一、需求背景

需要顯示 ViewModel 中的 Message/DpMessage，顯示內容根據其某些屬性來確定。程式碼結構抽象如下：

// Model
public class Message : INotifyPropertyChanged
{
    public string MSG;
    public string Stack;
}


// ViewModel
public class MessageViewModel : INotifyPropertyChanged
{
    public Message { get; set; }
    public static readonly DpMessageProperty = DependencyProperty.Register(...)
}
 

<TextBox Text="{Binding Message, Converter={x:static ShowDetailMessageConverter}}"/>
<TextBox Text="{Binding DpMessage, Converter={x:static ShowDetailMessageConverter}}"/>

以上程式碼，注意，兩個 Text 繫結的目標都是 MessageViewModel，繫結的 Path 分別是 Message、DpMessage。

當 Message 或者 DpMessage 變化（注意，變化指的是重新賦值，即，引用了新的 Message

問題來了：當 Message 或者 DpMessage 的屬性變化了呢，Message 類是實現了 INotifyPropertyChanged 的，屬性變化能觸發自身的變化（MessageViewModel.INotifyPropertyChanged）通知嗎？答案是，不能。即，Message .MSG 者 Message .Stack 變化了，View 不能得到更新通知！這不符合需求。

• 當然，這個問題可以通過更改繫結物件避過，即，將繫結物件直接設定為 Message 或者 DpMessage，然後使用 MultiBinding，將需要的屬性都繫結過去，也可以同樣實現需求，但是，這種方式的缺點多：繁瑣
  （如果涉及的屬性數量非常大呢）、不直觀（目標是 Message 整體，卻綁定了其屬性）等。

二、Message 屬性（實現 INotifyPropertyChanged）的解決方法

在 Message .PropertyChanged 中監測屬性變化，變化時主動呼叫 MessageViewModel.OnPropertyChanged。這是很簡單的。

三、DpMessage 依賴屬性的解決方法

不同於 INotifyPropertyChanged，依賴屬性無法通過 OnPropertyChanged 函式觸發屬性變更通知，這個函式僅作為回撥函式使用。因此，檢視 原始碼，看看.Net如何去觸發的通知，找到函式如下：

/// <summary>
/// This is to enable some performance-motivated shortcuts in property
/// invalidation.  When this is called, it means the caller knows the
/// value of the property is pointing to the same object instance as
/// before, but the meaning has changed because something within that
/// object has changed.
/// </summary>
/// <remarks>
/// Clients who are unaware of this will still behave correctly, if not
///  particularly performant, by assuming that we have a new instance.
/// Since invalidation operations are synchronous, we can set a bit
///  to maintain this knowledge through the invalidation operation.
/// This would be problematic in cross-thread operations, but the only
///  time DependencyObject can be used across thread in today's design
///  is when it is a Freezable object that has been Frozen.  Frozen
///  means no more changes, which means no more invalidations.
///
/// This is being done as an internal method to enable the performance
///  bug #1114409.  This is candidate for a public API but we can't
///  do that kind of work at the moment.
/// </remarks>
[FriendAccessAllowed] // Built into Base, also used by Framework.
internal void InvalidateSubProperty(DependencyProperty dp)
{
	// when a sub property changes, send a Changed notification 
	// with old and new value being the same, and with 
        // IsASubPropertyChange set to true
	NotifyPropertyChange(new DependencyPropertyChangedEventArgs(dp, 
            dp.GetMetadata(DependencyObjectType), GetValue(dp)));
}

注意函式說明部分，when a sub property changes, send a Changed notification with old and new value being the same, and with IsASubPropertyChange set to true，這完全符合我們的需求！！！這個函式本意是作為 Public API 的，但是由於效能的 bug #1114409，將其內部化了。那麼，我可以通過反射去呼叫它：

{
    this.InvokeInternal<DependencyObject>("NotifySubPropertyChange", new object[] { DpColorProperty });
}


/// <summary>
/// 反射呼叫指定型別的 Internal 方法。
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="caller"></param>
/// <param name="method"></param>
/// <param name="parameters"></param>
/// <returns></returns>
public static object InvokeInternal<T>(this T caller, string method, object[] parameters)
{
	MethodInfo methodInfo = typeof(T).GetMethod(method, BindingFlags.Instance | BindingFlags.NonPublic);
	return methodInfo?.Invoke(caller, parameters);
}

好了，完美解決！