如何在CollectionChanged上更新IValueConverter？ [英] How do I update an IValueConverter on CollectionChanged?

问题描述

这是一个解释我的问题的基本示例。假设我有

Here's a basic example to explain my problem. Let's say I have

ObservableCollection<int> Numbers {get; set;}

和一个返回数字总和的IValueConverter。

and an IValueConverter that returns the sum of Numbers.

通常我会做的是将IValueConverter更改为一个IMultiValueConverter，并将第二个值绑定到Numbers.Count像这样

Normally what I'd do is changed the IValueConverter into an IMultiValueConverter and bind a second value to Numbers.Count like this

<MultiBinding Converter="{StaticResource SumTheIntegersConverter}">
    <Binding Path="Numbers"     />
    <Binding Path="Numbers.Count"   />
</MultiBinding>

但是，我无法使用此方法来解决我的实际问题。似乎应该有一个更好的方式来更新绑定，当收集更改，我只是没有想到。

However I'm unable to use this method to solve my actual problem. It seems like there should be a better way to update the binding when the collection changes that I'm just not thinking of. What's the best way to get the value converter to run when items are added and removed to Numbers?

推荐答案

我最终做了一些事情，最好的办法是让值转换器运行像这样似乎工作。它远不是一个最佳的解决方案，我仍然对更好的东西感兴趣，但它似乎为我的目的。

I ended up doing something like this which seems to work. It's far from an optimal solution and I'd still be interested in something better but it seems to work for my purposes.

class CollectionChangedHandlingValueConverter : IValueConverter
{
    DependencyObject myTarget;
    DependencyProperty myTargetProperty;

    //If this ever needs to be called from XAML you can make it a MarkupExtension and use ProvideValue to set up the Target and TargetProperty
    public CollectionChangedHandlingValueConverter(DependencyObject target, DependencyProperty dp)
    {
        myTarget = target;
        myTargetProperty = dp;
    }

    #region IValueConverter Members

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        INotifyCollectionChanged collection = value as INotifyCollectionChanged;
        if (collection != null)
        {
            //It notifies of collection changed, try again when it changes
            collection.CollectionChanged += DataCollectionChanged;
        }

        //Do whatever conversions here
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }

    #endregion

    void DataCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if ((myTarget != null) && (myTargetProperty != null))
        {
            BindingOperations.GetBindingExpressionBase(myTarget, myTargetProperty).UpdateTarget();
        }
    }
}

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