Add docs about FuncValueConverter

README.adoc

@@ -83,7 +83,7 @@ Each sample is tagged with it's difficulty. The degree of difficulty describes h
 
 | link:src/Avalonia.Samples/MVVM/ValueConversionSample[ValueConverter Sample]
 | 🐥 Easy
-| Converter, Binding, MultiBinding, IValueConverter, IMultiValueConverter, MVVM
+| Converter, Binding, MultiBinding, IValueConverter, IMultiValueConverter, MVVM, FuncValueConverter
 
 | link:src/Avalonia.Samples/MVVM/ValidationSample[Validation Sample]
 | 🐥 Easy

src/Avalonia.Samples/MVVM/ValueConversionSample/Converter/FuncValueMathConverters.cs

@@ -6,17 +6,17 @@ namespace ValueConversionSample.Converter;
 /// A static class holding our FuncValueConverter
 /// </summary>
 /// <remarks>
-/// Consume it from XAML via <code>{x:Static conv:FuncValueMathConverters.MyConverter}</code>
+/// Consume it from XAML via <code>{x:Static conv:FuncValueStringConverters.MyConverter}</code>
 /// </remarks>
-public static class FuncValueMathConverters
+public static class FuncValueStringConverters
 {
     /// <summary>
-    /// Gets a Converter that returns true if a number is 0, otherwise false
+    /// Gets a Converter that returns true if a string is null or empty, otherwise false
     /// </summary>
-    public static FuncValueConverter<int, bool> IsZero { get; } = new FuncValueConverter<int, bool>(i => i == 0);
+    public static FuncValueConverter<string?, bool> IsEmpty { get; } = new FuncValueConverter<string?, bool>(s => string.IsNullOrEmpty(s));
     
     /// <summary>
-    /// Gets a Converter that returns true if a number is not 0, otherwise false
+    /// Gets a Converter that returns true if a string is not null or empty, otherwise false
     /// </summary>
-    public static FuncValueConverter<int, bool> IsNotZero { get; } = new FuncValueConverter<int, bool>(i => i != 0);
+    public static FuncValueConverter<string?, bool> IsNotEmpty { get; } = new FuncValueConverter<string?, bool>(s => !string.IsNullOrEmpty(s));
 }

src/Avalonia.Samples/MVVM/ValueConversionSample/README.adoc

@@ -27,7 +27,7 @@ toc::[]
 
 === Buzz-Words
 
-Converter, Binding, MultiBinding, IValueConverter, IMultiValueConverter, MVVM
+Converter, Binding, MultiBinding, IValueConverter, IMultiValueConverter, MVVM, FuncValueConverter
 
 
 == Before we start
@@ -172,6 +172,26 @@ Now we can add two https://docs.avaloniaui.net/docs/controls/numericupdown[[`Num
 
 TIP: `ConverterParameter` can only be a static value. You cannot bind to it and you cannot use `DynamicResource`. If you need more flexibility, consider to do your calculations in your `ViewModel` or use a `MultiValueConverter`.
 
+
+[NOTE]
+====
+If you want to, you can also define the converter as a static member and use it without the need to add it to the `Resources`
+
+[source,cs]
+----
+/// <summary>
+/// Gets a static instance of the MathAddConverter
+/// </summary>
+public static MathAddConverter AddConverter { get; } = new MathAddConverter();
+----
+
+[source,xml]
+----
+<NumericUpDown Grid.Row="1" Grid.Column="1"
+               Value="{Binding Number1, Converter={x:Static MathAddConverter.AddConverter}, ConverterParameter={StaticResource MyConverterParameter}}" />
+----
+====
+
 === Step 4: See it in action
 
 In your IDE select `[Run]` or `[Debug]` to see the result in action. Try to change the value in both input boxes and see how they update each other. 
@@ -391,6 +411,69 @@ In your IDE select `[Run]` or `[Debug]` to see the result in action. Try to chan
 image::_docs/result_02.png[IMultiValueConverter sample]
 
 
+== Solution 3 : Using FuncValueConverter
+
+In case you need a converter that only converts a given input in one direction without the need of a `ConverterParameter`, you can also use a `FuncValueConverter`. The benefit is that the converter is much shorter to write. In this sample we will add two `FuncValueConverter` which will check if a given string is empty or not. We will consume the converters to show and hide content based on some user inout.
+
+=== Step 1 : Create the FuncValueConverters
+
+TIP: We can use a static class here as the converters will not change.
+
+The `FuncValueConverter` has two generic parameters:
+
+TIn:: This parameter defines the expected input type. This can also be an array in case you want to use this converter in a `MultiBinding`.
+
+TOut:: This parameter defines the expected output type.
+
+The constructor expects a lambda defining the function to convert.
+
+[source,c#]
+----
+/// <summary>
+/// A static class holding our FuncValueConverter
+/// </summary>
+/// <remarks>
+/// Consume it from XAML via <code>{x:Static conv:FuncValueStringConverters.MyConverter}</code>
+/// </remarks>
+public static class FuncValueStringConverters
+{
+    /// <summary>
+    /// Gets a Converter that returns true if a string is null or empty, otherwise false
+    /// </summary>
+    public static FuncValueConverter<string?, bool> IsEmpty { get; } = new FuncValueConverter<string?, bool>(s => string.IsNullOrEmpty(s));
+    
+    /// <summary>
+    /// Gets a Converter that returns true if a string is not null or empty, otherwise false
+    /// </summary>
+    public static FuncValueConverter<string?, bool> IsNotEmpty { get; } = new FuncValueConverter<string?, bool>(s => !string.IsNullOrEmpty(s));
+}
+----
+
+=== Step 2: Setup the View
+
+Our view will be made of one `TextBox` called "Input" and two `TextBlocks` while only one of them is visible at a time using our converter bound to the length of the text typed.
+
+[source,xml]
+----
+<StackPanel Spacing="5">
+    <TextBox x:Name="Input" Watermark="Type anything" />
+    
+    <!-- Visible when no text input -->
+    <TextBlock Text="You can only see me if the TextBox is empty" 
+               IsVisible="{Binding #Input.Text, Converter={x:Static conv:FuncValueStringConverters.IsEmpty}}" />
+    
+    <!-- Visible when text is present -->
+    <TextBlock Text="{Binding #Input.Text.Length, StringFormat='You have typed {0:N0} chars'}" 
+               IsVisible="{Binding #Input.Text, Converter={x:Static conv:FuncValueStringConverters.IsNotEmpty}}" />
+</StackPanel>
+----
+
+=== Step 3: See it in action
+
+In your IDE select `[Run]` or `[Debug]` to see the result in action. Try to change the value in the input box and see how it updates the result below.
+
+image::_docs/result_03.png[FuncValueConverter sample]
+
 == Related 
 
 Avalonia has some really nice built-in `Converters` for you. Read more about  in the https://docs.avaloniaui.net/docs/data-binding/converting-binding-values[[Docs\]].

src/Avalonia.Samples/MVVM/ValueConversionSample/Views/MainWindow.axaml

@@ -77,11 +77,11 @@
                 
                 <!-- Visible when no text input -->
                 <TextBlock Text="You can only see me if the TextBox is empty" 
-                           IsVisible="{Binding #Input.Text.Length, Converter={x:Static conv:FuncValueMathConverters.IsZero}}" />
+                           IsVisible="{Binding #Input.Text, Converter={x:Static conv:FuncValueStringConverters.IsEmpty}}" />
                 
                 <!-- Visible when text is present -->
                 <TextBlock Text="{Binding #Input.Text.Length, StringFormat='You have typed {0:N0} chars'}" 
-                           IsVisible="{Binding #Input.Text.Length, Converter={x:Static conv:FuncValueMathConverters.IsNotZero}}" />
+                           IsVisible="{Binding #Input.Text, Converter={x:Static conv:FuncValueStringConverters.IsNotEmpty}}" />
             </StackPanel>
         </TabItem>
     </TabControl>