To have less redundant XAML markup i try to get a radiobutton-type selection control to be populated generically, i.e. i use an ItemsControl with an enum as ItemsSource and create a DataTemplate which shows which item is selected by checking whether the enum value of the item is the same as the current setting.
This alone cannot be done using a simple converter or DataTrigger because two bindings are needed, so i created a generic MutliValueConverter to check for equality:
<CheckBox.Visibility>
<MultiBinding Converter="{StaticResource EqualityComparisonConv}">
<Binding Path="Key"/>
<Binding Path="DisplayMode_Current" Source="{x:Static local:App.Settings}"/>
</MultiBinding>
</CheckBox.Visibility>
public class EqualityComparisonConverter : IMultiValueConverter
{
#region IMultiValueConverter Members
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
if (values.Length < 2) throw new Exception("At least two inputs are needed for comparison");
bool output = (bool)values.Skip(1).Aggregate(values[0], (x1, x2) => { return x1.Equals(x2); });
return output;
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
throw new NotSupportedException();
}
#endregion
}
The obvious problem here is that the converter returns a boolean which i would need to convert to Visibility first.
Wrapping the MultiBinding in another binding with just a converter does not work because the properties are not dependency properties (hence cannot have a binding assigned to them). I could think of a few workarounds like storing the bool in some Tag property, so i can use that as a new binding source, but i would be more interested in something like this:
<CheckBox.Visibility>
<local:PipeConverter Converter="{StaticResource BooleanToVisibilityConv}">
<MultiBinding Converter="{StaticResource EqualityComparisonConv}">
<Binding Path="Key"/>
<Binding Path="DisplayMode_Current" Source="{x:Static local:App.Settings}"/>
</MultiBinding>
</local:PipeConverter>
</CheckBox.Visibility>
This class would need to update its output when changes in the original binding occur and it would need to be able to expose its output value to the Visibility property but i do not know how to achieve either. One problem one runs into is that there is a need for dependency properties so inheriting from DependencyObject would be nice, but inheriting from a Binding class would also make sense because the PipeConverter should bind and needs to be set as the value of another dependency property.
Based on the idea (see publicgk's answer, option C) that one can create a converter which contains a collection of converters which internally are used in sequence i wrote a shoddy implementation which fits my needs. i.e. i can use a MultiValueConverter at the beginning and pipe the output into a list of normal converters:
[ContentProperty("Converters")]
public class GroupConverter : IValueConverter, IMultiValueConverter
{
private IMultiValueConverter _multiValueConverter;
public IMultiValueConverter MultiValueConverter
{
get { return _multiValueConverter; }
set { _multiValueConverter = value; }
}
private List<IValueConverter> _converters = new List<IValueConverter>();
public List<IValueConverter> Converters
{
get { return _converters; }
set { _converters = value; }
}
#region IValueConverter Members
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return GroupConvert(value, Converters);
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return GroupConvertBack(value, Converters.ToArray().Reverse());
}
private static object GroupConvert(object value, IEnumerable<IValueConverter> converters)
{
return converters.Aggregate(value, (acc, conv) => { return conv.Convert(acc, typeof(object), null, null); });
}
private static object GroupConvertBack(object value, IEnumerable<IValueConverter> converters)
{
return converters.Aggregate(value, (acc, conv) => { return conv.ConvertBack(acc, typeof(object), null, null); });
}
#endregion
#region IMultiValueConverter Members
private InvalidOperationException _multiValueConverterUnsetException =
new InvalidOperationException("To use the converter as a MultiValueConverter the MultiValueConverter property needs to be set.");
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
if (MultiValueConverter == null) throw _multiValueConverterUnsetException;
var firstConvertedValue = MultiValueConverter.Convert(values, targetType, parameter, culture);
return GroupConvert(firstConvertedValue, Converters);
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
if (MultiValueConverter == null) throw _multiValueConverterUnsetException;
var tailConverted = GroupConvertBack(value, Converters.ToArray().Reverse());
return MultiValueConverter.ConvertBack(tailConverted, targetTypes, parameter, culture);
}
#endregion
}
(As you can see i pretty much completely disregard the ConverterParameters, TargetTypes and CultureInfo parameters, further the ConvertBack methods are untested so i do not advise anyone to actually use this.)
XAML usage:
<vc:GroupConverter MultiValueConverter="{StaticResource EqualityComparisonConv}">
<StaticResource ResourceKey="BoolToVisibilityConv"/>
</vc:GroupConverter>
Three options:
Option A: Convert your bool to visibility in your multivalueconverter (its just one line)
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
if (values.Length < 2)
throw new Exception("At least two inputs are needed for comparison");
bool output = (bool)values.Skip(1).Aggregate(values[0], (x1, x2) =>
{ return x1.Equals(x2); });
return output ? Visibility.Visible : Visibility.Collapsed;
}
Option B: Use the existing booltovisibilityconverter programatically
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
if (values.Length < 2)
throw new Exception("At least two inputs are needed for comparison");
bool output = (bool)values.Skip(1).Aggregate(values[0], (x1, x2) =>
{ return x1.Equals(x2); });
System.Windows.Controls.BooleanToVisibilityConverter booltovisibilityconverter = new System.Windows.Controls.BooleanToVisibilityConverter();
return booltovisibilityconverter.Convert(output, System.Type.GetType("System.Boolean"), parameter, culture);
}
PS: You may want to cache the booltovisibilityconverter instead of creating it everytime.
Option C: Pipe your converters
Piping Value Converters in WPF
PS: Beaware that this article is quite old.
Related
In WPF one has the possibility to use a converter in binding, so that one can bind for instance a Visibility property of a control to a Boolean property in the view model.
For this specific pairing (Visibility and Boolean) WPF does offer an out-of-the-box converter called BooleanToVisibilityConverter.
But let's say I'd like to bind a Boolean property of a control to a Visibility property in the view model. Is there any way to use the standard BooleanToVisibilityConverter and tell the binding to invert it (to use ConvertBack instead on Convert and vice versa)?
Or do I have to write another converter for that case?
So, there is no built-in way of inverting the converter. We can, however, work around that by introducing a "shim" converter like this one:
public class InverterConverter : IValueConverter
{
public IValueConverter Converter { get; set; }
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return Converter.ConvertBack(value, targetType, parameter, culture);
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return Converter.Convert(value, targetType, parameter, culture);
}
}
With the usage as follows:
<ContentControl>
<ContentControl.Content>
<Binding>
<Binding.Converter>
<InverterConverter Converter="{StaticResource YourConverter}" />
</Binding.Converter>
</Binding>
</ContentControl.Content>
</ContentControl>
This, obviously, is some heavy syntax but we can simplify it with this little markup extension:
public class InvertedExtension : MarkupExtension
{
public IValueConverter Converter { get; set; }
public InvertedExtension(IValueConverter converter)
{
Converter = new InverterConverter() { Converter = converter };
}
public override object ProvideValue(IServiceProvider serviceProvider)
{
return Converter;
}
}
<ContentControl Content="{Binding Converter={Inverted {StaticResource MyConverter}}}" />
Is there any way to use the standard BooleanToVisibilityConverter and tell the binding to invert it (to use ConvertBack instead on Convert and vice versa)?
No.
Or do I have to write another converter for that case?
Yes.
You could implement a generic converter that accepts "true" and a "false" values of any type:
public class BooleanConverter<T> : IValueConverter
{
public T True { get; set; }
public T False { get; set; }
public virtual object Convert(object value, Type targetType, object parameter, CultureInfo culture) =>
value is bool && ((bool)value) ? True : False;
public virtual object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) =>
value is T && EqualityComparer<T>.Default.Equals((T)value, True);
}
...and derive from this for each type that you want to handle:
public class BooleanToVisibilityNegationConverter : BooleanConverter<Visibility>
{
public BooleanToVisibilityNegationConverter()
: base()
{
True = Visibility.Hidden;
False = Visibility.Visible;
}
}
I have a enum to string converter
public class EnumToStringConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter,
CultureInfo culture)
{
MailSettingsStateEnum enumValue = (MailSettingsStateEnum)value;
// extension method on the enum, to return a string based on enum.
return enumValue.Description();
}
// ConvertBack not relevant here.
}
I am using this in wpf xaml easily as follows to set the Content property of a label.
<Label Content="{Binding MailSettingState, Converter={StaticResource
EnumConverterString}}"
BorderBrush="{Binding MailSettingState, Converter={StaticResource
EnumConverterBorderBrush}}" />
Now as you can see, I have another property BorderBrush. I also have to set this based on the same enum. And so I had to write another converter EnumConverterBorderBrush
So is there a way by which I have only one converter, and it return an object which has two properties and i can use these properties in the xaml? I can create the converter, its easy, but I dont know how to use it in xaml. Say the converter returned an object and has tow property called MessageString(of type string), and another BorderBrush of the type Brush, how do I use it the xaml?
You can switch the output based on the targetType you receive in your converter.
So you could do something like this:
public class EnumToStringConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter,
CultureInfo culture)
{
var enumValue = (MailSettingsStateEnum)value;
switch(targetType)
{
case typeof(string)
return enumValue.Description();
case typeof(Brush)
return enumValue.GetBrush();
default:
throw new NotSupportedException("Type not supported")
}
}
// ConvertBack not relevant here.
}
Now you'll have one converter to rule them all!
converter should return object which match requested targetType. converter can return different values for input enum value depending on parameter. I think it is more flexible than relying on targetType only.
public class SpecEnumConverter: IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is Enum)
{
if ((string) parameter == "brush")
return "Red"; // return brush here!
// if not pre-defined parameter (null or any other), return description
return (int) value; // return enum description here!
}
return value;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
usage:
<Label Content="{Binding MailSettingState, Converter={StaticResource
EnumConverterSpec}}"
BorderBrush="{Binding MailSettingState, Converter={StaticResource
EnumConverterSpec}, ConverterParameter='brush'}" />
I already commented above, but here's the solution.
<Label DataContext="{Binding MailSettingState, Converter={converters:EnumConverter}}" Content="{Binding Label}" BorderBrush="{Binding BorderBrush}"/>
public class EnumConverter: MarkupExtension, IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var enumValue = (MailSettingsStateEnum) value;
return new ConvertedEnum { Label = enumValue.Description(),
BorderBrush = new BorderBrush()};
}
// ConvertBack not relevant here.
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this;
}
}
public class ConvertedEnum
{
public string Label {get; set;}
public BorderBrush {get; set;}
}
Separate converters still look prettier to me.
When you drag the slider, you get a ConvertBack (expected), but why do I then get a "Convert" straight after that? I'd only expect Convert to be called when its first initialized, or if it was raising a property change notification, but it doesn't.
<Window x:Class="WpfApplication10.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:l="clr-namespace:WpfApplication10"
Title="MainWindow">
<Slider Value="{Binding Value, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay, Converter={l:Converter}}"/>
</Window>
public class Converter : MarkupExtension, IValueConverter
{
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this;
}
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return value;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return value;
}
}
public partial class MainWindow : Window
{
public double Value { get; set; }
public MainWindow()
{
DataContext = this;
InitializeComponent();
}
}
This is likely occurring because you're using .NET 4, which changed the way bindings work a little bit : http://karlshifflett.wordpress.com/2009/05/27/wpf-4-0-data-binding-change-great-feature/
Whereas prior to 4 the binding wouldn't update back (assuming that it didn't need to), the new behavior is to do that by default. Rationale is explained in the linked blog.
Edit: I suppose I should ask if there is a reason you don't want it to convert back, or if it's just curiosity? If you need to prevent converting back, the clearest strategy is probably to keep track of your last converted value to parrot back :
public class Converter : MarkupExtension, IValueConverter
{
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this;
}
public object lastValue;
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return lastValue;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
lastValue = value;
return value;
}
}
There's also the option of setting the binding to OneWayToSource, which would keep it from setting back to the slider value... assuming you don't need to push data back to the slider.
I just started playing around with Silverlight and a supposedly simple thing like binding a Combobox is driving me nuts. I read a bunch of articles now but none really address the issue that I'm after or were made for Silverlight 2 and don't seem to work.
Let's say I have an entity object "User" which has a "UserStatus" field. In the database UserStatus field is defined as byte and in code it's defined as:
public enum UserStatus : byte
{
Active = 1,
Locked = 2,
Suspended = 3,
}
When ADO.NET entity framework creates the user entity it leaves the UserStatus field as byte. So, to address this I stumbled across IValueConverter and implemented the following:
public class EnumConverter : IValueConverter
{
#region IValueConverter Members
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
switch (parameter.ToString())
{
case "UserStatus":
return ((UserStatus)value).ToString();;
}
return "?";
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
#endregion
}
Now, I also need to supply the Combobox an ItemSource, so I implemented this:
internal static class EnumValueCache
{
private static readonly IDictionary<Type, object[]> Cache = new Dictionary<Type, object[]>();
public static object[] GetValues(Type type)
{
if (!type.IsEnum)
throw new ArgumentException("Type '" + type.Name + "' is not an enum");
object[] values;
if (!Cache.TryGetValue(type, out values))
{
values = type.GetFields()
.Where(f => f.IsLiteral)
.Select(f => f.GetValue(null))
.ToArray();
Cache[type] = values;
}
return values;
}
}
public class EnumValuesConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value == null)
return null;
switch (parameter.ToString())
{
case "UserStatus":
return EnumValueCache.GetValues(typeof(UserStatus));
}
return null;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
And then finally, this is how I added it to my XAML:
<ComboBox ItemsSource="{Binding UserStatus, Mode=TwoWay, Converter={StaticResource EnumValuesConverter}, ConverterParameter='UserStatus'}"
SelectedItem="{Binding UserStatus, Mode=TwoWay, Converter={StaticResource EnumConverter}, ConverterParameter='UserStatus'}" />
What happens now though is that the ItemsSource gets correctly bound and I see all the options in the dropdown, however, the SelectedItem is not set.
Even when I try to manually set SelectedItem to ="1" or "Active", none of them work.
Can anyone help me out and tell me what's wrong, why I can't seem to get the SelectedItem set?
Thanks,
Tom
I can see two problems with your code.
First, in silverlight 3 the ComboBox matches the object in the SelectedItem to the set of objects in the ItemsSource via the object.Equals method. However your GetValues method returns an array of Boxed enum values. Whereas your EnumConverter returns a string. Hence you asking Silverlight to compare a byte with a string, these are never equal.
Secondly, you need to place some code in the ConvertBack method if you are going to two way bind the SelectedItem (BTW there is no need for a twoway binding the ItemsSource).
I have a control that I want to show/hide, depending on the value of a boolean.
I have a NegatedBooleanConverter (switches true to false and vice versa) and I need to run this converter first.
I have a BooleanToVisibilityConverter and I need to run this converter after the NegatedBoolConverter.
How can I fix this problem? I want to do this in XAML.
edit: this is a possible solution.
That doesn't seem to work. It first converts the value with the separate converters and then does something with the converted values.
What I need is:
Convert the value with the first converter (this gives convertedValue).
Convert convertedValue with the second converter and it's this result that I need.
This is what I did:
public class CombiningConverter : IValueConverter
{
public IValueConverter Converter1 { get; set; }
public IValueConverter Converter2 { get; set; }
public object Convert(
object value, Type targetType, object parameter, CultureInfo culture)
{
object convertedValue =
Converter1.Convert(value, targetType, parameter, culture);
return Converter2.Convert(
convertedValue, targetType, parameter, culture);
}
public object ConvertBack(
object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
and I call it like this:
<converters:CombiningConverter
x:Key="negatedBoolToVisibilityConverter"
Converter1="{StaticResource NegatedBooleanConverter}"
Converter2="{StaticResource BoolToVisibilityConverter}" />
A MultiValueConverter might also be possible I think. Maybe I'll try that later.
Expanding on Natrium's great answer...
XAML
<conv:ConverterChain x:Key="convBoolToInverseToVisibility">
<conv:BoolToInverseConverter />
<BooleanToVisibilityConverter />
</conv:ConverterChain>
Class
/// <summary>Represents a chain of <see cref="IValueConverter"/>s to be executed in succession.</summary>
[ContentProperty("Converters")]
[ContentWrapper(typeof(ValueConverterCollection))]
public class ConverterChain : IValueConverter
{
private readonly ValueConverterCollection _converters= new ValueConverterCollection();
/// <summary>Gets the converters to execute.</summary>
public ValueConverterCollection Converters
{
get { return _converters; }
}
#region IValueConverter Members
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return Converters
.Aggregate(value, (current, converter) => converter.Convert(current, targetType, parameter, culture));
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return Converters
.Reverse()
.Aggregate(value, (current, converter) => converter.Convert(current, targetType, parameter, culture));
}
#endregion
}
/// <summary>Represents a collection of <see cref="IValueConverter"/>s.</summary>
public sealed class ValueConverterCollection : Collection<IValueConverter> { }
In this case, you don't need a converter chain. You just need a configurable converter. This is similar to Carlo's answer above, but explicitly defines the true and false values (which means you can use the same converters for Hidden, Visible or Collapsed conversions).
[ValueConversion(typeof(bool), typeof(Visibility))]
public class BoolToVisibilityConverter : IValueConverter
{
public Visibility TrueValue { get; set; }
public Visibility FalseValue { get; set; }
public BoolToVisibilityConverter()
{
// set defaults
FalseValue = Visibility.Hidden;
TrueValue = Visibility.Visible;
}
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return (bool)value ? TrueValue : FalseValue;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
Then in XAML:
<BoolToVisibilityConverter x:Key="BoolToVisibleConverter"
FalseValue="Hidden"
TrueValue="Visible" />
What we do in our project is make a regular BooleanToVisibilityConverter, said converter takes one parameter (anything at all, a string, an int, bool, whatever). If the parameter is set it inverts the result, if not, it spits out the regular result.
public class BooleanToVisibilityConverter : IValueConverter
{
#region IValueConverter Members
public object Convert(object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
bool? isVisible = value as bool?;
if (parameter != null && isVisible.HasValue)
isVisible = !isVisible;
if (isVisible.HasValue && isVisible.Value == true)
return Visibility.Visible;
else
return Visibility.Collapsed;
}
public object ConvertBack(object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new System.NotImplementedException();
}
#endregion
}
To answer my own question again: I have been using this solution for years now:
Piping Value Converters in WPF - CodeProject
It makes a new converter of 2 existing converters, calling the first one first, and then the second one etc etc.
I'm pretty pleased with this solution.
At this point I'd like to suggest ValueConverters.NET (NuGet) which has a ton of useful ValueConverters, including the ValueConverterGroup that can be used to combine ValueConverters.
BoolToValueConverters also offer fields to define the TrueValue, FalseValue as well as if the input IsInverted, so a ValueConverterGroup is not even necessary in most cases.
Just to illustrate how easy the life can get, here is a sample demonstration which shows a converter that displays an element if the binding is not null:
<Window ...
xmlns:vc="clr-namespace:ValueConverters;assembly=ValueConverters"
...>
...
<vc:ValueConverterGroup x:Key="IsNotNullToVisibilityConverter">
<vc:NullToBoolConverter IsInverted="True" />
<vc:BoolToVisibilityConverter />
</vc:ValueConverterGroup>
ValueConverters are a prime example of reinventions of the wheel in a lot of WPF applications. Why it has to be?
Also complex things can often be solved with StyleTriggers or within the ViewModels logic itself.
It almost never happens that I need to build a custom converter. In my opinion WPF has enough engineer requirements already.
Personally I would just make 1 single converter that does the full conversion. Unless you desperately need the converters (like the negation) in other places, it will be easier to maintain (imo) if the conversion is done once, in one place.
I think you may want to use a Multiconverter here instead of two separate converters. You should be able to reuse the logic from your existing converters. Check out this discussion for a start.
To address this specific problem, instead of using two converters your could write your own BoolToVisibilityConverter that uses the ConverterParameter (as a bool) to determine whether or not to negate the original Boolean.
I've just created what I've called ReversedBooleanToVisibilityConverter to basically do what those 2 would do for you but in one step.
Here is a combination of Natrium and metao answers to save you some time:
public class ComparisonConverter : IValueConverter
{
public object TrueValue { get; set; } = true;
public object FalseValue { get; set; } = false;
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return value?.Equals(parameter) == true? TrueValue : FalseValue;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return value?.Equals(TrueValue) == true ? parameter : Binding.DoNothing;
}
}
And how you use it:
<converter:ComparisonConverter x:Key="ComparisonConverter" />
<converter:ComparisonConverter TrueValue="{x:Static Visibility.Visible}"
FalseValue="{x:Static Visibility.Collapsed}"
x:Key="ComparisonToVisibilityConverter" />
...
<RadioButton IsChecked="{Binding Type, ConverterParameter={x:Static entities:LimitType.MinMax}, Converter={StaticResource ComparisonConverter}}"/>
<TextBox Visibility="{Binding Type, ConverterParameter={x:Static entities:LimitType.MinMax}, Converter={StaticResource ComparisonToVisibilityConverter}}"/>