Setting a property with an EventTrigger
I want to be able to set a property with an EventTrigger, there's a number of problems with this.
1) EventTriggers only support Actions, so I must use a storyBoard to set my properties.
2) Once I use a storyboard, I have two options:
- Stop: Once the animation has stopped the value reverts back to before the animation started
- HoldEnd: This locks the property, so that neither code, nor user interaction can change the property that the animation is holding.
In the below example, I want to set the IsChecked property to False when the button is clicked and I want the user to be able to change the IsChecked and/or I want to be able to change the property in code.
Example:
<EventTrigger
SourceName="myButton"
RoutedEvent="Button.Click">
<EventTrigger.Actions>
<BeginStoryboard>
<Storyboard>
<BooleanAnimationUsingKeyFrames
Storyboard.TargetName="myCheckBox"
Storyboard.TargetProperty="IsChecked"
FillBehavior="Stop">
<DiscreteBooleanKeyFrame
KeyTime="00:00:00"
Value="False" />
</BooleanAnimationUsingKeyFrames>
</Storyboard>
</BeginStoryboard>
</EventTrigger.Actions>
</EventTrigger>
I realize that I can use the "Completed" event after the storyboard completes to set the value to False. However, in this instance I want to contain the logic within the XAML, as this logic will be used on a custom control and is only specific to the UI.
Just create your own action.
namespace WpfUtil
{
using System.Reflection;
using System.Windows;
using System.Windows.Interactivity;
/// <summary>
/// Sets the designated property to the supplied value. TargetObject
/// optionally designates the object on which to set the property. If
/// TargetObject is not supplied then the property is set on the object
/// to which the trigger is attached.
/// </summary>
public class SetPropertyAction : TriggerAction<FrameworkElement>
{
// PropertyName DependencyProperty.
/// <summary>
/// The property to be executed in response to the trigger.
/// </summary>
public string PropertyName
{
get { return (string)GetValue(PropertyNameProperty); }
set { SetValue(PropertyNameProperty, value); }
}
public static readonly DependencyProperty PropertyNameProperty
= DependencyProperty.Register("PropertyName", typeof(string),
typeof(SetPropertyAction));
// PropertyValue DependencyProperty.
/// <summary>
/// The value to set the property to.
/// </summary>
public object PropertyValue
{
get { return GetValue(PropertyValueProperty); }
set { SetValue(PropertyValueProperty, value); }
}
public static readonly DependencyProperty PropertyValueProperty
= DependencyProperty.Register("PropertyValue", typeof(object),
typeof(SetPropertyAction));
// TargetObject DependencyProperty.
/// <summary>
/// Specifies the object upon which to set the property.
/// </summary>
public object TargetObject
{
get { return GetValue(TargetObjectProperty); }
set { SetValue(TargetObjectProperty, value); }
}
public static readonly DependencyProperty TargetObjectProperty
= DependencyProperty.Register("TargetObject", typeof(object),
typeof(SetPropertyAction));
// Private Implementation.
protected override void Invoke(object parameter)
{
object target = TargetObject ?? AssociatedObject;
PropertyInfo propertyInfo = target.GetType().GetProperty(
PropertyName,
BindingFlags.Instance|BindingFlags.Public
|BindingFlags.NonPublic|BindingFlags.InvokeMethod);
propertyInfo.SetValue(target, PropertyValue);
}
}
}
In this case I'm binding to a property called DialogResult on my viewmodel.
<Grid>
<Button>
<i:Interaction.Triggers>
<i:EventTrigger EventName="Click">
<wpf:SetPropertyAction PropertyName="DialogResult" TargetObject="{Binding}"
PropertyValue="{x:Static mvvm:DialogResult.Cancel}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
Cancel
</Button>
</Grid>
As much as I love XAML, for this kinds of tasks I switch to code behind. Attached behaviors are a good pattern for this. Keep in mind, Expression Blend 3 provides a standard way to program and use behaviors. There are a few existing ones on the Expression Community Site.
I modified Neutrino's solution to make the xaml look less verbose when specifying the value:
Sorry for no pictures of the rendered xaml, just imagine a [=] hamburger button that you click and it turns into [<-] a back button and also toggles the visibility of a Grid.
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
...
<Grid>
<Button x:Name="optionsButton">
<i:Interaction.Triggers>
<i:EventTrigger EventName="Click">
<local:SetterAction PropertyName="Visibility" Value="Collapsed" />
<local:SetterAction PropertyName="Visibility" TargetObject="{Binding ElementName=optionsBackButton}" Value="Visible" />
<local:SetterAction PropertyName="Visibility" TargetObject="{Binding ElementName=optionsPanel}" Value="Visible" />
</i:EventTrigger>
</i:Interaction.Triggers>
<glyphs:Hamburger Width="10" Height="10" />
</Button>
<Button x:Name="optionsBackButton" Visibility="Collapsed">
<i:Interaction.Triggers>
<i:EventTrigger EventName="Click">
<local:SetterAction PropertyName="Visibility" Value="Collapsed" />
<local:SetterAction PropertyName="Visibility" TargetObject="{Binding ElementName=optionsButton}" Value="Visible" />
<local:SetterAction PropertyName="Visibility" TargetObject="{Binding ElementName=optionsPanel}" Value="Collapsed" />
</i:EventTrigger>
</i:Interaction.Triggers>
<glyphs:Back Width="12" Height="11" />
</Button>
</Grid>
...
<Grid Grid.RowSpan="2" x:Name="optionsPanel" Visibility="Collapsed">
</Grid>
You can also specify values this way like in Neutrino's solution:
<Button x:Name="optionsButton">
<i:Interaction.Triggers>
<i:EventTrigger EventName="Click">
<local:SetterAction PropertyName="Visibility" Value="{x:Static Visibility.Collapsed}" />
<local:SetterAction PropertyName="Visibility" TargetObject="{Binding ElementName=optionsBackButton}" Value="{x:Static Visibility.Visible}" />
<local:SetterAction PropertyName="Visibility" TargetObject="{Binding ElementName=optionsPanel}" Value="{x:Static Visibility.Visible}" />
</i:EventTrigger>
</i:Interaction.Triggers>
<glyphs:Hamburger Width="10" Height="10" />
</Button>
And here's the code.
using System;
using System.ComponentModel;
using System.Reflection;
using System.Windows;
using System.Windows.Interactivity;
namespace Mvvm.Actions
{
/// <summary>
/// Sets a specified property to a value when invoked.
/// </summary>
public class SetterAction : TargetedTriggerAction<FrameworkElement>
{
#region Properties
#region PropertyName
/// <summary>
/// Property that is being set by this setter.
/// </summary>
public string PropertyName
{
get { return (string)GetValue(PropertyNameProperty); }
set { SetValue(PropertyNameProperty, value); }
}
public static readonly DependencyProperty PropertyNameProperty =
DependencyProperty.Register("PropertyName", typeof(string), typeof(SetterAction),
new PropertyMetadata(String.Empty));
#endregion
#region Value
/// <summary>
/// Property value that is being set by this setter.
/// </summary>
public object Value
{
get { return (object)GetValue(ValueProperty); }
set { SetValue(ValueProperty, value); }
}
public static readonly DependencyProperty ValueProperty =
DependencyProperty.Register("Value", typeof(object), typeof(SetterAction),
new PropertyMetadata(null));
#endregion
#endregion
#region Overrides
protected override void Invoke(object parameter)
{
var target = TargetObject ?? AssociatedObject;
var targetType = target.GetType();
var property = targetType.GetProperty(PropertyName, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static | BindingFlags.Instance);
if (property == null)
throw new ArgumentException(String.Format("Property not found: {0}", PropertyName));
if (property.CanWrite == false)
throw new ArgumentException(String.Format("Property is not settable: {0}", PropertyName));
object convertedValue;
if (Value == null)
convertedValue = null;
else
{
var valueType = Value.GetType();
var propertyType = property.PropertyType;
if (valueType == propertyType)
convertedValue = Value;
else
{
var propertyConverter = TypeDescriptor.GetConverter(propertyType);
if (propertyConverter.CanConvertFrom(valueType))
convertedValue = propertyConverter.ConvertFrom(Value);
else if (valueType.IsSubclassOf(propertyType))
convertedValue = Value;
else
throw new ArgumentException(String.Format("Cannot convert type '{0}' to '{1}'.", valueType, propertyType));
}
}
property.SetValue(target, convertedValue);
}
#endregion
}
}
EDIT: The Interactivity dll is no longer part of Blend and is now the "Microsoft.Xaml.Behaviors.Wpf" NuGet package. Code listed here: https://github.com/microsoft/XamlBehaviorsWpf
See: https://devblogs.microsoft.com/dotnet/open-sourcing-xaml-behaviors-for-wpf/
Steps to migrate from old Blend Microsoft.Expression.Interactions.dll to new opensource Interactivity dll (hopefully my old notes are correct ;p):
1. Install the "Microsoft.Xaml.Behaviors.Wpf" NuGet package.
2. Edit xaml files:
Replace 'http://schemas.microsoft.com/expression/2010/interactivity' and
'http://schemas.microsoft.com/expression/2010/interactions'
with 'http://schemas.microsoft.com/xaml/behaviors'.
Replace 'xmlns:ei="clr-namespace:Microsoft.Expression.Interactivity.Core;assembly=Microsoft.Expression.Interactions"' and
'xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"'
with 'xmlns:i="http://schemas.microsoft.com/xaml/behaviors"'.
3. Edit C# files:
Replace usings in c# files 'Microsoft.Xaml.Interactivity' and
'Microsoft.Xaml.Interactions'
with 'Microsoft.Xaml.Behaviors'.
Remove references to 'Microsoft.Expression.Interactions' and
'System.Windows.Interactivity'.