I am confused about where I put a dependency property when building a WPF application using the MVVM pattern. Does it go in the Model or the ViewModel?
Edit
After looking at early answers (thanks for those), I find that I am still confused, so I am giving more detail to help someone explain this to me.
I have a class called Station. It is used by surveyors and civil engineers to represent the length along a road. For the most part, a Station is just a double, but it has a few decorations. First off, the format is different. When distance along is greater than 100 feet, we add a + symbol as another way to format it. So 1234.56 feet down the road we might have Station 12+34.56. (I will skip the other decorations since this one is good enough for my point.)
Thus the logic for the special formatting lives in Station, a class in the Model. I want a TextBox in the View to take user input of 1234.56 and coerce it to a text value of "12+34.56". So I want a TextBox to give the user access to a value in the Model, so it needs to be a dependency property. (That is correct, isn't it?) But the business logic for coercing/parsing/understanding how to go back and forth between a TextBox and a Station should live in the Station class. (Right?)
Furthermore, I will later want to give the user the ability to set the station value by clicking on a graphical drawing of the road, including dynamically updating the value as the mouse moves and locking the value in upon issuing a data point. (Now you see why I tried to keep this brief.)
So is this not something that I would want to make a dependency property, especially with the dynamic data point possibly getting involved later? If not, how do I hook a text box item to a station using MVVM? (I really have researched this a lot before asking my question, but with no success.)
Paul
Typically, you wouldn't use Dependency Properties in either the ViewModel or the Model.
Dependency properties are really intended for View-related functionality only. You'd bind a View's DP to a ViewModel, which would instead implement INotifyPropertyChanged.
Putting a DP into the ViewModel or the Model itself would actually violate one of the main goals of MVVM, as this would couple the user interface technology (WPF) to your Model or application-specific types.
With MVVM, you prefer INotifyPropertyChanged properties over DependencyProperties.
Your Station class should implement the property with INotifyPropertyChanged. Your TextBox binding should use a converter to present and read the value in the format you wish.
public class Station : INotifyPropertyChanged
{
private decimal _value;
public decimal Value
{
get { return _value; }
set
{
if (_value == value) return;
_value = value;
NotifyPropertyChanged("Value");
}
}
/* INotifyPropertyChanged implementation */
}
public class StationConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
string formattedValue = // Add the plus here
return formattedValue;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
string numericValue = // Parse the optional '+' out of the value
decimal stationValue = decimal.Parse(numericValue);
}
}
XAML:
<UserControl.Resources>
<StationConverter Key="StationConverter" />
<TheViewModel Key="TheVM" />
<UserControl.Resources>
<TextBox Text="{Binding Path=Station.Value, Source={StaticResource TheVM}, Converter={StaticResource StationConverter}, Mode=TwoWay}"/>
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.
initially it looked a simple problem to me but the more I think more I am confused on what is the best way to achieve what I want.
We are making a WPF application following MVVM
so scenario is -
we have a class say MyClass which has a property of type Complex, it also has a property called Category of type int.
Public Class MyClass
Public Property Category As Integer
Public Property MyProperty As Complex
End Class
Now MyProperty can have a certain values only based on its category.
We have a list of an object which contains all possible values of MyProperty against each category.
My question is considering MVVM, where this list of possible items be placed?
should we make it part of the object like a new property which has all possible values of MyProperty and then have a check when we set the property ? or somewhere else ?
Public Property AvailablePropertyValues As IEnumerable(Of Complex)
Keeping it in object makes it real simple when we bind this object to view, as we don't have to filter the list but I know its trivial to filter lists for each item and we should not consider ease of creating view while modelling our objects.
Any ideas on how to model my object ?
I think you should have all the data that shows up on the screen come from the viewmodel via bindings, that includes the possible choices you want to select from, and the actual selected value, and then validate either through the setters (that's what i see setters to be used for, to insert custom validation logic based on the input value) or nicer, by implementing an IValidatableObject interface so you can also have UI notification of invalid values.
For IValidatableObject you can see the topic http://weblogs.asp.net/scottgu/class-level-model-validation-with-ef-code-first-and-asp-net-mvc-3 or http://msprogrammer.serviciipeweb.ro/2012/03/19/ivalidatableobject-and-idataerrorinfo/
I'm wondering how to bind a color property of an element to the ViewModel without leaking the view implementation (e.g. WPF) into the ViewModel and thus creating a dependency. For example, I have a TextBlock and I've bound its Foreground property like this:
<TextBlock Name="MyTextBlock" Foreground="{Binding Path=PropName}" />
Many sources like this, this, this, etc. use System.Windows.Media.Brush from within the ViewModel, like this:
public System.Windows.Media.Brush PropName
{
get
{
//assume presentation logic to determine correct color.
return System.Windows.Media.Brushes.Red;
}
}
I don't want my ViewModel to be tied to WPF (i.e. via System.Windows.Media.Brush) or any other presentation framework. Is there some way of doing it so that I can use a general or universal color type or even an RGB value and have it interpreted correctly in the XAML from the binding?
I would create an enumeration of colors.
e.g:
enum Colors {Red, Green, Yellow, Pink, Blue};
Then if you are developing with WPF, you could just create a ValueConverter, where you determine if enumeration is Red you can convert it the way you want it and return it for control. That way you can separate the UI logic with viewmodel. Viewmodel tells what color and UI handles the rest.
If you want custom colors, upper answer is a good way of handling it.
Just Bind string with the Hex into Background property of the element like this(Course you'll need a Notify implementation)
XAML
<Border Background="{Binding anycolor}" />
Code
public string anycolor { get; set; }
If you don't want to reference those namespaces maybe you can return a string and use a converter containing something like :
var color = (Color)ColorConverter.ConvertFromString("Red");
or RGB
var color = (SolidColorBrush)(new BrushConverter().ConvertFrom("#ffaacc"));
I might not understand the question very well though.
This is my new favorite creation (sorry about the VB):
Imports System.Windows.Data
Public Class MappingConverter
Implements IValueConverter
Public Property Mappings As New List(Of Mapping)
Public Function Convert(value As Object, targetType As Type, parameter As Object, culture As Globalization.CultureInfo) As Object Implements IValueConverter.Convert
Return Mappings.Where(Function(m) m.Source = value.ToString).FirstOrDefault
End Function
Public Function ConvertBack(value As Object, targetType As Type, parameter As Object, culture As Globalization.CultureInfo) As Object Implements IValueConverter.ConvertBack
Throw New NotImplementedException
End Function
End Class
Public Class Mapping
Public Property Source As String
Public Property Target As Object
End Class
And used like this:
<UserControl.Resources>
<wpflib:MappingConverter x:Key="SelectionHighlightConverter">
<wpflib:MappingConverter.Mappings>
<wpflib:Mapping Source="True" Target="{x:Static Brushes.CornflowerBlue}" />
<wpflib:Mapping Source="False" Target="{x:Static Brushes.White}" />
</wpflib:MappingConverter.Mappings>
</wpflib:MappingConverter>
</UserControl.Resources>
When you use that converter on a binding of a boolean property, the True or False will converted to the specified color. It can actually map any string convertable value (int, string, boolean, etc) to any object you want.
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.
I am building an application that can be used by many users. Each user is classified to one of the next Authentication levels:
public enum AuthenticationEnum
{
User,
Technitian,
Administrator,
Developer
}
Some controls (such as buttons) are exposed only to certain levels of users.
I have a property that holds the authentication level of the current user:
public AuthenticationEnum CurrentAuthenticationLevel { get; set; }
I want to bind this property to the 'Visibilty' property of some controls and pass a parameter to the Converter method, telling it what is the lowest authentication level that is able to see the control.
For example:
<Button Visibility="{Binding Path=CurrentAuthenticationLevel, Converter={StaticResource AuthenticationToVisibility}, ConverterParameter="Administrator"}"/>
means that only 'Administrator' and 'Developer' can see the button.
Unfortunately, the above code passes "Administrator" as a string. Of course I can use switch/case inside the converter method and convert the string to AuthenticationEnum. But this is ugly and prone to maintenance errors (each time the enum changes - the converter method would require a change also).
Is there a better way to pass a nontrivial object as a parameter?
ArsenMkrt's answer is correct,
Another way of doing this is to use the x:Static syntax in the ConverterParameter
<Button ...
Visibility="{Binding Path=CurrentAuthenticationLevel,
Converter={StaticResource AuthenticationToVisibility},
ConverterParameter={x:Static local:AuthenticationEnum.Administrator}}"/>
And in the converter
public class AuthenticationToVisibility : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
AuthenticationEnum authenticationEnum = (AuthenticationEnum)parameter;
//...
}
}
User
(AuthenticationEnum)Enum.Parse(typeof(AuthenticationEnum),parameter)
to parse string as enumerator