WPF中的双向绑定调用属性两次

本文关键字：两次 属性 调用 绑定 WPF | 更新日期: 2023-09-27 18:10:37

我一直在使用wpf和双向数据绑定来更好地理解它，我注意到当一个文本框有双向数据绑定到一个属性时，该属性被调用两次。我通过在调用属性时向输出窗口写入一个值来验证这一点。我的代码如下:-

我xaml 

<Page
    x:Class="_2waybindTest.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:_2waybindTest"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <TextBox HorizontalAlignment="Left" Margin="55,93,0,0" TextWrapping="Wrap" Text="{Binding TestProperty, Mode=TwoWay}" VerticalAlignment="Top" Width="540"/>
        <Button Content="Button" HorizontalAlignment="Left" Margin="55,31,0,0" VerticalAlignment="Top" Click="Button_Click_1"/>
        <TextBox HorizontalAlignment="Left" Margin="55,154,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="540"/>
    </Grid>
</Page>

我的简单视图模型类来测试

public class viewmodel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private string _TestProperty;
    public void SetTestProperty()
    {
        this.TestProperty = "Set Test Property";
    }
    public string TestProperty{
        get
        {
            return this._TestProperty;
        }
        set
        {
            this._TestProperty = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("TestProperty"));
            }
            Debug.WriteLine("this._TestProperty = " + this._TestProperty);
        }
    }
}

我的xaml代码后面

/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class MainPage : Page
{
    public MainPage()
    {
        this.InitializeComponent();
        DataContext = new viewmodel();
    }
    /// <summary>
    /// Invoked when this page is about to be displayed in a Frame.
    /// </summary>
    /// <param name="e">Event data that describes how this page was reached.  The Parameter
    /// property is typically used to configure the page.</param>
    protected override void OnNavigatedTo(NavigationEventArgs e)
    {
    }
    private void Button_Click_1(object sender, RoutedEventArgs e)
    {
        var vm = (viewmodel)DataContext;
        vm.SetTestProperty();
    }
}

为什么被调用两次。这是预期行为吗?

WPF中的双向绑定调用属性两次

一般来说，在触发propertyChanged事件之前，您应该检查value是否实际更改了，否则您可能会陷入绑定更新的无限循环。在您的例子中，textbox可能正在检查更改，从而防止了这种循环。

public string TestProperty{
    set
    {
        if(this._TestProperty == value)
        {
            return;
        }
        this._TestProperty = value;
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs("TestProperty"));
        }            
    }
}
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