TextBlock Text 属性在更新其源 Binding 属性时未更新？

Question

我在 XAML 中有以下代码 sn-p。

<Grid>

    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="01*"/>
        <ColumnDefinition Width="03*"/>
        <ColumnDefinition Width="01*"/>
    </Grid.ColumnDefinitions>

    <Button Name="btnPrevious" Grid.Column="0" Content="Previous" Click="btnPrevious_Click"/>
    <TextBlock Name="txtBlockName" Grid.Column="1" Text="{Binding SelectedName, UpdateSourceTrigger=PropertyChanged}" FontSize="30" VerticalAlignment="Center" HorizontalAlignment="Center"/>
    <Button Name="btnNext" Grid.Column="2" Content="Next"  Click="btnNext_Click"/>

</Grid>

这会生成如下输出，如图 1 所示。

这背后的代码如下所示。

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    public List<string> namesList = new List<string>();

    public string SelectedName
    {
        get
        {
            return namesList[1];
        }
        set
        {
            if (value != namesList[1])
            {
                namesList[1] = value;
                NotifyPropertyChanged("SelectedName");
            }
        }
    }

    public MainWindow()
    {
        InitializeComponent();

        namesList.Add("ABC");
        namesList.Add("DEF");
        namesList.Add("GHI");


        this.DataContext = this;
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        string str = namesList[0];
        namesList[0] = namesList[1];
        namesList[1] = str;

        this.IsEnabled = false;
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        string str = namesList[2];
        namesList[2] = namesList[1];
        namesList[1] = str;

        this.IsEnabled = false;
    }



    protected void NotifyPropertyChanged(String propertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

}

TextBlock 的 Text 属性绑定到 SelectedName 属性。并且不会在更新 SelectedName 属性时进行更新。尽管我的类实现了INotifyPropertyChanged 接口并定义了它的NotifyPropertyChanged 行为，但它仍然不起作用。

在调试时，我设置了一些断点来观察SelectedName 属性值，我观察到它正在根据要求更新，但TextBlock 的Text 属性没有更新。如图2所示。

我在网上看到了很多问题和解决方案，但没有一个能解决我的问题。任何帮助将不胜感激。

Answer 1

到目前为止给出的两个答案中，唯一一个甚至接近我认为合理的替代方案的是this answer 中提出的第二个选项（即在 之后“如果你想“更新”SelectedName你可以”）。

也就是说，在我看来，您最好更改您的数据结构，以便更接近地模拟用户界面中实际发生的情况。如果您花时间去做，您的代码会更容易编写和理解，并且会更简单。

在这个特定的示例中，这意味着，由于（出现）您希望能够在三个不同的文本值之间导航，您应该使用该属性来简单地呈现当前文本值，并使用索引变量来保持跟踪选择了哪个值。例如：

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    // Don't make fields public. If you do want to expose the list
    // use a public read-only property. And even there, think carefully
    // about whether you want callers to be able to modify the list; you
    // can return a `ReadOnlyCollection<string>` that wraps your list if
    // you only want callers to be able to examine the contents, rather than
    // modify it.
    //
    // Also, make any field that you never change "readonly".
    private readonly List<string> namesList = new List<string>();

    // Here's the index that keeps track of which string is selected
    private int _index;

    public string SelectedName
    {
        get { return namesList[_index]; }
    }

    public MainWindow()
    {
        InitializeComponent();

        namesList.Add("ABC");
        namesList.Add("DEF");
        namesList.Add("GHI");

        this.DataContext = this;
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        if (_index > 0)
        {
            _index--;
            NotifyPropertyChanged(nameof(SelectedName));
        }

        // I don't know what you expect this to do. You are setting the window's
        // "IsEnabled" property to false, which doesn't seem useful. More likely,
        // you intend to set the "Previous" button's enabled state, but even there
        // you really only want to set it to false if the _index value is 0. If the
        // _index value had been 2 and was just set to 1, you still want the
        // "Previous" button enabled. This would actually be an excellent place for
        // you to learn how to implement an ICommand, to have its CanExecute() method
        // return a value based on the _index value (where the "Previous" ICommand
        // object would return true unless _index is 0, and the "Next" ICommand
        // object would return true unless _index is namesList.Count - 1). Then you
        // can bind the button's "Command" property to the appropriate ICommand
        // object for the button and WPF will automatically deal with enabling
        // or disabling the button according to the command's state
        this.IsEnabled = false;
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        if (_index < namesList.Count - 1)
        {
            _index++;
            NotifyPropertyChanged(nameof(SelectedName));
        }

        // See comment above.
        this.IsEnabled = false;
    }

    protected void NotifyPropertyChanged(String propertyName)
    {
        PropertyChanged?.DynamicInvoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

请注意，对于其他两个答案中提出的任何更改，您仍然会遇到与每个文本值之间的导航相关的错误。最初单击按钮可能会起作用，但之后您的数据结构将变得不连贯，您将无法获得您想要的结果。

此答案中的示例解决了所有这些问题，以及您最初的担忧。

Answer 2

从未调用过SelectedName Setter 属性。因此，您可以使用如下所示的 BindingExpression.UpdateSource() 方法更新 TextBlock.Text，

 private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        string str = namesList[0];
        namesList[0] = namesList[1];
        namesList[1] = str;
        BindingExpression be = txtBlockName.GetBindingExpression(TextBlock.TextProperty);
        be.UpdateSource();

        this.IsEnabled = false;
    }

Answer 3

您永远不会设置“SelectedName”，您可以将其设为只读。

永远不会引发PropertyChanged。

如果你想让绑定注意到它，你应该这样做

NotifyPropertyChanged("SelectedName");

在您更改了 _namesLIst 的索引 #1 后，

public partial class MainWindow : Window , INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private readonly List<string> _namesList = new List<string>();

    public string SelectedName
    {
        get { return _namesList[1]; }            
    }

    public MainWindow()
    {
        InitializeComponent();

        _namesList.Add("ABC");
        _namesList.Add("DEF");
        _namesList.Add("GHI");

        DataContext = this;
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        var str = _namesList[0];
        _namesList[0] = _namesList[1];
        _namesList[1] = str;
        NotifyPropertyChanged(nameof(SelectedName));
        IsEnabled = false;
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        var str = _namesList[2];
        _namesList[2] = _namesList[1];
        _namesList[1] = str;
        NotifyPropertyChanged(nameof(SelectedName));

        IsEnabled = false;
    }

    private void NotifyPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

}

如果你想“更新”SelectedName，你可以

    public string SelectedName
    {
        get { return _namesList[1]; }
        set
        {
            if (value == _namesList[1]) return;
            _namesList[1] = value;
            NotifyPropertyChanged("SelectedName");
        }
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        var str = _namesList[0];
        _namesList[0] = _namesList[1];
        SelectedName = str;            
        IsEnabled = false;
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        var str = _namesList[2];
        _namesList[2] = _namesList[1];
        SelectedName = str;            
        IsEnabled = false;
    }