I'd like to be able to display an index value from within a DataTemplate, but I don't want the data to be persisted or backed by the model or viewmodel. In other words, if the order of the items in the OC changes, I don't want to have to recalculate the indexes. The value should be intrinsically tied to the underlying index in the OC. It is okay if the index is 0-based (in fact, I'd expect it).
One method that others have used is the AlternationIndex AP, but this has its own pitfalls for certain situations.
One last thought: I can't help but think that a converter is going to be helpful in a final solution.
I would use a converter to do this.
The trick is giving it the source collection, either on the ConverterParameter or a Dependency Property. At that point, conversion is as simple as using IndexOf.
Here's a sample converter that does this:
public class ItemToIndexConverter : IValueConverter
{
public object Convert(...)
{
CollectionViewSource itemSource = parameter as CollectionViewSource;
IEnumerable<object> items = itemSource.Source as IEnumerable<object>;
return items.IndexOf(value as object);
}
public object ConvertBack(...)
{
return Binding.DoNothing;
}
}
You can make the implementation strongly typed, return a formatted string as a number, etc. The basic pattern will be as above though.
This implementation uses the parameter approach, as making a DP is more messy in my view. Because you can't bind ConverterParameter, I have it set to a static resource that is bound to the collection:
<CollectionViewSource x:Key="collectionSource" Source="{Binding Path=MyCollection}" />
...
<TextBlock Text="{Binding Converter={StaticResource ResourceKey=ItemToIndexConverter},
ConverterParameter={StaticResource ResourceKey=collectionSource}}"/>
Related
This question already has answers here:
Binding ConverterParameter
(3 answers)
Closed 4 years ago.
I am trying to bind the text of a textblock based on two things -
Object ShoppingList
Object Items(which is a property of the ShoppingList object. Type is List).
I want to invoke the converter as I want the text to be dependent on change in the value of either of the above.The only way that I could think of was this way as shown below. But this is not possible as I cannot bind the ConverterParameter to the object ShoppingList as it is not a dependency property .
<TextBlock
Margin="5"
TextWrapping="Wrap"
Text="{Binding Items, Converter={StaticResource ABCDConverter}, ConverterParameter="???" />
Below is the converter I had written
Convert(Items obj, object par, xyz culture)
{
if (obj != null && par!=null)
{
var parameter = (ShoppingList)par;
// Different Logic to determine the string to be returned
}
return string.Empty;
}
In simple words, how can I invoke the converter based on changes in either of Items or ShoppingList
Sounds like you're looking for MultiBinding, paired with an IMultiValueConverter.
That being said, I would strongly suggest that you determine the needed value in your ViewModel since you already have all the needed properties there and you know when and how they change. Once you have your derived property, just use regular binding to bind to it in the View. Using MultiBindings will generally tend to break your separation of concerns as you will end up adding specific logic to the converter that really should be in the ViewModel. One case where I did find MultiBindings indispensable was in a group header template (i.e. for a DataGrid). It would be pretty well impossible to get the values from the ViewModel as you don't know how many groups you have or what they will contain at runtime as there are just too many moving parts. In this case, MultiBindings were great.
On the other hand, if you are just targeting a specific element that you know about at design time, you should generally avoid using MultiBinding for the reasons stated above. In addition, MutliBinding are quite verbose compared to regular Bindings, making your XAML harder to read which creates a greater potential for bugs and limits future extensibility and maintainability.
But if you must, here's a simple example:
XAML
<Window x:Class="testapp.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:testapp"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<local:MyMultiConverter x:Key="multiTextConverter"/>
</Window.Resources>
<Grid>
<TextBlock>
<TextBlock.Text>
<MultiBinding Converter="{StaticResource multiTextConverter}">
<Binding Path="someProp"/>
<Binding Path="someOtherProp" />
</MultiBinding>
</TextBlock.Text>
</TextBlock>
</Grid>
</Window>
Converter
public class MyMultiConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType,
object parameter, CultureInfo culture)
{
string ret = null;
if(values.Count() > 1)
{
string value1 = values[0] as string;
string value2 = values[1] as string;
ret = value1 + value2;
}
return ret;
}
public object[] ConvertBack(object value, Type[] targetTypes,
object parameter, CultureInfo culture)
{
}
}
Now, if you're binding to a List, and, assuming you care when items are added or removed, you will rather need to use an ObservableCollection as it implements the INotifyCollectionChanged Interface see here.But this is not quite enough, as the binding will not be subscribing to that event. You will need to listen to the change in collection in your DataContext (ViewModel?). Then create some dummy property, and when the collection changes, just increment the dummy property (with INotifyPropertyChanged of course) and use that property in the multibinding as a third binding. This way, the TextBlock text will get updated if either a property changes, or your list changes.
If you don't care about the changes in the collection, only when you assign a whole new collection, then the sample provided will work as is.
I'm having an issue with C# using WPF.
Just being brief here.
The following code below collects names via Entity Framework into a list.
This is in my MainWindow.xaml.cs file.
public ObservableCollection<string> FruitInfo
{
get
{
using (var context = new Fruit())
{
ObservableCollection<string> fruits= new ObservableCollection<string>();
foreach (var item in context.Fruits.OrderBy(s => s.FruitName))
{
fruits.Add(item.FruitName);
}
return fruits;
}
}
}
In my MainWindow.xaml file, I have the following:
<GroupBox Grid.Row="0" Grid.Column="0" Margin="5" Header="Fruit Info" >
<ComboBox Margin="5" SelectedItem="{Binding FruitInfo}"/>
</GroupBox>
When running my project, I see that the Combo Box does not populate the fruits.
Any ideas why I'm not seeing this?
All thoughts appreciated
You should bind the ItemsSource of the ComboBox to your collection, and the SelectedItem to another string that will represent the user's selection.
First:
<GroupBox Grid.Row="0" Grid.Column="0" Margin="5" Header="Fruit Info" >
<ComboBox Margin="5" ItemsSource="{Binding FruitInfo}" SelectedItem="{Binding SelectedFruit}"/>
</GroupBox>
Second: Make a SelectedFruit in your ViewModel
public string SelectedFruit { get; set; }
Ok, I understand what your trying to do, even though I'm still not sure why you're trying to do it.
The idea of using using is that it creates the variable for you, and the disposes of it when you finish the block of code you're running.
Now, you're creating a variable in that block, and return it ... and then, the system tries to dispose of it. So your return collection must be implicitly convertible to System.IDisposable, which I doubt yours is.
Putting that aside, you should follow emedbo advice. You will bind the source to the collection, and have another property for the selected index (since you're using mvvm).
I wouldn't get the data like that inside a using inside a getter, since it feels like that data you're getting might be deleted, and if it's not, then the whole use of your using is a bit wrong.
Not to mention it's not very readable, and you should aim for readability in most cases.
I don't use the Entity Framework, but I think the pattern for the FruitInfo property is missing of an important piece.
The problem is that the binding mechanism does not realize about the new ObservableCollection, because it expect some "notification" way to be alerted. That is, you have several ways to solve your problem:
use a DependencyPropety instead of an ordinary property: every time you set the property the bound controls are also notified.
I'd recommend this solution: reliable and versatile.
implement the INotifyPropertyChanged interface in the class exposing the FruitInfo property (e.g. MainWindow), then fire a PropertyChanged event on any actual FruitInfo's value changing.
This way is also valuable, but it looks useless adding a thing already exposed in any DependencyObject-derived class. The INotifyPropertyChanged fits perfectly for the POCO classes (Plain-Old CLR-Objects).
give a name to the combobox, then set the ItemsSource property explicitly.
It works fine, but you'd lose the benefits of the data-context inheritance, especially within templates.
the pattern you used creates the collection in a "lazy" fashion: consider avoiding the lazy-way, and set the FruitInfo value before the combobox is created/bound.
Doable, but typically may be applied in a few cases. Also requires that you know for sure the sequence of the objects creation. Keep as latest way.
== UPDATE
Try to modify your code as follows:
private ObservableCollection<string> _fruits = new ObservableCollection<string>();
public ObservableCollection<string> FruitInfo
{
get
{
using (var context = new Fruit())
{
this._fruits.Clear();
foreach (var item in context.Fruits.OrderBy(s => s.FruitName))
{
this._fruits.Add(item.FruitName);
}
return this._fruits;
}
}
}
I'm attempting to databind to an entry in a dictionary where the key is the enum. I've consulted this question, but the answer doesn't work for me. Here are the non-boilerplate parts of my code:
SomePage.xaml:
<!-- Here I try all the ways I can think of. None of them produce any text -->
<TextBlock Text="{Binding Data[0]}" />
<TextBlock Text="{Binding Data[EnumValueA]}" />
<TextBlock Text="{Binding Data[SomeEnum.EnumValueA]}" />
<TextBlock Text="{Binding Data[(local:SomeEnum)EnumValueA]}" />
<TextBlock Text="{Binding Data[(local:SomeEnum)SomeEnum.EnumValueA]}" />
SomePage.xaml.cs:
public SomePage() {
DataContext = new SomeVM();
InitializeComponent();
}
SomeVM.cs:
public enum SomeEnum {
EnumValueA, EnumValueB
}
public class SomeVM {
public Dictionary<SomeEnum, int> Data { get; private set; }
public SomeVM() {
Data = new Dictionary<SomeEnum, int> {
{SomeEnum.EnumValueA, 1337}
};
}
}
Why does this databinding not work?
It is not possible. See http://msdn.microsoft.com/en-us/library/cc645024(v=vs.95).aspx#indexdata
Indexer
Indexers can be used for accessing properties in a path and obtaining
items from a list, but with some notable restrictions:
List item
Numeric integer indexers are supported.
Beginning in Silverlight 4, string indexers are supported.
Only one-dimensional array indexing is supported.
The type being indexed must implement or inherit IList. (List is
accepted, because it implements IList. However IList is not
accepted.)
Numeric integer indexes are specified by declaring the zero-based
index within bracket ([]) characters after the property name; for
example, Employees[2].
Property path evaluation first attempts to use integer indexing for a
collection. If that indexing is not valid for the collection, then the
information within [] is processed as a string. String indexing from a
property path generally expects a collection / business object that is
a dictionary with string keys. String indexing supports binding to
dynamic data objects where the CLR structure of the data source can
change, but the string keys represent a data contract that can still
be bound to by a client UI.
Validation uses indexers to access items of an attached property's
collection as part of its property path usage. The validation
structure for the application can declare UI states within templates
that are used only when validation errors are raised, and can then
reference the active error objects in that context. For example, the
following is a property path for a binding that accesses the first
item in a Validation.Errors collection; the context of the property
path is modified by RelativeSource so that the errors are checked only
at run time on the applied template:
<TextBlock Text="{Binding RelativeSource={RelativeSource
TemplatedParent}, Path=(Validation.Errors)[0].Exception.Message }">
I'm writing a ViewModel library which works with my WPF Custom Controls.
My problem is that my DomainModel has a large amount of Data Types:
Cd,Pens,Gadgets,Books,ecc.
All these Data Types are enumerated with an enum (I have more or less 1 hundred DataTypes), and each data Type corresponds to a DB table.
So the idea is to have a ViewModel library which exposes one property for each data type, thus my UI controls can directly bind the properties of my viewModel. The viewModel for each property return an ObservableCollection.
For instance, if I'd like to have my combo box populated with the "Gadgets" data, in my XAML I 'll have something like :
<my:XCombo ItemsSource="{Binding Gadgets}" .... />
and in my ViewModel I'll have :
public ObservableCollection<Gadgets> Gadgets
{
get
{
//get gadgets data from my domain model
return _model.GetData(DataEnum.Gadgets);
}
}
Now, in order to do that, I need in my ViewModel one property for each enumeration value, but I'd like to avoid to put 1 hundred property accessors. I'm lazy and this can be very error prone.
I know, in c#4 we have dynamic properties, so in this way I can avoid to write 100 property accessors, but I MUST use .net 3.5 which has no dynamic properties, i cannot use .net 4 ;(
Is there anyone who has already had this problem or any suggestion?
Thanks a lot in advance.
You could try to use an indexer property which returns the respective data
public IList this[DataEnum type]
{
return _model.GetData(type);
}
Then bind it using that:
ItemsSource="{Binding [Gadgets]}"
I have an enumeration that has values like
HomeRun, StolenBase, FirstBase, etc.
I want to display these values in a combobox, with a space inserted before the capital letters, so it will display as 'Home Run', 'Stolen Base', etc.
I already have code that can do the formatting for me, and I have added that code to the 'Convert' method of an IValueConverter implementation.
My question is, where do I need to use this converter (in the xaml) such that not only the dropdown, but also the displayed value, will have this formatting? Do I need to implement ConvertBack as well?
I am well aware of setting 'descriptions' for the enumeration and using the popular EnumToDescriptionConverter, but I'd rather stay away from that.
I'm not sure if there is a better way, but you can achieve what you want using an ItemTemplate
<ComboBox.ItemTemplate>
<DataTemplate>
<ContentPresenter
Content="{Binding Converter={StaticResource baseballEnumConverter}}"/>
</DataTemplate>
</ComboBox.ItemTemplate>
This will display the converted value in your ComboBox.
The SelectedValue of the ComboBox will still be the Enum value. You won't need to implement ConvertBack.
[updated] The key point of my answer is that the enum values are converted totally. I think this way is eaier than the coverting each enum value.[/updated]
Where do I need to use this converter (in the xaml) such that not only the dropdown, but also the displayed value, will have this
formatting?
At Binding ItemsSource of ComboBox(ItemsSource="{Binding Source={x:Null}, Converter={StaticResource converter}}"), Please check the following code.
Do I need to implement ConvertBack as well?
No, you don't., because at runtime you cannot modify the enumeration, and even though it can do, you cannot change ItemsSource of ComboBox in VIEW, which means Binding Mode is OneWay.
XAML
<Window x:Class="WpfApplication2.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="clr-namespace:WpfApplication2"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<local:MyEnumConverter x:Key="converter"/>
</Window.Resources>
<StackPanel>
<ComboBox ItemsSource="{Binding Source={x:Null}, Converter={StaticResource converter}, Mode=OneWay}"></ComboBox>
</StackPanel>
</Window>
Code
public enum MyEnum
{
HomeRun, StolenBase, FirstBase
}
[ValueConversion(typeof(object), typeof(List<string>))]
public class MyEnumConverter:IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var names = Enum.GetNames(typeof (MyEnum)).ToArray();
//Add some code to support the thing you want to do(add blank in front of Capital...)
return names;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotSupportedException();
}
}
If you want the selected value of the ComboBox to be converted back to an enum then you will need to implement ConvertBack.
I'd personally go with the description attribute pattern that you mentioned, because
the obvious questions have already been asked, and
You aren't limited to simply inserting spaces at uppercase letters - you can use whatever description you want.
But assuming you want to go with this pattern, you just need to write your converter correctly. I'd suggest something like this:
// Convert method
var formattedNames = new List<string>();
foreach (var value in Enum.GetValues(typeof(Things)))
{
// Format is a method to convert the enum value to the display string
var formattedName = Format(value);
formattedNames.Add(formattedName);
}
// return a list of strings that you can bind to
return formattedNames;
// ConvertBack method
// Unformat is a method to revert the display string back to the enum value
var value = Unformat(formattedValue);
return Enum.Parse(typeof(Things), value);
You could also create a simple class to hold both the display value and the enum, and then set the DisplayPath property on the combo box appropriately
class DisplayEnum
{
public string DisplayValue { get;set; }
public MyEnum ActualValue { get;set; }
}
<ComboBox DisplayMemberPath=DisplayValue ...
Edit
I realise that this won't work because the ConvertBack is attempting to convert a string to an enum, but the actual binding set is a List<string>. I'll leave it here because it is a start in the right direction.
I believe you'd need two converters
to convert the enum type into a set of enum values, and
to convert an enum value to a string. This second converter should implement the ConvertBack method.
As I pointed out, if you don't implement ConvertBack then you won't be able to bind the SelectedValue back to your enum Property on your ViewModel.
You will need to make a dictionary or some other lookup structure that maps the Enum value to the string representation.
There is an hint that you can use as a start :
http://geekswithblogs.net/jawad/archive/2005/06/24/EnumDropDown.aspx
I developped my own enum binding helpers starting with that idea.