手动刷新WPF触发器依赖属性

Question

我有一个自定义的UserControl，继承自RichTextBox。这个类有一个依赖属性Equation，可以进行双向绑定。

当用户将项目拖放到控件上时，我会更改Equation。这会将更改正确地传播到绑定的另一端，从而触发属性更改通知，但UI不会更改。如果我将绑定更改为不同的对象，然后再次绑定回来，它就会显示更新后的Equation。

如何在不更改绑定的情况下强制刷新？现在我正在设置Equation=null，然后再重新设置，这样可以起作用，但似乎有些hackish。肯定有更优雅的方法。

以下是控件的相关部分。我希望在更改Equation（Equation.Components.Add(txt)）之后调用OnEquationChanged回调函数。

public class EquationTextBox : RichTextBox
{
    protected override void OnDrop(DragEventArgs e)
    {
        if (e.Data.GetDataPresent(DataFormats.StringFormat))
        {
            string str = (string)e.Data.GetData(DataFormats.StringFormat);

            EquationText txt = new EquationText(str);
            //// Preferred /////
            Equation.Components.Add(txt);
            //// HACK /////
            Equation eqn = this.Equation;
            eqn.Components.Add(txt);
            this.Equation = null;
            this.Equation = eqn;
            ///////////////
            Console.WriteLine("Dropping " + str);
        }
    }

    public Equation Equation
    {
        get { return (Equation)GetValue(EquationProperty); }
        set { SetValue(EquationProperty, value); }
    }

    private static void onEquationChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        string prop = e.Property.ToString();
        EquationTextBox txtBox = d as EquationTextBox;
        if(txtBox == null || txtBox.Equation == null)
            return;

        FlowDocument doc = txtBox.Document;
        doc.Blocks.Clear();
        doc.Blocks.Add(new Paragraph(new Run(txtBox.Equation.ToString())));
    }

    public static readonly DependencyProperty EquationProperty =
        DependencyProperty.Register("Equation",
                                    typeof(Equation),
                                    typeof(EquationTextBox),
                                    new FrameworkPropertyMetadata(null,
                                                                  FrameworkPropertyMetadataOptions.AffectsRender,
                                                                  new PropertyChangedCallback(onEquationChanged)));

    private bool mIsTextChanged;
}

这是双向绑定中的另一个端点属性。由于Equation.Components.Add(txt)调用，上述代码会触发equation_PropertyChanged事件。

public Equation Equation
{
    get{ return mEquation; }
    set { mEquation = value; NotifyPropertyChanged(); }
}

private void equation_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    NotifyPropertyChanged("Equation");
}

private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
    if (PropertyChanged != null)
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}

编辑 --------------------------

根据评论，我尝试使用此调度程序（请注意，这是我第一次尝试使用调度程序）：

        string str = (string)e.Data.GetData(DataFormats.StringFormat);

        EquationText txt = new EquationText(str);
        Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal, new Action(() =>
            {
                Equation.Components.Add(txt); 
                NotifyPropertyChanged("Equation");
            }));

但仍然没有UI更新。

编辑2 --------------------------

双向绑定是在XAML中完成的。

<l:EquationTextBox x:Name="ui_txtVariableEquation" Grid.Row="0" Grid.Column="2" 
                             Grid.RowSpan="3" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
                                   AllowDrop="True"
                                   Equation="{Binding SelectedVariableVM.Variable.Equation, Mode=TwoWay}">
                </l:EquationTextBox>

与Equation类中的Components对象相关的信息

public class Equation : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public Equation()
    {
        mComponents = new ObservableCollection<EquationComponent>();

        mComponents.CollectionChanged += new NotifyCollectionChangedEventHandler(components_CollectionChanged);
    }

    public Equation(string eqn) : this()
    {
        mComponents.Add(new EquationText(eqn));
    }

    public ObservableCollection<EquationComponent> Components
    {
        get{ return mComponents; }
        set{ mComponents = value; NotifyPropertyChanged();}
    }

    public override string ToString()
    {
        string str = "";
        for(int i=0; i<mComponents.Count; i++)
            str += mComponents[i].ToString();

        return str;
    }

    private void components_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        NotifyPropertyChanged("Components");
    }

    private ObservableCollection<EquationComponent> mComponents;

    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

public class Variable : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public Variable(string name = "var", VariableType type = VariableType.UnknownType) :
        this(name, "", 0, type)
    {
    }

并且...

    public class Variable : INotifyPropertyChanged
{
    public Variable(string name, string unit, object val, VariableType type)
    {
        mEquation = new Equation(name + " = " + val.ToString() + 
        mEquation.PropertyChanged += new PropertyChangedEventHandler(equation_PropertyChanged);

    }

    ...
    public Equation Equation
    {
        get{ return mEquation; }
        set { mEquation = value; NotifyPropertyChanged(); }
    }

    private void equation_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        NotifyPropertyChanged("Equation");
    }

    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }


    private Equation mEquation;
    ...
}

当Equation类中触发事件时，会调用Variable.equation_PropertyChanged方法。

- ryan0270

---

好的。它在一个事件处理程序中。你尝试过用异步调度器包装NotifyPropertyChanged的调用吗？ - Gayot Fow

不，我对那些不熟悉。我会查一下并尝试一下。 - ryan0270

看到我的编辑了吗？还是没有成功（假设我操作正确）。 - ryan0270

确保您的绑定源实际上实现了 INotifyPropertyChanged 接口。我有一个同事因为他实现了 PropertyChanged 事件但忘记在类签名中添加接口而花费数小时调试数据绑定问题。 - Mike Strobel

是的，绑定源在类定义中确实列出了INotifyPropertyChanged。 - ryan0270

Answer 1

我认为问题在于绑定生成的值实际上没有改变（仍然是相同的Equation对象）。如果DP值不改变，那么您的DP更改处理程序将不会被调用。

也许，在您的DP更改处理程序中，您应该订阅新方程式的PropertyChanged事件，并在基础属性更改时重新构建文档：

private static void onEquationChanged(
    DependencyObject d,
    DependencyPropertyChangedEventArgs e)
{
    var txtBox = d as EquationTextBox;
    if (txtBox == null)
        return;

    var oldEquation = e.OldValue as Equation;
    if (oldEquation != null)
        oldEquation.PropertyChanged -= txtBox.OnEquationPropertyChanged;

    var newEquation = e.NewValue as Equation;
    if (newEquation != null)
        newEquation.PropertyChanged += txtBox.OnEquationPropertyChanged;

    txtBox.RebuildDocument();
}

private void OnEquationPropertyChanged(object sender, EventArgs e)
{
    RebuildDocument();
}

private void RebuildDocument()
{
    FlowDocument doc = this.Document;

    doc.Blocks.Clear();

    var equation = this.Equation;
    if (equation != null)
        doc.Blocks.Add(new Paragraph(new Run(equation.ToString())));
}

- Mike Strobel