maui-data-binding（毛伊岛数据绑定）

.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

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.

<!-- ✅ 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

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

Enforce binding warnings as errors

Add to the .csproj:

<WarningsAsErrors>XC0022;XC0025</WarningsAsErrors>

Binding Modes

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

<!-- ✅ 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:

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

Set BindingContext in XAML:

<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):

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

INotifyPropertyChanged and ObservableObject

Manual implementation

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)

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):

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:

<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:

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

Multi-Binding

Combine multiple source values with IMultiValueConverter:

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

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

<!-- Square box: Height = Width -->
<BoxView WidthRequest="100"
         HeightRequest="{Binding Source={RelativeSource Self}, Path=WidthRequest}" />

StringFormat

Use Binding.StringFormat for simple display formatting without a converter:

<Label Text="{Binding Price, StringFormat='Total: {0:C2}'}" />
<Label Text="{Binding DueDate, StringFormat='{0:MMM dd, yyyy}'}" />

Wrap the format string in single quotes when it contains commas or braces.

Binding Fallbacks

• FallbackValue — used when the binding path cannot be resolved or the converter throws.
• TargetNullValue — used when the bound value is null.

<Label Text="{Binding MiddleName, TargetNullValue='(none)',
              FallbackValue='unavailable'}" />
<Image Source="{Binding AvatarUrl, TargetNullValue='default_avatar.png'}" />

.NET 9+ Code Bindings (AOT-safe)

Fully AOT-safe, no reflection:

label.SetBinding(Label.TextProperty,
    static (PersonViewModel vm) => vm.FullName);

entry.SetBinding(Entry.TextProperty,
    static (PersonViewModel vm) => vm.Age,
    mode: BindingMode.TwoWay,
    converter: new IntToStringConverter());

Threading

MAUI automatically marshals PropertyChanged to the UI thread — you can raise it from any thread. However, direct ObservableCollection mutations (Add / Remove) from background threads may crash:

// ✅ Safe — PropertyChanged is auto-marshalled
await Task.Run(() => Title = "Loaded");

// ⚠️ ObservableCollection.Add — dispatch to UI thread
MainThread.BeginInvokeOnMainThread(() => Items.Add(newItem));

Common Pitfalls

References

• Data binding overview
• Compiled bindings
• Value converters
• Relative bindings
• Multi-bindings
• CommunityToolkit.Mvvm