A WPF ComboBox/ListBox itemtemplate/datatemplate question please.
Let's say that I set DisplayMember="Name", this is equivalent to
<DataTemplate>
<TextBox Text="{Binding Name}" />
</DataTemplate>
"Name" is a field in my view model.
Now, if DisplayMember="{Binding Name}", "Name" is no longer a property in my view model but instead contains the name of the property in my view model that I want to display. Using an ItemTemplate, how would I set this up? Thank you in advance.
DisplayMemberPath is a property of the ItemsControl. And its binding path will be resolved relative to the ItemsControl's DataContext, not its items.
Example (pseudo code without INPC interface implementation):
public class ViewModel
{
// This sets the string name of the property (or the path to it)
// to be displayed in the items.
public string DisplayNameProperty {get; set;} = "Name" // Or = "Surname"
// Items Source
public IEnumerable Items {get; set;}
}
<ItemsControl ItemsSource="{Binding Items}"
DisplayMemberPath="{Binding DisplayNameProperty}"/>
The binding in the DisplayMemberPath property will allow you to control from the ViewModel which property will be displayed in ItemsControl's items.
But in practice, such a task is extremely rare.
Personally, in my practice, such a need has never arisen.
Related
DataContext and Source seem to be very similar to me.
What are the advantages and disadvantages?
When to use which one?
With Source:
<TextBlock Text="{Binding Name, Source={StaticResource Person}}" />
Or the solution with DataContext:
public partial class DataContextSample : Window
{
public string Name {get; set;}
public DataContextSample()
{
InitializeComponent();
this.DataContext = this;
}
}
<TextBlock Text="{Binding Name}" />
A binding with out a specified Source binds to the DataContext property of the element.
The DataContext is a special property which, if not set, is redirected to the element's parent's DataContext. This prevents duplicate xaml (always setting the Source in every binding) and makes all bindings relative so it is easier to change the UI without having to adjust all Sources in the bindings.
I am trying to bind a property of my DataContext to the SelectedItem on a ComboBox like this:
<ComboBox x:Name="ElementSelector"
ItemsSource="{Binding Source={StaticResource Elements}}"
DisplayMemberPath="ElementName"
SelectedItem="{Binding ValueElement, Mode=TwoWay}">
where the Elements resource is a CollectionViewSource (don't know, whether this matters).
When everything is initialized, the property ValueElement of the DataContext is set to the first item in the CollectionViewSource. What I want, is to initialize it the other way around: I would like to set SelectedItem of the ComboBox to the value of the property or null if no matching item is contained.
How can this be done?
EDIT - Additional information:
The ComboBox is part of a DataTemplate:
<DataTemplate x:Key="ReferenceTemplate"
DataType="viewModels:ElementMetaReferenceViewModel">
<StackPanel Orientation="Horizontal">
<StackPanel.Resources>
<ResourceDictionary>
<views:ElementsForReferenceViewSource x:Key="Elements"
Source="{Binding DataContext.CurrentProject.Elements, ElementName=Root}"
ReferenceToFilterFor="{Binding}"/>
</ResourceDictionary>
</StackPanel.Resources>
<TextBlock Text="{Binding PropertyName}"/>
<ComboBox x:Name="ElementSelector"
ItemsSource="{Binding Source={StaticResource Elements}}"
DisplayMemberPath="ElementName"
SelectedItem=""{Binding ValueElement, Mode=TwoWay}" />
</StackPanel>
</DataTemplate>
The ElementsForReferenceViewSource simply derives from CollectionViewSource and implements an additional DependencyProperty which is used for filtering.
The DataContext of the items in the CollectionViewSource look like this:
public class ElementMetaReferenceViewModel : ViewModelBase<ElementMetaReference, ElementMetaReferenceContext>
{
...
private ElementMetaViewModel _valueElement;
public ElementMetaViewModel ValueElement
{
get { return _valueElement; }
set
{
if (value == null) return;
_valueElement = value;
Model.TargetElement = value.Model;
}
}
...
}
For people encountering the same issue
The above code works as expected. The solution was getting the stuff behind the scenes right. Make sure, that the instance of the ViewModel which is the value of the property you want to bind to is definitely contained in the CollectionViewSource.
In my case the issue was deserializing an object tree incorrectly, so objects were instantiated twice. Then for each object a distinct ViewModel was initialized and then obviously the value of the property was not contained in the list.
Remark
To check whether this is an issue in your case, you can try the following:
Override the ToString() methods of the ViewModels displayed in the ComboBox like this:
public override string ToString()
{
return "VM"+ Model.GetHashCode().ToString();
}
Then you can easily compare the items in the source collection with the value on your property. Not the most professional way, but it did the job for me.
Is there a way to bind a Silverlight control to an object (or database table's row) which contains the values of several control's properties, doing so without by define the binding for each property?
For instance:
Let's say I have the class (or entity based on database table's row) with the following values:
class TextBlockValues
{
public string Text{get; set;}
public string HorizontalAlignment{get; set;}
public string VerticalAlignment{get; set;}
}
I want to bind it to a TextBlock in my silverlight application (again without explicit specify the binding for each property).
Thank you for your time.
There are two parts in a binding: DataContext and the actual Binding objects. Once you set up the data context for an item, all the properties, and children will automatically use that.
For example:
<TextBlock Name="CaptionText" Text="{Binding Text}" HorizontalAlignment="{Binding HorizontalAlignment}" Height="20" TextAlignment="Center" FontStretch="Expanded" FontSize="13" />
And in the .cs file:
CaptionText.DataContext = myObject;
If I understand your question right the answer is no. Even though you can set the control's DataContext you still have to bind which property in the control binds to what in the class.
Consider this container:
public class ItemInfo : DependencyObject
{
public string Name { get; set; }
public ObservableCollection<SomeDataItem> DataValues { get; set; }
...
Dependency object registration and event handling
...
}
public class MyItemSource : ObservableCollection<ItemInfo>
{
...
}
Now, I wish to display this data in a listview where the control that displays the item is custom. For that, I'd set the MyItemSource to listview's ItemSource and define a ItemTemplate. However, it seems that I have no access to ItemInfo in the ItemTemplate. This is my XAML:
<Grid>
<ListBox ItemsSource="{StaticResource MyStaticDataSource}"
Grid.IsSharedSizeScope="True">
<ListBox.ItemTemplate>
<DataTemplate>
<local:ItemInfoUserControl x:Name="itemInfoUserControl"
Name = "{Binding Name}" <--- this doesn't work
Data = "{Binding DataValues}" <--- this doesn't work
Width="300" Height="200"
Grid.Row="1">
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
Instead of binding to ItemSource's ItemInfo, it binds to the ItemInfoUserControl properties, which is not what I wish it to do. Is there a way to bind properties in itemtemplate to a property in itemsource ? Or is there an alternative approach to what i'm ultimately trying to accomplish ?
Thanks!
Well, first of all you need to use proper binding syntax:
Name = "{Binding Name}"
and
Data = "{Binding DataValues}"
Instead of just "Name = "Binding Name"" and "Data = "Binding DataValues"". Note the addition of "{" and "}" around your binding expression.
This might be enough to solve your problem, as long as Name and DataValues are DependencyProperties in ItemInfoUserControl. If not, you'll need to implement them as DependencyProperties in order to be able to bind to them in XAML. See here for a good MSDN article on defining custom dependency properties.
Edit: Also, just noticed -- you're setting both x:Name and Name. From this article on MSDN:
If Name is available as a property on the class, Name and x:Name can be used interchangeably as attributes, but a parse exception will result if both are specified on the same element. If the XAML is markup compiled, the exception will occur on the markup compile, otherwise it occurs on load.
Remove x:Name="itemInfoUserControl" and see if that helps.
I've got a ComboBox with an ItemsSource which I've bound to a List(Of String).
What I'd like to do is have the XAML update a String property when the SelectedValue of the ComboBox changes. I've seen a whole bunch of examples for TextBoxes which use
Text="{Binding Path=MyString}"
sort of stuff, but I don't really think that'll be the way to go if, in future, I need to change the ItemsSource to a List(Of ObscureObject)...
Binding to the selected property of a combobox is fairly simple.
XAML :
<ComboBox ItemsSource={Binding Path=MyCollection} SelectedItem={Binding Path=MyItem}/>
CodeBehind :
public List<string> MyCollection {get; set;}
public string MyItem {get; set;}
If you want to insert text into the selected item, you'll need to use INotifyPropertyChanged
as for your scalability issue, its a fairly minor change to update the type of a property to reflect a collection. Otherwise you could try binding to an Object although that would mean you would constantly have to recast the object back to the state you want.
You can use SelectedItem property of ComboBox to achieve this.
<ComboBox ItemsSource="{Binding Path=YouList}"
SelectedItem="{Binding Path=MyString}" />
When you change your list in future you will have to bind the SelectedItem with a property of your objects type.
Have a look at this article for more details -
http://japikse.blogspot.com/2008/10/wpf-combobox-selecteditem-selectedvalue.html