Binding a ComboBox entirely in XAML - silverlight

Question

I have a ComboBox on a silverlight control, that I want to bind. Sounds simple, except what I'm finding is that because the data for the ItemsSource comes from a web service asynchronously, I need to use the code behind to bind the SelectedValue only after the data has come back.
The collection that the data goes in implements INotifyCollectionChanged and INotifyPropertyChanged, so it should all be working, and indeed the combo box loads properly, but there is no value pre-selected.
What I think is happening is that the SelectedValue is getting bound before the collection has loaded - when the combobox is empty - so nothing is selected, and then later when the data comes in, the combobox is populated, but it is not checking the selected value again.
So whilst I have this working if I use code behind to hook up events and creating bindings in code, I'd like to move this all to XAML with something like:
<ComboBox HorizontalAlignment="Stretch" Margin="5,3,9,127" Name="cboCategoryID" Grid.Row="4" Grid.Column="1"
ItemsSource="{StaticResource Categories}"
SelectedValue="{Binding CategoryID, Mode=TwoWay, NotifyOnValidationError=True, ValidatesOnDataErrors=True}"
SelectedValuePath="CategoryID"
DisplayMemberPath="Caption"
VerticalAlignment="Center">
</ComboBox>
This correctly loads items, but doesn't bind the selected value. If I put the following code in the code-behind, it all works:
public MainControl()
{
InitializeComponent();
CategoryCollection cats = new CategoryCollection();
cats.Dispatcher = this.Dispatcher;
cats.LoadComplete += new EventHandler(cats_LoadComplete);
cboCategoryID.ItemsSource = cats;
cats.LoadAll();
}
private void cats_LoadComplete(object sender, EventArgs e)
{
cboCategoryID.SetBinding(ComboBox.SelectedValueProperty, new System.Windows.Data.Binding("CategoryID"));
}
Is there a way to do this without resorting to code behind?

Answer 1

Are you using mvvm? If so, you can try to set the ItemsSource and SelectedItem in the callback of the web service, or take a look at this post from Kyle.
http://blogs.msdn.com/b/kylemc/archive/2010/06/18/combobox-sample-for-ria-services.aspx

Answer 2

you are already using a collection that notifies of changes, so if the value that you are binding the SelectedValue to is notifying of changes, then all you have to do is set that property after the values are loaded from the webservice. it SHOULD update the combobox automatically, allowing you to do your binding purely in xaml.
public myObject CategoryID { get {....}
set {
this.categoryID = value;
RaisePropertyChanged("CategoryID");}
public void DataLoadedHandler()
{
CategoryID = 34; // this will cause the binding to update
}

Answer 3

take a look at this simple sample:
XAML:
<UserControl
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:sdk="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Data" xmlns:local="clr-namespace:StackoverflowQuestions.Silverlight" xmlns:sdk1="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk" x:Class="StackoverflowQuestions.Silverlight.MainPage"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="400">
<UserControl.Resources>
<DataTemplate x:Key="Item">
<TextBlock Text="{Binding PropertyToBeWatched}" />
</DataTemplate>
</UserControl.Resources>
<Grid x:Name="LayoutRoot" Background="White">
<!--<sdk:DataGrid ItemsSource="{Binding MyList}" RowStyle="{StaticResource Style1}">
<sdk:DataGrid.Columns>
<sdk:DataGridTextColumn Binding="{Binding PropertyToBeWatched}" Header="Property1"/>
</sdk:DataGrid.Columns>
</sdk:DataGrid>-->
<ComboBox ItemsSource="{Binding MyList}" SelectedItem="{Binding SelectedItem, Mode=TwoWay}" Height="50" VerticalAlignment="Top" ItemTemplate="{StaticResource Item}" />
</Grid>
</UserControl>
Codebehind:
public partial class MainPage : UserControl, INotifyPropertyChanged
{
private ObservableCollection _myList;
private CustomClass _selectedItem;
public event PropertyChangedEventHandler PropertyChanged;
public ObservableCollection<CustomClass> MyList
{
get { return _myList ?? (_myList = new ObservableCollection<CustomClass>()); }
set
{
_myList = value;
RaisePropertyChanged("MyList");
}
}
public CustomClass SelectedItem
{
get { return _selectedItem; }
set
{
_selectedItem = value;
RaisePropertyChanged("SelectedItem");
}
}
protected void RaisePropertyChanged(string propertyname)
{
var handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyname));
}
public MainPage()
{
InitializeComponent();
this.DataContext = this;
MyList.Add(new CustomClass() { PropertyToBeWatched = "1"});
MyList.Add(new CustomClass() { PropertyToBeWatched = "2" });
MyList.Add(new CustomClass() { PropertyToBeWatched = "2" });
MyList.Add(new CustomClass() { PropertyToBeWatched = "2" });
SelectedItem = MyList[1]; //Here is where it happens
}
}
By binding the SelectedItem of the ComboBox to an entity, we can achieve what you want. This works TwoWay ofcourse.
Hope this helps. :D