Silverlight DataStateBehavior, initial value not used

问题

I am trying to use the Silverlight DataStateBehavior, it works fine in most cases where I click a button which sets say a 'Selected' Property in the view model to either false or true. The DataStateBehavior then tells the VisualStateManager to go to the relevant state.

Like this:

   <Button...>
   <i:Interaction.Behaviors>
             <id:DataStateBehavior Binding="{Binding Selected}" Value="True" TrueState="SelectedVisualState" FalseState="DeselectedVisualState"/>
          </i:Interaction.Behaviors>
   </Button>

The above works fine. What I am trying to do though is to get it to set the correct state when the application loads, if I were to set the 'Selected' property on the view model to true by default I wouldn't see any changes in the UI until I clicked the button to change the viewmodel property.

I know there are several classes involved with the DataState stuff including:

• BindingListener.cs
• ConverterHelper.cs
• DataStateBehavior.cs
• DataStateSwitchBehavior.cs
• DataTrigger.cs

Any clues would be good, Thanks

回答1:

I'm actually going to add a second answer which I just tried, and seems to be cleaner since it can be done all in XAML and without a custom behavior. I'll leave the other answer just as a reference for an alternative solution since they both work.

<i:Interaction.Triggers>
    <i:EventTrigger EventName="Loaded">
        <ic:GoToStateAction StateName="SelectedVisualState"/>
    </i:EventTrigger>
</i:Interaction.Triggers>

You will just need at add a reference to the Microsoft.Expression.Interactions assembly that is part of the Blend SDK.

xmlns:ic="clr-namespace:Microsoft.Expression.Interactivity.Core;assembly=Microsoft.Expression.Interactions"

回答2:

Try this extension of the DataStateBehavior class. When the target element loads, the DataStateBehavior will be evaluate as if the property has been updated.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;

namespace Gusdor.Wpf
{
    /// <summary>
    /// Fix for data state behavior. Behavior will trigger transitions when target element loads.
    /// </summary>
    class DataStateBehaviorFix: Microsoft.Expression.Interactivity.Core.DataStateBehavior
    {
        public bool UseTransitionsOnLoad { get; set; }

        protected override void OnAttached()
        {
            base.OnAttached();

            AssociatedObject.Loaded += AssociatedObject_Loaded;
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();

            AssociatedObject.Loaded -= AssociatedObject_Loaded;
        }

        void AssociatedObject_Loaded(object sender, System.Windows.RoutedEventArgs e)
        {
            Evaluate();
        }

        void Evaluate()
        {
            if (Value == null)
            {
                GotoState(Binding == null, this.AssociatedObject);
            }
            else GotoState(Value.Equals(Binding), this.AssociatedObject);
        }

        /// <summary>
        /// Attempts to change to the named state. Walks up tree to first match.
        /// </summary>
        /// <param name="flag"></param>
        /// <param name="element"></param>
        void GotoState(bool flag, FrameworkElement element)
        {          
            string stateName = flag ? TrueState : FalseState;

            if (HasState(element, stateName))
            {
                bool ret = System.Windows.VisualStateManager.GoToElementState(element, stateName, UseTransitionsOnLoad);
            }
            else if (element.Parent as FrameworkElement != null)
                GotoState(flag, element.Parent as FrameworkElement);
        }
        /// <summary>
        /// Checks if an element has the state named
        /// </summary>
        /// <param name="element"></param>
        /// <param name="stateName"></param>
        /// <returns></returns>
        bool HasState(FrameworkElement element, string stateName)
        {
            var groups = Microsoft.Expression.Interactivity.VisualStateUtilities.GetVisualStateGroups(element).Cast<VisualStateGroup>();

            return groups.Any(p => p.States.Cast<VisualState>().Any(s => s.Name == stateName));
        }
    }
}

回答3:

One way I have solved this problem is to make a behavior you can add to your control to put it into an initial visual state upon loading. Here is a simple example:

public class InitialVisualStateBehavior : Behavior<Control>
{
    public static readonly DependencyProperty InitialStateProperty = DependencyProperty.Register(
        "InitialState",
        typeof(string),
        typeof(InitialVisualStateBehavior),
        null);

    public string InitialState
    {
        get { return (string)GetValue(InitialStateProperty); }
        set { SetValue(InitialStateProperty, value); }
    }

    protected override void OnAttached()
    {
        base.OnAttached();

        if (this.AssociatedObject != null)
        {
            this.AssociatedObject.Loaded += new RoutedEventHandler(AssociatedObject_Loaded);
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        if (this.AssociatedObject != null)
        {
            this.AssociatedObject.Loaded -= AssociatedObject_Loaded;
        }
    }

    private void AssociatedObject_Loaded(object sender, RoutedEventArgs e)
    {
        VisualStateManager.GoToState(this.AssociatedObject, this.InitialState, false);
    }
}

You would then just add that behavior to the UserControl-level in XAML:

<i:Interaction.Behaviors>
    <myi:InitialVisualStateBehavior InitialState="SelectedVisualState" />
</i:Interaction.Behaviors>

You could also easily modify this to accept a comma separated list of initial states which you could then split and loop through if you needed to put the control in a bunch of different mutually exclusive states after loading.

This might also be refactored into a TriggerAction that you could just trigger off of the Loaded event of the control, I'm not sure which way would be cleaner.

来源：https://stackoverflow.com/questions/2112749/silverlight-datastatebehavior-initial-value-not-used