当PropertyChanged被触发时，WPF依赖属性设置器不触发，但是源值没有改变 [英] WPF dependency property setter not firing when PropertyChanged is fired, but source value is not changed

问题描述

我的自定义文本框中有一个int依赖属性，它保存一个后备值。它绑定到一个int？ DataContext上的属性。

如果我在DataContext中引发PropertyChanged事件，并且source属性的值不会更改（保持为空），则依赖属性的setter不会被触发。 >

这是一个问题，因为我想更新PropertyChanged上的自定义Textbox（清除文本），即使源属性保持不变。但是，我没有找到任何我想要的绑定选项（有一个UpdateSourceTrigger属性，但是我想在这里更新目标，而不是源）。
也许有更好的方式通知Textbox需要清除其文本，我可以接受任何建议。

根据要求简单）

DataContext（source）：

  private int？ _foo 
 
 public int？ Foo 
 {
 get 
 {
 //绑定正在工作，因为_foo被检索（在此处命中一个断点）。 
 // RaisePropertyChanged（Foo）从别处调用，即使_foo的值没有被更改
 return _foo; 
} 
 set 
 {
 //断点被用户输入，所以绑定正在工作
 _foo = value; 
 RaisePropertyChanged（Foo）; 
} 
} 
  

自定义文本框（目标）：

  public double？值
 {
 get 
 {
 return（double？）GetValue（ValueProperty）; 
} 
 set 
 {
 //当Foo为空且Value也为空时，断点不会在此处
 SetValue（ValueProperty，value）; 
 
 //这是当值设置为null时需要运行的代码块
 if（value == null&！String.IsNullOrEmpty（Text））
 {
 Text = String.Empty; 
} 
 
 
 
 public static readonly DependencyProperty ValueProperty = DependencyProperty.Register（Value，typeof（double？），typeof（CustomTextbox），new PropertyMetadata （null，ValueChangedHandler））; 
 
 private static void ValueChangedHandler（DependencyObject dependecyObject，DependencyPropertyChangedEventArgs e）
 {
 //当Foo为空且Value也为空时，断点不会在此处
 } 
  

解决方案

XAML将直接调用SetValue，而不是调用你的财产安置者。我不记得具体细节，但是我之前遇到过类似的问题。您不应该在Value的setter中放置任何逻辑，而是在依赖属性更改时定义回调，并从中更新值。

I have an int dependency property on my custom Textbox, which holds a backing value. It is bound to an int? property on the DataContext.

If I raise the PropertyChanged event in my DataContext, and the source property's value is not changed (stays null), then the dependency property's setter is not fired.

This is a problem, because I want to update the custom Textbox (clear the text) on PropertyChanged, even if the source property stays the same. However, I didn't find any binding option that does what I want (there is an UpdateSourceTrigger property, but I want to update the target here, not the source). Maybe there is a better way to inform the Textbox that it needs to clear its text, I'm open to any suggestions.

Source, as requested (simplified)

DataContext (source):

  private int? _foo;

  public int? Foo
  {
      get
      {
          // The binding is working, because _foo is retrieved (hits a breakpoint here).
          // RaisePropertyChanged("Foo") is called from elsewhere, even if _foo's value is not changed
          return _foo;
      }
      set
      {
          // Breakpoint is hit on user input, so the binding is working
          _foo = value;
          RaisePropertyChanged("Foo");
      }
  }

Custom Textbox (target):

public double? Value
{
    get
    {
        return (double?)GetValue(ValueProperty);
    }
    set
    {
            // When Foo is null and Value is also null, the breakpoint is not hit here
            SetValue(ValueProperty, value);

            // This is the piece of code that needs to be run whenever Value is set to null
            if (value == null && !String.IsNullOrEmpty(Text)) 
            {
                Text = String.Empty;
            }
        }
    }

    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value", typeof(double?), typeof(CustomTextbox), new PropertyMetadata(null, ValueChangedHandler));

    private static void ValueChangedHandler(DependencyObject dependecyObject, DependencyPropertyChangedEventArgs e)
        {
           // When Foo is null and Value is also null, the breakpoint is not hit here
        }

解决方案

The XAML will call SetValue directly, instead of calling your property setter. I can't remember exactly the specifics, but I ran into a similar problem a while ago. You should not put any logic in the setter for Value, instead define a callback for when the Dependency Property changes, and update the value from there.

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