What causes the problem:
~~~~~~~~~~~~~~~~~~~~~

What is the problem:
~~~~~~~~~~~~~~~~~
If the converter is used, then the bindableObj.GetValues() in SetInternalValue will always contain old values, meaning that a change caused by the converter is not considered (SetInternalValue is invoked, but GetValues will deliver the old value). Then, the AllTrueConverter in the example above will receive wrong values (I didn’t really get why, the converter ANormalConverter is invoked and returns the correct value, but AllTrueConverter receives the old value then)

How I fixed the problem for me:
~~~~~~~~~~~~~~~~~~~~~~~~~~
1) Create an extra class for providing the BindingPropertyChangedDelegate in MultiBinding.cs, which will allow us to replace the value at the matching index:
private sealed class PropertyChangedMappingProvider
{
public PropertyChangedMappingProvider(int index, MultiBinding reference)
{
this.ChangedDelegate = (bindableObj, o, n) =>
{
reference.SetInternalValue(bindableObj, index, o, n);
};
}

public BindableProperty.BindingPropertyChangedDelegate ChangedDelegate;
}

2) Change the assignment of the BindingPropertyChangedDelegate in Method “Apply”:
PropertyChangedMappingProvider changedProvider = new PropertyChangedMappingProvider(i, this);
var property = BindableProperty.Create($”{nameof(MultiBinding)}Property-{Guid.NewGuid():N}”, typeof(object),
typeof(MultiBinding), default(object), propertyChanged: changedProvider.ChangedDelegate);

3) Change SetInternalValue:
private void SetInternalValue(BindableObject source, int index = -1, object oldValue = null, object newValue = null)
{
if (source == null || isApplying) return;

if (index >=0 && index < Properties.Length)
{
var values = source.GetValues(Properties);
// Be sure to override the changed property with the new value
values[index] = newValue;
internalValue.Value = values;
}
else
{
internalValue.Value = source.GetValues(Properties);
}
}

I struggled a lot to find the cause and I guess this is a very, very dirty fix, though it was interesting to debug through it and see how all glues together.
Maybe you might consider to provide a more elegant solution than this in the future ;)

Warm regards,
Mathias Lackner

Hi Gregory,

Sorry for the confusion. It sounds like you figured it out. I have updated the code in github and in this post to reflect the latest API in Xamarin.Forms 2.5.1

Well, I finally realized that an additional optional parameter was added to the Apply and Unapply methods.
It works now after adding those!

So I just now reverted back to Xamarin.Forms 2.5.0.280555 and the project compiles just fine once again. I’m perplexed.

Although in browsing the source online, it appears to still be there. If that be the case, I’m not quite sure why upgrading to the latest Xamarin.Forms would cause the project to fail compilation.

Thank you for your comment. The GitHub repository has been updated to the latest Xaramin.Forms nuget (v2.3.4.270). If you are still seeing compile issues please open an issue on the GitHub repository. https://github.com/Keboo/Xamarin.Forms.Proxy

Thanks for your work and explanation.

I got your simpler version of MultiBinding.cs working. (https://gist.github.com/Keboo/0d6e42028ea9e4256715)
For the moment that will do for me.

In the future I might get back on this topic and study the rest.

Hi Jim,

The converter is a very simple class that simply grabs the first value that is not null. Here is the complete code for it:

using System;
using System.Globalization;
using System.Linq;
using Xamarin.Forms.Proxy;

namespace MultiBindingExample
{
public class FirstNotNullConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
return values.FirstOrDefault(x => x != null) ?? parameter;
}
}
}

Additional information: Position 22:8. Type helper:FirstNotNullConverter not found in xmlns clr-namespace:Ximon.Helpers;assembly=Ximon

Yes, I checked my namespace and it is OK.

I copied the code directly from GitHub (I used the NON-SETTER version)

And My XAML:

….

Any ideas? Thanks!!!