x:Bind in XAML

Purpose

AXSG supports WinUI-style x:Bind for Avalonia through source generation, typed semantic binding, generated runtime helpers, and language-service support.

Use x:Bind when you want:

  • compile-time validation of members, methods, and source scopes
  • direct access to root members and typed template items
  • source-aware binding to named elements, ancestors, self, templated parent, or explicit source expressions
  • stronger editor tooling than plain runtime path bindings
  • generated TwoWay/BindBack behavior without reflection

This is an Avalonia-adapted x:Bind model. It follows the WinUI/UWP/Uno concept closely, but the runtime integration is implemented on top of Avalonia bindings and generated helpers.

Requirements

  • The root must be x:Class-backed.
  • Template scopes should declare x:DataType.
  • The compiler must be able to resolve the source type, either from scope or from explicit DataType=.

Full syntax

{x:Bind path,
         Mode=OneWay|TwoWay|OneTime,
         BindBack=MethodOrSetterCompatibleExpression,
         ElementName=SomeControl,
         RelativeSource={RelativeSource Self|TemplatedParent|FindAncestor,...},
         Source={x:Static ...}|{x:Reference ...}|SomeExpression,
         DataType=local:SomeType,
         Converter={StaticResource SomeConverter},
         ConverterCulture='en-US',
         ConverterLanguage='en-US',
         ConverterParameter=SomeValue,
         StringFormat='Value: {0}',
         FallbackValue='...',
         TargetNullValue='...',
         Delay=250,
         Priority=LocalValue,
         UpdateSourceTrigger=PropertyChanged|LostFocus|Explicit}

ConverterLanguage is accepted as an alias of ConverterCulture.

Supported forms

Root members and methods

<TextBlock Text="{x:Bind Title}" />
<TextBlock Text="{x:Bind FormatTitle(Title)}" />
<TextBlock Text="{x:Bind BuildSummary(FirstName, LastName, Count)}" />

Template-item binding

<ItemsControl ItemsSource="{x:Bind Items}">
  <ItemsControl.ItemTemplate>
    <DataTemplate x:DataType="vm:ItemViewModel">
      <TextBlock Text="{x:Bind Name}" />
    </DataTemplate>
  </ItemsControl.ItemTemplate>
</ItemsControl>

Named elements

<TextBox x:Name="Editor" />
<TextBlock Text="{x:Bind Text, ElementName=Editor}" />

Relative sources

<TextBlock Tag="{x:Bind Tag, RelativeSource={RelativeSource Self}}" />
<TextBlock Text="{x:Bind Title, RelativeSource={RelativeSource TemplatedParent}, DataType=local:ParentView}" />
<TextBlock Text="{x:Bind DataContext.Title, RelativeSource={RelativeSource FindAncestor, AncestorType=local:ShellView}}" />

Explicit Source=

<TextBlock Text="{x:Bind Value, Source={x:Static local:BindingSources.Current}, DataType=local:StaticSource}" />
<TextBlock Text="{x:Bind Text, Source={x:Reference Editor}}" />

Static members, indexers, pathless bind

<TextBlock Text="{x:Bind helpers:UiHelpers.Prefix}" />
<TextBlock Text="{x:Bind Items[SelectedIndex].Name}" />
<ContentPresenter Content="{x:Bind}" />

Event handlers

<Button Click="{x:Bind HandlePrimaryClick}" />
<Button Click="{x:Bind CaptureEditorText(Editor.Text)}" />
<Button Click="{x:Bind Perform(), ElementName=ActionButton}" />

Source-resolution model

x:Bind source selection is expression-based and compiler-resolved.

Resolution order is:

  1. explicit ElementName, RelativeSource, or Source
  2. typed template source when inside a typed template
  3. root object when outside a typed template

Supported source kinds in the current implementation:

  • root object
  • typed template/data-context source
  • named element
  • target object (RelativeSource Self)
  • templated parent (RelativeSource TemplatedParent)
  • ancestor lookup (RelativeSource FindAncestor)
  • explicit source expression (Source=...)

DataType= can override the semantic source type when the runtime source is known but the compiler cannot infer a useful CLR type from the source expression alone.

TwoWay and bind-back semantics

Simple TwoWay:

<TextBox Text="{x:Bind Alias, Mode=TwoWay}" />

Explicit bind-back method:

<TextBox Text="{x:Bind SearchDraft, Mode=TwoWay, BindBack=ApplySearchDraft}" />

Key behavior:

  • the forward path is generated from the evaluated x:Bind expression
  • the reverse path is generated separately through the bind-back contract
  • this is not just a plain Avalonia runtime setter shortcut

Delay and update source trigger

AXSG supports Avalonia-oriented Delay and UpdateSourceTrigger options on x:Bind.

Example:

<TextBox Text="{x:Bind SearchDraft,
                        Mode=TwoWay,
                        BindBack=ApplySearchDraft,
                        Delay=250,
                        UpdateSourceTrigger=Explicit}" />

UpdateSourceTrigger=Explicit stores pending bind-back values until you flush them:

Bindings.Update();

or:

SourceGenMarkupExtensionRuntime.UpdateXBind(this);

Generated lifecycle surface

For class-backed roots that use x:Bind, AXSG emits:

Bindings.Initialize();
Bindings.Update();
Bindings.StopTracking();

Intended usage:

  • Initialize() reattaches generated x:Bind tracking
  • Update() flushes pending explicit values and refreshes active x:Bind bindings
  • StopTracking() detaches generated x:Bind subscriptions

Converters and formatting

Supported conversion and formatting options:

  • Converter
  • ConverterCulture
  • ConverterLanguage
  • ConverterParameter
  • StringFormat
  • FallbackValue
  • TargetNullValue

These are part of generated runtime descriptors and are applied before final target assignment/coercion.

Diagnostics

Representative x:Bind diagnostics include:

  • missing x:Class root for generated x:Bind
  • unresolved named elements or source expressions
  • unsupported mode values
  • invalid event expression for the target delegate
  • unresolved members, methods, or type tokens

Language-service support

x:Bind participates in:

  • completion
  • hover
  • go to definition
  • find references
  • rename propagation
  • signature help

The language service understands:

  • root vs template source scope
  • ElementName, RelativeSource, and Source
  • inline DataType=
  • converter and formatting options

Runtime and hot reload

x:Bind is compiled into generated evaluators and reflection-free runtime descriptors.

Hot reload uses the generated object-graph path and resets generated x:Bind state when the root graph is repopulated.

How x:Bind compares to Avalonia bindings

Compared with normal Avalonia {Binding} / {CompiledBinding}:

  • x:Bind is expression-oriented, not just path-oriented
  • x:Bind can call methods and use static members directly
  • default source semantics are root/template-item oriented instead of ordinary data-context-only lookup
  • TwoWay reverse flow uses generated bind-back plumbing

Compared with inline C#:

  • x:Bind is the better fit for binding-like expressions and event call expressions
  • inline C# is better when you need multi-statement logic or explicit code blocks

Current intentional gaps

Not currently part of the validated surface:

  • WinUI x:Load integration
  • Uno-style POCO-target x:Bind outside Avalonia binding application