c# - 为什么当字符串相等时此 AppendText 依赖属性不打印？

Question

如果字符串相同，则不会打印/绑定(bind)/发布到 WPF 表单文本框中的 View 。例如，如果我使用 random 来生成我正在制作成字符串的字节数组，那么它确实会发布到 View 中。

这是 View 绑定(bind)到的我的 ViewModel:

   public class ViewModel : INotifyPropertyChanged
   {
      public StringBuilder Data
      {
         get { return _data; }
         set
         {
            _data = value;
            OnPropertyChanged("Data");
         }
      }

      private Service service = new Service();
      private StringBuilder _data;

      public ViewModel()
      {
         service.BytesArrived += ServiceOnBytesArrived;
         ThreadPool.QueueUserWorkItem(starupService);
      }

      private void starupService(object state)
      {
         service.Start();
      }

      private void ServiceOnBytesArrived(byte[] bytes)
      {
         var sBuilder = new StringBuilder();
         foreach (var b in bytes)
         {
            sBuilder.Append(b.ToString() + ", ");
         }

         Data = sBuilder;
      }

      public event PropertyChangedEventHandler PropertyChanged;

      protected virtual void OnPropertyChanged(string propertyName)
      {
         var handler = PropertyChanged;
         if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
      }
   }

这是为我打印字节的服务(如果使用随机代替则工作正常:

   public class Service
   {
      public void Start()
      {
         var random = new Random(DateTime.Now.Minute);

       while (true)
       {
        //random.NextBytes(bytes);
        for (int i = 0; i < 10; i++)
        {
           bytes[i] = 0;
           Thread.Sleep(10);
        }
        //Thread.Sleep(100);
        BytesArrived(bytes);
       }
    }

  private byte[] bytes = new byte[10];
  public event Action<byte[]> BytesArrived;
}

这是我正在使用的使用 AppendText 的依赖属性:

   public static class TextBoxAttachedBehaviors
   {
      #region AppendText Attached Property

      public static string GetAppendText(TextBox textBox)
      {
         return (string)textBox.GetValue(AppendTextProperty);
      }

      public static void SetAppendText(
         TextBox textBox, string value)
      {
         textBox.SetValue(AppendTextProperty, value);
      }

      public static readonly DependencyProperty AppendTextProperty =
         DependencyProperty.RegisterAttached(
            "AppendText",
            typeof(string),
            typeof(TextBoxAttachedBehaviors),
            new UIPropertyMetadata(null, OnAppendTextChanged));

      private static void OnAppendTextChanged(DependencyObject d,
                                              DependencyPropertyChangedEventArgs e)
      {
         if (e.NewValue == null)
            return;
         TextBox textBox = d as TextBox;
         textBox.AppendText(e.NewValue.ToString());
      }

      #endregion
   }

XAML:

<TextBox attachedBehaviors:TextBoxAttachedBehaviors.AppendText="{Binding TextBoxAppend}"/>

如果您有 ReSharper，它将提供替换 namespace 的功能，例如attachedBehaviors: 带有实际附加行为的类链接，在我的例子中是 xmlns:attachedBehaviors="clr-namespace:Support.NetworkMonitor.AttachedBehaviors"。

Answer 1

DependencyProperties 在触发通知之前比较它们的旧值和新值，并且仅在确实存在差异时才触发它。解决方案很简单:在设置字符串之前将 AppendText 设置为 null，例如

  public StringBuilder Data
  {
     get { return _data; }
     set
     {
        _data = null;
        OnPropertyChanged("Data");
        _data = value;
        OnPropertyChanged("Data");
     }
  }