相当于IDeserializationCallback.OnDeserialization的XAML阅读器

Question

我已经向持久化类添加了新字段，并且需要确保在从磁盘加载旧版本的XAML序列化文件时将它们设置为合理的默认值。以前，在使用BinaryFormatter时，我会使用OnDeserialization方法来确定在持久化类中添加了新字段时应该设置哪些默认值(使用OptionalField属性)。例如：

    /// <summary>
    /// Runs when the entire object graph has been deserialized.
    /// </summary>
    /// <param name="sender">The object that initiated the callback. The functionality for this parameter is not currently implemented.</param>
    public override void
    OnDeserialization
        (object sender)
    {

到目前为止，在写入XAML文件时，我似乎找不到任何等效的东西，例如：

            using (TextReader reader = File.OpenText(filePath))
            {
                protocol = (Protocol)XamlServices.Load(reader);
            }

我希望确保在协议类型中不包含新的可选字段的旧文件(在上面的示例代码中)具有合理的默认值。我到处寻找，但似乎找不到任何明显的东西(例如https://ludovic.chabant.com/devblog/2008/06/25/almost-everything-you-need-to-know-about-xaml-serialization-part-2/)，有没有类似的东西？

Answer 1

XamlServices在内部使用XamlObjectWriter。此类型有一个包含各种回调的XamlObjectWriterSettings参数。这些都不是XamlServices公开的，但是它的功能很容易被复制。

我没有对此进行过广泛的测试，但这似乎是有效的：

public static object LoadXaml(TextReader textReader)
{
    var settings = new XamlObjectWriterSettings
    {
        AfterBeginInitHandler = (s, e) => Debug.Print($"Before deserializing {e.Instance}"),
        AfterEndInitHandler = (s, e) => Debug.Print($"After deserializing {e.Instance}")
    };

    using (var xmlReader = XmlReader.Create(textReader))
    using (var xamlReader = new XamlXmlReader(xmlReader))
    using (var xamlWriter = new XamlObjectWriter(xamlReader.SchemaContext, settings))
    {
        XamlServices.Transform(xamlReader, xamlWriter);
        return xamlWriter.Result;
    }
}

e.Instance包含被反序列化的对象。不确定哪个回调最适合您的目的。它们更等同于[OnDeserializing]/[OnDeserialized]属性，因为它们是在单个对象反序列化时调用的，而不是像IDeserializationCallback.OnDeserialization那样在整个图完成之后调用。

下面是一个在序列化期间提供事件的类的更完整的实现。XamlObjectReader不像XamlObjectWriter那样支持回调，所以这使用了一种变通方法。由于注释中解释的原因，它只在序列化对象之前而不是在序列化对象之后引发事件。

public class CallbackXamlService
{
    // Default settings that XamlService uses
    public XmlWriterSettings XmlWriterSettings { get; set; }
        = new XmlWriterSettings { Indent = true, OmitXmlDeclaration = true };

    public event EventHandler<XamlObjectEventArgs> BeforeDeserializing;
    public event EventHandler<XamlObjectEventArgs> AfterDeserializing;
    public event EventHandler<XamlObjectEventArgs> BeforeSerializing;
    // AfterSerializing event doesn't seem to be easily possible, see below

    public object LoadXaml(TextReader textReader)
    {
        var settings = new XamlObjectWriterSettings
        {
            BeforePropertiesHandler = (s, e) => BeforeDeserializing?.Invoke(this, e),
            AfterPropertiesHandler = (s, e) => AfterDeserializing?.Invoke(this, e)
        };

        using (var xmlReader = XmlReader.Create(textReader))
        using (var xamlReader = new XamlXmlReader(xmlReader))
        using (var xamlWriter = new XamlObjectWriter(xamlReader.SchemaContext, settings))
        {
            XamlServices.Transform(xamlReader, xamlWriter);
            return xamlWriter.Result;
        }
    }

    public string SaveXaml(object instance)
    {
        var stringBuilder = new StringBuilder();
        using (var textWriter = new StringWriter(stringBuilder))
            SaveXaml(textWriter, instance);
        return stringBuilder.ToString();
    }

    public void SaveXaml(TextWriter textWriter, object instance)
    {
        Action<object> beforeSerializing = (obj) => BeforeSerializing?.Invoke(this, new XamlObjectEventArgs(obj));

        // There are no equivalent callbacks on XamlObjectReaderSettings
        // Using a derived XamlObjectReader to track processed objects instead
        using (var xmlWriter = XmlWriter.Create(textWriter, XmlWriterSettings))
        using (var xamlXmlWriter = new XamlXmlWriter(xmlWriter, new XamlSchemaContext()))
        using (var xamlObjectReader = new CallbackXamlObjectReader(instance, xamlXmlWriter.SchemaContext, null, beforeSerializing))
        {
            XamlServices.Transform(xamlObjectReader, xamlXmlWriter);
            xmlWriter.Flush();
        }
    }

    private class CallbackXamlObjectReader : XamlObjectReader
    {
        public Action<object> BeforeSerializing { get; }

        //private Stack<object> instanceStack = new Stack<object>();

        public CallbackXamlObjectReader(object instance, XamlSchemaContext schemaContext, XamlObjectReaderSettings settings, Action<object> beforeSerializing)
            : base(instance, schemaContext, settings)
        {
            BeforeSerializing = beforeSerializing;
        }

        public override bool Read()
        {
            if (base.Read())
            {
                if (NodeType == XamlNodeType.StartObject)
                {
                    //instanceStack.Push(Instance);
                    BeforeSerializing?.Invoke(Instance);
                }
                // XamlObjectReader.Instance is not set on EndObject nodes
                // EndObject nodes do not line up with StartObject nodes when types like arrays and dictionaries
                // are involved, so using a stack to track the current instance doesn't always work.
                // Don't know if there is a reliable way to fix this without possibly fragile special-casing,
                // the XamlObjectReader internals are horrendously complex.
                //else if (NodeType == XamlNodeType.EndObject)
                //{
                //    object instance = instanceStack.Pop();
                //    AfterSerializing(instance);
                //}
                return true;
            }
            return false;
        }
    }
}