Update(contactList1) method you start a Dispatcher.BeginInvoke() for every item in contactList1, but if you enter Update(contactList2) you abort only once the LastOperation, which is not the complete foreach but an single Items.Add() .

Try something like this (not tested):

public void Update(IEnumerable<Contact> contacts)
{
    if (LastOperation != null) LastOperation.Abort();

    Items.Clear();
    LastOperation = Dispatcher.BeginInvoke(
        DispatcherPriority.Background,
        new ParameterizedThreadStart(o => { foreach (var item in o) Items.Add(item); }),
        contacts);
}