Add FallBackValues to Binding page

[Tamnac]

I don't really know how this works for non string content, so someone may want to add on.

Also not entirely sure if this should go on this page or somewhere under How to guides > Data Binding > ...

Currently there's a note mentioning in passing under one of those pages, which is insufficient and I was unable to find it without help from the community on telegram. I think this is valuable enough to warrant a proper section.

[timunie]

For non-string fallback:

<UserControl.Resources>
   <system:Int32 x:Key="MyFallback">42</system:Int32>
</UserControl.Resources>

<NumericUpDown Value="{Binding MyValue, FallbackValue={StaticResource MyFallback}}" />

Please try it on your own first, I have written that out of my head. Not that I made a mistake here.

[Tamnac]

@timunie That does work, but numbers are trivial to write as strings, and in fact FallbackValue=42 works as well. I'm more concerned about objects that don't have a full string representation, like an image or color or any custom data type. I don't know if it's possible to include those, or if it even makes sense given you cannot directly write them in xaml

[stevemonaco]

FYI, I'm currently working to refresh this particular doc to clean up language, add all Binding properties, etc, and will PR when ready. I have not yet written an example for FallbackValue because it's hard to demonstrate without subproperty bindings. Not sure what other scenarios FallbackValue is used while using compiled bindings. (Edit: also when converters return UnsetValue)

[Tamnac]

@stevemonaco even a basic example is better than nothing. If you'll include it in your rewrite, I'll close this pr

[stevemonaco]

@stevemonaco even a basic example is better than nothing. If you'll include it in your rewrite, I'll close this pr

I will. I'm waiting for clarification on a different binding topic (UpdateSourceTrigger) in the same document before submitting. As of now, the section looks like the following (might receive some editing for repetition):

FallbackValue

FallbackValue is used when the property binding cannot be made or when a converter returns AvaloniaProperty.UnsetValue.

A common use case is when a parent property in a subproperty binding is null. If Student is null below, the FallbackValue will be used:

<TextBlock Text="{Binding Student.Name, FallbackValue=Cannot find name}"/>

:::tip ReflectionBinding can bind to arbitrary types without regard to compile-time safety. When the binding cannot be made, FallbackValue may be useful to substitute a value. :::

[Tamnac]

Alright, thanks