maui-data-binding

What this skill does

# .NET MAUI Data Binding

Wire UI controls to ViewModel properties with compile-time safety, correct
change notification, and minimal overhead. Prefer compiled bindings everywhere
and treat binding warnings as build errors.

## When to Use

- Adding `x:DataType` compiled bindings to a new or existing page
- Implementing `INotifyPropertyChanged` or CommunityToolkit `ObservableObject`
- Creating or consuming `IValueConverter` / `IMultiValueConverter`
- Choosing the correct `BindingMode` for a control property
- Setting `BindingContext` in XAML or code-behind
- Using relative bindings (`Self`, `AncestorType`, `TemplatedParent`)
- Applying `StringFormat`, `FallbackValue`, or `TargetNullValue`
- Writing AOT-safe code bindings with `SetBinding` and lambdas (.NET 9+)

## When Not to Use

- **CollectionView layouts / templates** — use the `maui-collectionview` skill
- **Shell navigation parameters** — use the `maui-shell-navigation` skill
- **Service registration / DI** — use the `maui-dependency-injection` skill
- **Property-change-triggered animations** — use built-in [.NET MAUI animation APIs](https://learn.microsoft.com/dotnet/maui/user-interface/animation/basic)

## Inputs

- A .NET MAUI project targeting .NET 8 or later
- XAML pages or C# code-behind where bindings are declared
- A ViewModel class (or plan to create one)

---

## Compiled Bindings — x:DataType Placement

Compiled bindings are **8–20× faster** than reflection-based bindings and are
required for NativeAOT / trimming. Enable them with `x:DataType`.

### Placement rules

Set `x:DataType` **only where `BindingContext` is set**:

1. **Page / View root** — where you assign `BindingContext`.
2. **DataTemplate** — which creates a new binding scope.

Do **not** scatter `x:DataType` on arbitrary child elements. Adding
`x:DataType="x:Object"` on children to escape compiled bindings is an
anti-pattern — it disables compile-time checking and reintroduces reflection.

```xml
<!-- ✅ Correct: x:DataType at the page root -->
<ContentPage xmlns:vm="clr-namespace:MyApp.ViewModels"
             x:DataType="vm:MainViewModel">
    <StackLayout>
        <Label Text="{Binding Title}" />
        <Slider Value="{Binding Progress}" />
    </StackLayout>
</ContentPage>

<!-- ❌ Wrong: x:DataType scattered on children -->
<ContentPage x:DataType="vm:MainViewModel">
    <StackLayout>
        <Label Text="{Binding Title}" />
        <Slider x:DataType="x:Object" Value="{Binding Progress}" />
    </StackLayout>
</ContentPage>
```

### DataTemplate always needs its own x:DataType

```xml
<CollectionView ItemsSource="{Binding People}">
    <CollectionView.ItemTemplate>
        <DataTemplate x:DataType="model:Person">
            <Label Text="{Binding FullName}" />
        </DataTemplate>
    </CollectionView.ItemTemplate>
</CollectionView>
```

### Enforce binding warnings as errors

| Warning | Meaning |
|---------|---------|
| **XC0022** | Binding path not found on the declared `x:DataType` |
| **XC0023** | Property is not bindable |
| **XC0024** | `x:DataType` type not found |
| **XC0025** | Binding used without `x:DataType` (non-compiled fallback) |

Add to the `.csproj`:

```xml
<WarningsAsErrors>XC0022;XC0025</WarningsAsErrors>
```

---

## Binding Modes

Set `Mode` explicitly **only** when overriding the default. Most properties
already have the correct default:

| Mode | Direction | Use case |
|------|-----------|----------|
| `OneWay` | Source → Target | Display-only (default for most properties) |
| `TwoWay` | Source ↔ Target | Editable controls (`Entry.Text`, `Switch.IsToggled`) |
| `OneWayToSource` | Target → Source | Read user input without pushing back to UI |
| `OneTime` | Source → Target (once) | Static values; no change-tracking overhead |

```xml
<!-- ✅ Defaults — omit Mode -->
<Label Text="{Binding Score}" />
<Entry Text="{Binding UserName}" />
<Switch IsToggled="{Binding DarkMode}" />

<!-- ✅ Override only when needed -->
<Label Text="{Binding Title, Mode=OneTime}" />
<Entry Text="{Binding SearchQuery, Mode=OneWayToSource}" />

<!-- ❌ Redundant — adds noise -->
<Label Text="{Binding Score, Mode=OneWay}" />
<Entry Text="{Binding UserName, Mode=TwoWay}" />
```

---

## BindingContext and Property Paths

Every `BindableObject` inherits `BindingContext` from its parent unless
explicitly set. Property paths support dot notation and indexers:

```xml
<Label Text="{Binding Address.City}" />
<Label Text="{Binding Items[0].Name}" />
```

Set `BindingContext` in XAML:

```xml
<ContentPage xmlns:vm="clr-namespace:MyApp.ViewModels"
             x:DataType="vm:MainViewModel">
    <ContentPage.BindingContext>
        <vm:MainViewModel />
    </ContentPage.BindingContext>
</ContentPage>
```

Or in code-behind (preferred with DI):

```csharp
public MainPage(MainViewModel vm)
{
    InitializeComponent();
    BindingContext = vm;
}
```

---

## INotifyPropertyChanged and ObservableObject

### Manual implementation

```csharp
public class MainViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler? PropertyChanged;

    private string _title = string.Empty;
    public string Title
    {
        get => _title;
        set
        {
            if (_title != value)
            {
                _title = value;
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Title)));
            }
        }
    }
}
```

### CommunityToolkit.Mvvm (recommended)

```csharp
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

public partial class MainViewModel : ObservableObject
{
    [ObservableProperty]
    private string _title = string.Empty;

    [RelayCommand]
    private async Task LoadDataAsync() { /* ... */ }
}
```

The source generator creates the `Title` property, `PropertyChanged` raise,
and `LoadDataCommand` automatically.

---

## Value Converters — IValueConverter

Implement `Convert` (source → target) and `ConvertBack` (target → source):

```csharp
public class IntToBoolConverter : IValueConverter
{
    public object? Convert(object? value, Type targetType,
        object? parameter, CultureInfo culture)
        => value is int i && i != 0;

    public object? ConvertBack(object? value, Type targetType,
        object? parameter, CultureInfo culture)
        => value is true ? 1 : 0;
}
```

Declare in XAML resources and consume:

```xml
<ContentPage.Resources>
    <local:IntToBoolConverter x:Key="IntToBool" />
</ContentPage.Resources>

<Switch IsToggled="{Binding Count, Converter={StaticResource IntToBool}}" />
```

`ConverterParameter` is always passed as a **string** — parse inside `Convert`:

```xml
<Label Text="{Binding Score, Converter={StaticResource ThresholdConverter},
              ConverterParameter=50}" />
```

---

## Multi-Binding

Combine multiple source values with `IMultiValueConverter`:

```xml
<Label>
    <Label.Text>
        <MultiBinding Converter="{StaticResource FullNameConverter}">
            <Binding Path="FirstName" />
            <Binding Path="LastName" />
        </MultiBinding>
    </Label.Text>
</Label>
```

```csharp
public class FullNameConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType,
        object parameter, CultureInfo culture)
    {
        if (values.Length == 2 && values[0] is string first
            && values[1] is string last)
            return $"{first} {last}";
        return string.Empty;
    }

    public object[] ConvertBack(object value, Type[] targetTypes,
        object parameter, CultureInfo culture)
        => throw new NotSupportedException();
}
```

---

## Relative Bindings

| Source | Syntax | Use case |
|--------|--------|----------|
| Self | `{Binding Source={RelativeSource Self}, Path=WidthRequest}` | Bind to own properties |
| Ancestor | `{Binding BindingContext.Title, Source={RelativeSource AncestorType={x:Type ContentPage}}}` | Reach parent BindingContext |
| TemplatedParent | `{Binding Source={RelativeSource TemplatedParent}, Path