动态赋值DataContext时Avalonia控件OnAttached()方法不触发问题求助

问题场景 #

使用Avalonia TextEditor时，通过附加属性实现Text与ViewModel的绑定，遇到以下异常情况：

• TextEditor位于UserControl内的TabControl标签页中。应用初始加载时，TextEditorDocumentBindingBehavior.OnAttached()触发，但此时ViewModel为NULL，方法直接返回（符合预期，因为SelectedItem.SelectedChildItem初始为NULL）。
• 父UserControl加载完成后，SelectedItem.SelectedChildItem获得值，转换器触发并返回对应的ViewModel作为DataContext，但此时OnAttached()并未触发，导致ViewModel中的文本无法显示在TextEditor中。
• 切换到第二个标签页再切回第一个标签页时，OnAttached()触发，ViewModel中的Source文本能正常显示。

请教问题 #

1. 该现象的原因是什么？
2. 是否需要强制触发OnAttached()？
3. 在父控件动态为ViewModel赋值DataContext时，如何确保OnAttached()触发以加载初始值？

ViewModel 属性代码 #

[Reactive] public string? Source {get; set;} = string.Empty;
public string? SourceText
{
    get { return Source; }
    set
    {
        if (value != Source)
        {
            Source = value;
        }
    }
}

TextEditorProperties 附加属性代码 #

public class TextEditorProperties
{
    public static readonly AttachedProperty<string> BindTextToProperty =
        AvaloniaProperty.RegisterAttached<TextEditor, string>("BindTextTo", typeof(TextEditorProperties));
    public static string GetBindTextTo(TextEditor p_textEditor)
    {
        return p_textEditor.GetValue(BindTextToProperty);
    }
    public static void SetBindTextTo(TextEditor p_textEditor, string p_value)
    {
        p_textEditor.SetValue(BindTextToProperty, p_value);
    }
}

TextEditorDocumentBindingBehavior 行为实现代码 #

public class TextEditorDocumentBindingBehavior : Behavior<TextEditor>
{
    private TextDocument? TextEditorDocument;
    protected override void OnAttached()
    {
        base.OnAttached();
        if (AssociatedObject is TextEditor textEditor)
        {
            if (AssociatedObject.DataContext is MyViewModel viewModel)
            {
                var propertyToBind = TextEditorProperties.GetBindTextTo(AssociatedObject);
                if (string.IsNullOrWhiteSpace(propertyToBind))
                {
                    throw new InvalidOperationException("BindTextTo property cannot be null or empty.");
                }
                TextEditorDocument = new TextDocument();
                TextEditorDocument.TextChanged += (sender, e) =>
                {
                    var property = viewModel.GetType().GetProperty(propertyToBind);
                    if (property != null)
                    {
                        if (sender is not TextDocument textDocument) return;
                        property.SetValue(viewModel, textDocument.Text);
                    }
                };
                var initialText = GetInitialText(viewModel, propertyToBind);
                TextEditorDocument.Text = initialText;
                AssociatedObject.TextArea.Document = TextEditorDocument;
            }
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        if (TextEditorDocument != null)
        {
            TextEditorDocument.TextChanged -= (sender, e) => { };
            TextEditorDocument = null;
        }
    }

    private string? GetInitialText(MyViewModel p_viewModel, string p_propertyToBind)
    {
        var property = p_viewModel.GetType().GetProperty(p_propertyToBind);
        if (property != null)
        {
            return property.GetValue(p_viewModel) as string;
        }
        return string.Empty;
    }
}

XAML 定义 #

父UserControl

<UserControl>
<TabControl>
<TabItem Header="One" >
    <Grid>
        <AvaloniaEdit:TextEditor Name="TextEditorTest"
                                 behaviors:TextEditorProperties.BindTextTo="SourceText"
                                 FontFamily="Cascadia Code,Consolas,Menlo,Monospace">
            <AvaloniaInteractive:Interaction.Behaviors>
                <behaviors:TextEditorDocumentBindingBehavior/>
            </AvaloniaInteractive:Interaction.Behaviors>
        </AvaloniaEdit:TextEditor>
    </Grid>
    </ScrollViewer>
</TabItem>

<TabItem Header="Two"> </TabItem>
</TabControl>
</UserControl>  

MyUserControl

<MyUserControl >
    <MyUserControl.DataContext>
        <Binding Path="SelectedItem.SelectedChildItem" Converter="{StaticResource SelectedChildItemToMyViewModelConverter}"/>  
    </MyUserControl.DataContext> 
</MyUserControl>   

问题解答 #

1. 现象原因 #

• Behavior的OnAttached()方法仅在行为附加到控件且控件已加载时触发一次。初始加载时，TabControl的第一个TabItem虽然已创建，但因为DataContext为NULL，行为逻辑直接跳过。
• 当父控件的SelectedItem.SelectedChildItem赋值后，TextEditor的DataContext更新，但此时行为已经完成OnAttached()的执行，不会自动重新触发。
• 切换标签页时，TabItem会经历卸载和重新加载的过程，行为会重新附加到TextEditor，此时DataContext已经存在，所以OnAttached()能正常执行并加载文本。

2. 是否需要强制触发OnAttached() #

不需要强制触发OnAttached()，这不符合Behavior的设计逻辑。OnAttached()用于初始化一次性资源，重复触发可能导致资源泄漏或重复绑定。正确做法是监听DataContext的变化，在变化时重新执行绑定逻辑。

3. 解决方案：监听DataContext变化 #

修改TextEditorDocumentBindingBehavior，添加DataContext变化监听，在DataContext更新时重新初始化绑定：

public class TextEditorDocumentBindingBehavior : Behavior<TextEditor>
{
    private TextDocument? TextEditorDocument;
    private IDisposable? _dataContextChangedSubscription;

    protected override void OnAttached()
    {
        base.OnAttached();
        if (AssociatedObject is TextEditor textEditor)
        {
            // 订阅DataContext变化事件
            _dataContextChangedSubscription = textEditor.GetObservable(TextEditor.DataContextProperty)
                .Subscribe(OnDataContextChanged);
            
            // 初始检查当前DataContext
            OnDataContextChanged(textEditor.DataContext);
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        _dataContextChangedSubscription?.Dispose();
        CleanUpExistingDocument();
    }

    private void OnDataContextChanged(object? newDataContext)
    {
        // 清理旧的文档资源
        CleanUpExistingDocument();
        
        if (newDataContext is MyViewModel viewModel)
        {
            var propertyToBind = TextEditorProperties.GetBindTextTo(AssociatedObject);
            if (string.IsNullOrWhiteSpace(propertyToBind))
            {
                throw new InvalidOperationException("BindTextTo property cannot be null or empty.");
            }
            
            TextEditorDocument = new TextDocument();
            TextEditorDocument.TextChanged += (sender, e) =>
            {
                var property = viewModel.GetType().GetProperty(propertyToBind);
                if (property != null && sender is TextDocument textDocument)
                {
                    property.SetValue(viewModel, textDocument.Text);
                }
            };
            
            var initialText = GetInitialText(viewModel, propertyToBind);
            TextEditorDocument.Text = initialText;
            AssociatedObject.TextArea.Document = TextEditorDocument;
        }
    }

    private void CleanUpExistingDocument()
    {
        if (TextEditorDocument != null)
        {
            TextEditorDocument.TextChanged -= (sender, e) => { };
            TextEditorDocument = null;
        }
    }

    private string? GetInitialText(MyViewModel p_viewModel, string p_propertyToBind)
    {
        var property = p_viewModel.GetType().GetProperty(p_propertyToBind);
        return property?.GetValue(p_viewModel) as string ?? string.Empty;
    }
}

关键修改点：

• 订阅TextEditor.DataContextProperty的变化，每次DataContext更新时调用OnDataContextChanged方法。
• 在OnDataContextChanged中先清理旧的TextDocument资源，再根据新的ViewModel重新初始化绑定。
• 初始OnAttached时立即检查一次DataContext，覆盖初始加载的场景。

内容的提问来源于stack exchange，提问作者JohnB