WPF data binding - what am I missing? - wpf

I am trying to grasp the concepts of WPF data binding through a simple example, but it seems I haven't quite gotten the point of all of it.
The example is one of cascading dropdowns; the XAML is as follows:
<Window x:Class="CascadingDropDown.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="496" Width="949" Loaded="Window_Loaded">
<Grid>
<ComboBox Name="comboBox1" ItemsSource="{Binding}" DisplayMemberPath="Key" SelectionChanged="comboBox1_SelectionChanged" />
<ComboBox Name="comboBox2" ItemsSource="{Binding}" DisplayMemberPath="Name" />
</Grid>
</Window>
This is the code of the form:
public partial class MainWindow : Window
{
private ObservableCollection<ItemA> m_lstItemAContext = new ObservableCollection<ItemA>();
private ObservableCollection<ItemB> m_lstItemBContext = new ObservableCollection<ItemB>();
private IEnumerable<ItemB> m_lstAllItemB = null;
public MainWindow()
{
InitializeComponent();
this.comboBox1.DataContext = m_lstItemAContext;
this.comboBox2.DataContext = m_lstItemBContext;
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
var lstItemA = new List<ItemA>() { new ItemA("aaa"), new ItemA("bbb"), new ItemA("ccc") };
var lstItemB = new List<ItemB>() { new ItemB("aaa", "a11"), new ItemB("aaa", "a22"), new ItemB("bbb", "b11"), new ItemB("bbb", "b22") };
initPicklists(lstItemA, lstItemB);
}
private void initPicklists(IEnumerable<ItemA> lstItemA, IEnumerable<ItemB> lstItemB)
{
this.m_lstAllItemB = lstItemB;
this.m_lstItemAContext.Clear();
lstItemA.ToList().ForEach(a => this.m_lstItemAContext.Add(a));
}
#region Control event handlers
private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
ComboBox ddlSender = (ComboBox)sender;
ItemA itemaSelected = (ItemA)ddlSender.SelectedItem;
var lstNewItemB = this.m_lstAllItemB.Where(b => b.KeyA == itemaSelected.Key);
this.m_lstItemBContext.Clear();
lstNewItemB.ToList().ForEach(b => this.m_lstItemBContext.Add(b));
}
private void comboBox2_?(object sender, ?EventArgs e)
{
// disable ComboBox if empty
}
#endregion Control event handlers
}
And these are my data classes:
class ItemA
{
public string Key { get; set; }
public ItemA(string sKey)
{
this.Key = sKey;
}
}
class ItemB
{
public string KeyA { get; set; }
public string Name { get; set; }
public ItemB(string sKeyA, string sName)
{
this.KeyA = sKeyA;
this.Name = sName;
}
}
So whenever an item is selected in comboBox1, the appropriate items are supposed to show up in comboBox2. This is working with the current code, though I'm not sure whether my way of re-populating the respective ObservableCollection is ideal.
What I haven't been able to achieve is actually reacting to changes in the underlying collection of comboBox2, for example to deactivate the control when the list is empty (i.e. when "ccc" is selected in comboBox1).
Of course, I can use an event handler on the CollectionChanged event of the ObservableCollection, and that would work in this example, but in a more complex scenario, where the ComboBox' DataContext might change to a completely different object (and possibly back), that would mean a two-fold dependency - I would always have to not only switch the DataContext, but also the event handlers back and forth. This doesn't seem right to me, but I am probably simply on an entirely wrong track about this.
Basically, what I am looking for is an event firing on the control rather than the underlying list; not the ObservableCollection announcing "my contents have changed", but the ComboBox telling me "something happenend to my items".
What do I need to do, or where do I have to correct my perception of the whole concept ?

Here is the cleaner (perhaps not the much optimized) way to acheive this, keeping your business model untouched, and using ViewModel and XAML only when possible :
View Model :
public class WindowViewModel : INotifyPropertyChanged
{
private ItemA selectedItem;
private readonly ObservableCollection<ItemA> itemsA = new ObservableCollection<ItemA>();
private readonly ObservableCollection<ItemB> itemsB = new ObservableCollection<ItemB>();
private readonly List<ItemB> internalItemsBList = new List<ItemB>();
public WindowViewModel()
{
itemsA = new ObservableCollection<ItemA> { new ItemA("aaa"), new ItemA("bbb"), new ItemA("ccc") };
InvokePropertyChanged(new PropertyChangedEventArgs("ItemsA"));
internalItemsBList = new List<ItemB> { new ItemB("aaa", "a11"), new ItemB("aaa", "a22"), new ItemB("bbb", "b11"), new ItemB("bbb", "b22") };
}
public ObservableCollection<ItemA> ItemsA
{
get { return itemsA; }
}
public ItemA SelectedItem
{
get { return selectedItem; }
set
{
selectedItem = value;
ItemsB.Clear();
var tmp = internalItemsBList.Where(b => b.KeyA == selectedItem.Key);
foreach (var itemB in tmp)
{
ItemsB.Add(itemB);
}
InvokePropertyChanged(new PropertyChangedEventArgs("SelectedItem"));
}
}
public ObservableCollection<ItemB> ItemsB
{
get { return itemsB; }
}
public event PropertyChangedEventHandler PropertyChanged;
public void InvokePropertyChanged(PropertyChangedEventArgs e)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, e);
}
}
Code Behind :
public partial class Window1
{
public Window1()
{
InitializeComponent();
DataContext = new WindowViewModel();
}
}
and XAML :
<StackPanel>
<ComboBox Name="comboBox1" ItemsSource="{Binding ItemsA}" DisplayMemberPath="Key" SelectedItem="{Binding SelectedItem, Mode=TwoWay}" />
<ComboBox Name="comboBox2" ItemsSource="{Binding ItemsB}" DisplayMemberPath="Name">
<ComboBox.Style>
<Style TargetType="{x:Type ComboBox}">
<Setter Property="IsEnabled" Value="true"/>
<Style.Triggers>
<DataTrigger Binding="{Binding ItemsB.Count}" Value="0">
<Setter Property="IsEnabled" Value="false"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ComboBox.Style>
</ComboBox>
</StackPanel>
copying-pasting this should work.
Few random thoughts :
1) in WPF, try to always use MVVM pattern and never put code in code-behind files for event handlers. For user actions (like button clicks) use the Commands pattern. For other user actions for which commands are not available, think as much as you can in a "binding-way" : you can do a lot since you can intercept event from the view in VM properties setters (in your example I use the SelectedItem property setter).
2) Use XAML as much as you can. WPF framework provides a very powerful binding and triggers system (in your example, the enabling of combobox don't needs any line of C#).
3) ObservableCollection are made to be exposed by the view model to the view via binding. They are also meant to be used in conjunction with their CollectionChanged event that you can handle in the view model. Take benefit of that (in your example, I play with Observable collection in the VM, where this playing should happen, and any changes in the collection gets reflected in the view via DataBinding).
Hopes this will help !

Basically, what I am looking for is an event firing on the control rather than the underlying list; not the ObservableCollection announcing "my contents have changed", but the ComboBox telling me "something happenend to my items"
if you wanna use MVVM pattern then i would say NO. not the control should give the information, but your viewmodel should.
taking an ObservableCollection is a good step at first. in your specail case i would consider to create just one list with ItemA and i would add a new List property of type ItemB to ItemA.
class ItemA
{
public string Key { get; set; }
public ItemA(string sKey)
{
this.Key = sKey;
}
public IEnumerable<ItemB> ListItemsB { get; set;}
}
i assume ItemA is the parent?
class ItemB
{
public string Name { get; set; }
public ItemB(string sName)
{
this.Name = sName;
}
}
you have a collection of ItemA and each ItemA has its own list of depending ItemB.
<ComboBox x:Name="cbo_itemA" ItemsSource="{Binding ListItemA}" DisplayMemberPath="Key"/>
<ComboBox ItemsSource="{Binding ElementName=cbo_itemA, Path=SelectedItem.ListItemsB}"
DisplayMemberPath="Name" />

Do you need the Keys collection? If not i'd suggest creating it dynamically from the items by grouping via CollectionView:
private ObservableCollection<object> _Items = new ObservableCollection<object>()
{
new { Key = "a", Name = "Item 1" },
new { Key = "a", Name = "Item 2" },
new { Key = "b", Name = "Item 3" },
new { Key = "c", Name = "Item 4" },
};
public ObservableCollection<object> Items { get { return _Items; } }
<StackPanel>
<StackPanel.Resources>
<CollectionViewSource x:Key="ItemsSource" Source="{Binding Items}">
<CollectionViewSource.GroupDescriptions>
<PropertyGroupDescription PropertyName="Key"/>
</CollectionViewSource.GroupDescriptions>
</CollectionViewSource>
</StackPanel.Resources>
<StackPanel.Children>
<ComboBox Name="keyCb" ItemsSource="{Binding Source={StaticResource ItemsSource}, Path=Groups}" DisplayMemberPath="Name"/>
<ComboBox ItemsSource="{Binding ElementName=keyCb, Path=SelectedItem.Items}" DisplayMemberPath="Name"/>
</StackPanel.Children>
</StackPanel>
The first ComboBox shows the keys which are generated by grouping by the Key-property, the second binds to the selected item's subitems in the first ComboBox, showing the Name of the item.
Also see the CollectionViewGroup reference, in the fist CB i use the Name in the second the Items.
Of course you can create these key-groups manually as well by nesting items in a key-object.

Related

Display Hierarchical Data in TreeView

I am simply trying to display hierarchical Data in a TreeView, but I just can't figure out how to make it display more than the First two Levels. (And i have read almost evry TreeView post, maybe the problem is my (miss)understatement of the Bindings in this case)
I have simplyfied my Datastructure for this test:
public class Node
{
public List<Node> Children { get; set; }
public Node Parent { get; set; }
public string Expression { get; set; }
}
Xaml currently looks like this: (Please notice that I have changed it several times now, but this here is the original state i have come up with: )
<Window x:Class="Klammern_Test.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:Klammern_Test"
Title="MainWindow" Height="439" Width="402">
<Window.Resources>
<HierarchicalDataTemplate DataType="{x:Type local:Node}">
<TreeViewItem ItemsSource="{Binding Children}" Header="{Binding Expression}"/>
</HierarchicalDataTemplate>
</Window.Resources>
<Grid>
<TreeView ItemsSource="{Binding Root}" Margin="12,41,12,12" Name="treeView" />
</Grid>
</Window>
And this is how I am trying to Populate my Treeview:
public partial class MainWindow : Window
{
public Node Root { get; set; }
public MainWindow()
{
InitializeComponent();
DataContext = this;
}
private void StartButton_Click(object sender, RoutedEventArgs e)
{
Parser = new StringParser();
Root = Parser.Parse(Tbx_Eingabe.Text);
treeView.Items.Add(PopulateTreeView(Root));
}
private TreeViewItem PopulateTreeView(Node node)
{
TreeViewItem treeViewItem = new TreeViewItem();
treeViewItem.IsExpanded = true;
treeViewItem.Header = node.Expression;
foreach (Node child in node.Children)
{
treeViewItem.Items.Add(new TreeViewItem() { Header = child.Expression });
if (child.Children.Count > 0)
{
PopulateTreeView(child);
}
}
return treeViewItem;
}
}
What am I missing?
EDIT:
After trying around with almulo's hints, I found this with the Snoop-tool, but I can't tell what it means at all, I have found no other red line and no entry to the Binding Errors column at all.
Using a TreeViewItem inside a HierarchicalDataTemplate which is what your TreeView's TreeViewItems will use to create themselves is... confusing.
Inside your HierarchicalDataTemplate you should just add the controls you want the item "header" to have. In this case, I guess it should be a TextBlock since you just wanna show some text.
Then use the HierarchicalDataTemplate.ItemSource property to bind the children of your node.
<Window.Resources>
<HierarchicalDataTemplate DataType="{x:Type local:Node}"
ItemsSource="{Binding Children}">
<TextBlock Text="{Binding Expression}" />
</HierarchicalDataTemplate>
</Window.Resources>
Also, in you code-behind, you shouldn't manipulate the TreeView.Items or the TreeViewItem.Items directly, since you are already using Bindings and the ItemsSource properties.
Instead, remove the PopulateTreeView method and let your Root property work as items source for the TreeView. But in order for this to work, you'll have to notify the view when the Root property changes its value.
To do so, implement the INotifyPropertyChanged interface and fire the PropertyChanged event every time Root changes.
EDIT: The ItemsControl property ItemsSource expects a collection (more specifically, an IEnumerable), so Root needs to be one. Even if it has a single item, like this:
public class MainWindow : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
// ...
private void StartButton_Click(object sender, RoutedEventArgs e)
{
Parser = new StringParser();
Root = new Node[] { Parser.Parse(Tbx_Eingabe.Text) };
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("Root"));
}
// ...
}

ComboBox does not select proper item when bound to CompositeCollection

I have a ComboBox bound to a collection of animals. From it I select my favourite animal. I need a static null item above the bound items. I declare it using a CompositeCollection. When the ComboBox is bound it does not select my initial favourite animal. How can I fix that? Similar problem here but still unresolved.
Observations:
Binding to the static item works i.e. if I don't have an initial favourite animal the static item gets selected.
The problem disappears if the static item is removed. Of course this would make the CompositeCollection and this whole question obsolete.
I already applied these measures:
A CollectionContainer cannot bind directly to a property as outlined here.
The composite collection is also moved to a static resource as suggested here.
Complete C# code and XAML to demonstrate the problem:
using System;
using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;
namespace WpfApplication1
{
public class Animal
{
public int Id { get; set; }
public string Name { get; set; }
}
public class Zoo
{
private IEnumerable<Animal> _animals = new Animal[]
{
new Animal() { Id = 1, Name = "Tom" },
new Animal() { Id = 2, Name = "Jerry" }
};
public Zoo(int initialId)
{
FavouriteId = initialId;
}
public int FavouriteId { get; set; }
public IEnumerable<Animal> Animals { get { return _animals; } }
}
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private void BindComboBox(object sender, RoutedEventArgs e)
{
// Selecting the static item by default works.
//DataContext = new Zoo(-1);
// Selecting "Jerry" by default does not work.
DataContext = new Zoo(2);
}
}
}
XAML
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1">
<Window.Resources>
<CollectionViewSource x:Key="AnimalsBridge" Source="{Binding Path=Animals}" />
<CompositeCollection x:Key="AnimalsWithNullItem">
<local:Animal Id="-1" Name="Pick someone..."/>
<CollectionContainer Collection="{Binding Source={StaticResource AnimalsBridge}}" />
</CompositeCollection>
</Window.Resources>
<StackPanel>
<Button Content="Bind" Click="BindComboBox"/>
<ComboBox x:Name="cmbFavourite"
SelectedValue="{Binding Path=FavouriteId}"
SelectedValuePath="Id" DisplayMemberPath="Name"
ItemsSource="{StaticResource AnimalsWithNullItem}"/>
</StackPanel>
</Window>
I dont have a solution to your problem but rather an alternative. I personally have view models dedicated to each view. I would then have a property on the view model to add the null value as required. I prever this method since it allows for better unit testing of my viewmodel.
For your example add:
public class ZooViewModel
{
.....
public IEnumerable<Animal> Animals { get { return _animals; } }
public IEnumerable<Animal> AnimalsWithNull { get { return _animals.WithDefault(new Animal() { Id = -1, Name = "Please select one" }); } }
}
The magic component
public static class EnumerableExtend {
public static IEnumerable<T> WithDefault<T>(this IEnumerable<T> enumerable,T defaultValue) {
yield return defaultValue;
foreach (var value in enumerable) {
yield return value;
}
}
}
Then in your XAML you just bind to
ComboBox x:Name="cmbFavourite"
SelectedValue="{Binding Path=FavouriteId}"
SelectedValuePath="Id" DisplayMemberPath="Name"
ItemsSource="{Binding AnimalsWithNull }"/>
Now you are binding directly to the source and can control the binding as normal. Also note because we are using "yield" we are not creating a new enum but rather just iterating over the existing list.

Silverlight MVVM Combobox Binding getting broken

I have a combo box defined as such
<ComboBox Name="RoomDropDown" Visibility="{Binding Path=RoomDropDownVisible,Mode=OneWay,Converter={StaticResource BoolVisibilityConvertor}}"
ItemsSource="{Binding Path=RoomList,Mode=OneWay}" DisplayMemberPath="display" SelectedValuePath="display" SelectedValue="{Binding Path=Room,Mode=TwoWay}"/>
There are 2 properties defined in the ViewModel, RoomList which is List and the Room property which is a string.
First time when i run the app everything works fine, and the Drop Down gets the correct values as well as the correct values is selected. However on a certain conditions the RoomList property is changed to a different source & the Room property is also changed. The problem that is now happening is the Combo Box is showing the correct values but the selected value is not getting selected. Worse, we can live with that, but the setter is also not firing when the value is manually changed in the DropDown.
Any pointers on what is going wrong here?
Followup:
Don't think I managed to get the exact problem across, here is some sample code that I wanted to add to illustrate the problem:
<Grid x:Name="LayoutRoot" Background="White">
<StackPanel VerticalAlignment="Center" Width="100">
<ComboBox Name="TestBox" Height="20" Width="100" ItemsSource="{Binding Path=ComboSource}" DisplayMemberPath="display" SelectedValuePath="code"
SelectedValue="{Binding Path=ComboSelection,Mode=TwoWay}"/>
<Button Content="Click Here" Click="Button_Click" />
</StackPanel>
</Grid>
public MainPage()
{
InitializeComponent();
this.Loaded += (s, e) =>
{
var temp = new List<Binding>();
temp.Add(new Binding() { code = "1", display = "One" });
temp.Add(new Binding() { code = "2", display = "Two" });
this.ComboSource = temp;
this.ComboSelection = "1";
this.DataContext = this;
};
}
private static readonly DependencyProperty ComboSelectionProperty =
DependencyProperty.Register("ComboSelectionProperty", typeof(string), typeof(MainPage), new PropertyMetadata(null));
public string ComboSelection
{
get { return (string)GetValue(ComboSelectionProperty); }
set
{
SetValue(ComboSelectionProperty, value);
this.RaisePropertyChanged("ComboSelection");
}
}
private static readonly DependencyProperty ComboSourceProperty =
DependencyProperty.Register("ComboSourceProperty", typeof(List<Binding>), typeof(MainPage), new PropertyMetadata(null));
public List<Binding> ComboSource
{
get
{
return (List<Binding>)GetValue(ComboSourceProperty);
}
set
{
SetValue(ComboSourceProperty, value);
this.RaisePropertyChanged("ComboSource");
}
}
private void Button_Click(object sender, RoutedEventArgs e)
{
var temp = new List<Binding>();
temp.Add(new Binding() { code = "3", display = "Three" });
temp.Add(new Binding() { code = "4", display = "Four" });
this.ComboSource = temp;
this.ComboSelection = "3";
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
private void RaisePropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
#endregion
}
public class Binding
{
public string code {get; set;}
public string display { get; set; }
}
Not strictly MVVM, but to explain the problem, when the button click event is fired, the Combosource is changed with a new selection being made, however that selection does not bind and the problem i mentioned above starts happening.
Your SelectedValuePath is "display", which I assume is a string property of the Room class. But you're binding SelectedValue to the Room property of your viewmodel, and I assume this property is of type Room... So the SelectedValue is of the type string, and you're binding it to a property of type Room: it can't work because there is no conversion between those types.
Instead of using the SelectedValue property, why not use the SelectedItem ?
<ComboBox Name="RoomDropDown" Visibility="{Binding Path=RoomDropDownVisible,Mode=OneWay,Converter={StaticResource BoolVisibilityConvertor}}"
ItemsSource="{Binding Path=RoomList,Mode=OneWay}" DisplayMemberPath="display" SelectedItem="{Binding Path=Room,Mode=TwoWay}"/>
There seems to be a bug in ComboBox data binding where the binding will completely break if the data it is binding to SelectedValue becomes null.
Place a breakpoint in the ComboSelection setter and see if it is ever getting set to null. If this is the source of the problem, add this to your setter:
public string ComboSelection
{
// .....
set
{
if(value == null)
return;
// .....
}
}
On a side note, you probably don't need use a dependency property to back ComboSelection. The data binding for this should work just fine on a normal property as long as you keep using PropertyChanged.

Silverlight Relating Two Datagrids

My question: How do I bind the SelectedItem from a primary datagrid to the ItemsSource for a secondary datagrid?
In detail:
I have two datagrids on my view. The first shows a collection of teams and the second shows as list of people in the selected team.
When I select a team from the grid I can see that the SelectedTeam property is getting updated correctly, but the People grid is not getting populated.
Note: I am not able to use nested grids, or the cool master-detail features provided in the SL data-grid.
UPDATE: Replacing the parent datagrid with a ComboBox gives completely different results and works perfectly. Why would ComboBox.SelectedItem and DataGrid.SelectedItem behave so differently?
Thanks,
Mark
Simple Repro:
VIEW:
<UserControl x:Class="NestedDataGrid.MainPage"
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"
mc:Ignorable="d"
xmlns:data="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Data">
<StackPanel x:Name="LayoutRoot">
<TextBlock Text="Teams:" />
<data:DataGrid ItemsSource="{Binding Teams}"
SelectedItem="{Binding SelectedTeam, Mode=TwoWay}"
AutoGenerateColumns="False">
<data:DataGrid.Columns>
<data:DataGridTextColumn Header="Id" Binding="{Binding TeamId}" />
<data:DataGridTextColumn Header="Desc" Binding="{Binding TeamDesc}" />
</data:DataGrid.Columns>
</data:DataGrid>
<TextBlock Text="Peeps:" />
<data:DataGrid ItemsSource="{Binding SelectedTeam.People}"
AutoGenerateColumns="False">
<data:DataGrid.Columns>
<data:DataGridTextColumn Header="Id"
Binding="{Binding PersonId}" />
<data:DataGridTextColumn Header="Name"
Binding="{Binding Name}" />
</data:DataGrid.Columns>
</data:DataGrid>
</StackPanel>
</UserControl>
CODE_BEHIND:
using System.Windows.Controls;
namespace NestedDataGrid
{
public partial class MainPage : UserControl
{
public MainPage()
{
InitializeComponent();
this.LayoutRoot.DataContext = new ViewModel();
}
}
}
VIEWMODEL:
using System.Collections.ObjectModel;
namespace NestedDataGrid
{
public class ViewModel: ObjectBase
{
public ViewModel()
{
ObservableCollection<Person> RainbowPeeps = new ObservableCollection<Person>()
{
new Person(){ PersonId=1, Name="George"},
new Person(){ PersonId=2, Name="Zippy"},
new Person(){ PersonId=3, Name="Bungle"},
};
ObservableCollection<Person> Simpsons = new ObservableCollection<Person>()
{
new Person(){ PersonId=4, Name="Moe"},
new Person(){ PersonId=5, Name="Barney"},
new Person(){ PersonId=6, Name="Selma"},
};
ObservableCollection<Person> FamilyGuyKids = new ObservableCollection<Person>()
{
new Person(){ PersonId=7, Name="Stewie"},
new Person(){ PersonId=8, Name="Meg"},
new Person(){ PersonId=9, Name="Chris"},
};
Teams = new ObservableCollection<Team>()
{
new Team(){ TeamId=1, TeamDesc="Rainbow", People=RainbowPeeps},
new Team(){ TeamId=2, TeamDesc="Simpsons", People=Simpsons},
new Team(){ TeamId=3, TeamDesc="Family Guys", People=FamilyGuyKids },
};
}
private ObservableCollection<Team> _teams;
public ObservableCollection<Team> Teams
{
get { return _teams; }
set
{
SetValue(ref _teams, value, "Teams");
}
}
private Team _selectedTeam;
public Team SelectedTeam
{
get { return _selectedTeam; }
set
{
SetValue(ref _selectedTeam, value, "SelectedTeam");
}
}
}
}
ASSOCIATED CLASSES:
using System;
using System.ComponentModel;
namespace NestedDataGrid
{
public abstract class ObjectBase : Object, INotifyPropertyChanged
{
public ObjectBase()
{ }
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void _OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler pceh = PropertyChanged;
if (pceh != null)
{
pceh(this, new PropertyChangedEventArgs(propertyName));
}
}
protected virtual bool SetValue<T>(ref T target, T value, string propertyName)
{
if (Object.Equals(target, value))
{
return false;
}
target = value;
_OnPropertyChanged(propertyName);
return true;
}
}
public class Person: ObjectBase
{
private int _personId;
public int PersonId
{
get { return _personId; }
set
{
SetValue(ref _personId, value, "PersonId");
}
}
private string _name;
public string Name
{
get { return _name; }
set
{
SetValue(ref _name, value, "Name");
}
}
}
public class Team : ObjectBase
{
private int _teamId;
public int TeamId
{
get { return _teamId; }
set
{
SetValue(ref _teamId, value, "TeamId");
}
}
private string _teamDesc;
public string TeamDesc
{
get { return _teamDesc; }
set
{
SetValue(ref _teamDesc, value, "TeamDesc");
}
}
private ObservableCollection<Person> _people;
public ObservableCollection<Person> People
{
get { return _people; }
set
{
SetValue(ref _people, value, "People");
}
}
}
}
UPDATE
Replacing the first datagrid with a combobox and eveything works OK. Why would DataGrid.SelectedItem and ComboBox.SelectedItem behave so differently?
<StackPanel x:Name="LayoutRoot">
<TextBlock Text="Teams:" />
<ComboBox SelectedItem="{Binding SelectedTeam, Mode=TwoWay}"
ItemsSource="{Binding Teams}"/>
<TextBlock Text="{Binding SelectedTeam}" />
<TextBlock Text="Peeps:" />
<data:DataGrid ItemsSource="{Binding SelectedTeam.People}" />
</StackPanel>
Having done some tests.
First I just wanted to confirm that the Binding itself is working. It works quite happly when the second DataGrid is swapped out for a ListBox. I've gone so far to confirm that the second DataGrid is having its ItemsSource property changed by the binding engine.
I've also swapped out the first DataGrid for a ListBox and then the second DataGrid starts working quite happly.
In addition if you wire up the SelectionChanged event on the first datagrid and use code to assign directly to the second datagrid it starts working.
I've also removed the SelectedItem binding on the first Grid and set up an ElementToElement bind to it from the on the ItemsSource property of the second Grid. Still no joy.
Hence the problem is narrowed down to SelectedItem on one DatGrid to the ItemsSource of another via the framework binding engine.
Reflector provides a possible clue. The Data namespace contains an Extensions static class targeting DependencyObject which has an AreHandlersSuspended method backed bye a static variable. The which the code handling changes to the ItemsSource property uses this method and does nothing if it returns true.
My unconfirmed suspicion is that in the process of the first Grid assigning its SelectedItem property it has turned on the flag in order to avoid an infinite loop. However since this flag is effectively global any other legitmate code running as a result of this SelectedItem assignment is not being executed.
Anyone got SL4 and fancy testing on that?
Any MSFTers lurking want to look into?
If SL4 still has it this will need reporting to Connect as a bug.
A better solution is to use add DataGridRowSelected command. This fits the MVVM pattern a whole lot better than my previous mouse click example.
This was inspired by some code from John Papa, I have created a detailed post about this http://thoughtjelly.blogspot.com/2009/12/binding-selecteditem-to-itemssource.html.
[Sits back contented and lights a cigar]
Mark
I had the same problem, and "fixed" it by adding this to my code-behind.
Code behind:
private void DataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (_model != null)
{
_model.RefreshDetail();
}
}
Model:
public void RefreshDetail()
{
RaisePropertyChanged("Detail");
}
I have a work-around. It involves a bit of code behind, so won't be favoured by purist MVVM zealots! ;-)
<StackPanel x:Name="LayoutRoot">
<TextBlock Text="Teams:" />
<data:DataGrid x:Name="dgTeams"
SelectedItem="{Binding SelectedTeam, Mode=TwoWay}"
ItemsSource="{Binding Teams}" />
<TextBlock Text="{Binding SelectedTeam}" />
<TextBlock Text="Peeps:" />
<data:DataGrid x:Name="dgPeeps" />
</StackPanel>
Code Behind:
public partial class MainPage : UserControl
{
public MainPage()
{
InitializeComponent();
this.LayoutRoot.DataContext = new ViewModel();
dgTeams.MouseLeftButtonUp += new MouseButtonEventHandler(dgTeams_MouseLeftButtonUp)
}
void dgTeams_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
DataGridRow row = DependencyObjectHelper.FindParentOfType<DataGridRow>(e.OriginalSource as DependencyObject);
///get the data object of the row
if (row != null && row.DataContext is Team)
{
dgPeeps.ItemsSource = (row.DataContext as Team).People;
}
}
}
The FindParentOfType method is detailed here: http://thoughtjelly.blogspot.com/2009/09/walking-xaml-visualtree-to-find-parent.html.
Hope this helps someone else.

WPF ComboBox resets selected item when item-source changes

Consider the following XAML:
<ComboBox Name="CompanyComboBox"
HorizontalAlignment="Stretch"
ItemsSource="{Binding Path=GlobalData.Companies}"
SelectedValuePath="Id"
SelectedValue="{Binding Customer.CompanyId, ValidatesOnDataErrors=True}"
DisplayMemberPath="Name" />
GlobalData.Companies is a collection (IEnumerable<Company>) of companies; this collection can be reloaded on background (it is downloaded from a webservice). When this happens, ComboBox correctly reloads items via binding. However as a side-effect, it also resets the selected item!
I have used Reflector to inspect combo-box sources and apparently this is intended behavior.
Is there any "nice" way how to get around this? What I want to achieve, is that if the user selects "Company A" and reloads list of companies afterwards, then "Company A" stays selected (assuming it is in the new list).
Please try with the following code.
Enable the following property to the combo box
IsSynchronizedWithCurrentItem="True"
Maybe you can use ObservableCollection<Company> instead of your IEnumerable<Company>? Then, on background change you would only Add / Remove items that are new / absent in the new list, selected item should stay, unless it was removed by the change.
You can update your observable collection in a separate thread with a small hack-around.
hmm, I don't know if it is a "nice" way, but if you can access the selected item before the reload occurs, you can save it (or its key or something), and select it programatically again after the reload is done.
quick mockup:
var selectedItem = myCombo.SelectedItem;
DoReload();
myCombo.SelectedItem = selectedItem;
But I assume you mean another way than this manual work around?
Hope this helps anyway...
UPDATE
Ok I see, from a background thread.
Are you using an ICollectionView to bind your combobox too? If so, you can use the CurrentItem property to keep a reference. I made a quick mockup, and this is working on my setup. this assumes you have a reference to your UI:
XAML
<Grid VerticalAlignment="Top">
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<ComboBox ItemsSource="{Binding Items}" IsSynchronizedWithCurrentItem="True" Grid.Column="0" Grid.Row="0" DisplayMemberPath="Name"/>
<Button Command="{Binding UpdateCommand}" Grid.Column="1" Grid.Row="0">Update</Button>
</Grid>
View/ViewModel
public partial class Window1 : Window {
public Window1() {
InitializeComponent();
this.DataContext = new ViewModel(this);
}
}
public class ViewModel
{
private readonly Window1 window;
private ObservableCollection<Item> items;
private ICollectionView view;
public ViewModel(Window1 window) {
this.window = window;
items = new ObservableCollection<Item>
{
new Item("qwerty"),
new Item("hello"),
new Item("world"),
};
view = CollectionViewSource.GetDefaultView(items);
}
public ObservableCollection<Item> Items { get { return items; } }
public ICommand UpdateCommand {
get { return new RelayCommand(DoUpdate); }
}
public Item SelectedItem { get; set; }
private void DoUpdate(object obj) {
var act = new Func<List<Item>>(DoUpdateAsync);
act.BeginInvoke(CallBack, act);
}
private List<Item> DoUpdateAsync() {
return new List<Item> {
new Item("hello"),
new Item("world"),
new Item("qwerty"),
};
}
private void CallBack(IAsyncResult result) {
try {
var act = (Func<List<Item>>)result.AsyncState;
var list = act.EndInvoke(result);
window.Dispatcher.Invoke(new Action<List<Item>>(delegate(List<Item> lst) {
var current = lst.Single(i => i.Name == ((Item)view.CurrentItem).Name);
Items.Clear();
lst.ForEach(Items.Add);
view.MoveCurrentTo(current);
}), list);
} catch(Exception exc){ Debug.WriteLine(exc); }
}
}
public class Item {
public Item(string name) {
Name = name;
}
public string Name { get; set; }
}
You will need to do some handling in case the selected item is no longer in the list.
The IsSynchronizedWithCurrentItem property is important here, else it won't work!
Also, the way the reference to the main window is made should be by a DI-framework.
As Yacoder pointed out this has to do with object equality. As long as you bind SelectedValue instead of SelectedItem you can define the ItemsSource as an anonymous type collection. Then this problem will not occur (and it is also faster if you need to read the values from a database).

Resources