使用低级 API 创建属性访问者

Learn how to use the IPropertyBagVisitor and IPropertyVisitor interfaces to create a property visitor from an example.

阅读时间7 分钟

最后更新于 1 个月前

此示例演示了如何使用具有

IPropertyBagVisitor

和

IPropertyVisitor

接口的低级 API 来创建属性访问者。此示例等同于使用

PropertyVisitor

基类创建属性访问者的示例。

示例概述

此示例包含创建属性访问程序的分步说明，此访问程序将对象的当前状态打印到控制台。

假设您具有以下类型：


public class Data{    public string Name = "Henry";    public Vector2 Vec2 = Vector2.one;    public List<Color> Colors = new List<Color> { Color.green, Color.red };    public Dictionary<int, string> Dict = new Dictionary<int, string> {{5, "zero"}};}

创建如下所示的实用程序方法

DebugUtilities

：


public static class DebugUtilities{    public static void PrintObjectDump<T>(T value)    {        // Magic goes here.    }}

使用

Data

对象调用

PrintObjectDump

方法，如下所示：


DebugUtilities.PrintObjectDump(new Data());

将以下内容打印到控制台：


- Name {string} = Henry- Vec2 {Vector2} = (1.00, 1.00)- Colors {List<Color>}  - [0] = {Color} RGBA(0.000, 1.000, 0.000, 1.000)  - [1] = {Color} RGBA(1.000, 0.000, 0.000, 1.000)- Dict {Dictionary<int, string>}  - [5] {KeyValuePair<int, string>}    - Key {int} = 5    - Value {string} = five

创建访问者

首先，创建一个实现

IPropertyBagVisitor

的

DumpObjectVisitor

类。在类中，使用 StringBuilder 构建表示对象当前状态的字符串。

创建一个实现
```
IPropertyBagVisitor
```
接口的
```
DumpObjectVisitor
```
类。
向该类添加
```
StringBuilder
```
字段。
添加一个
```
Reset
```
方法，用于清除
```
StringBuilder
```
并重置缩进级别。

添加一个

GetDump

方法，返回对象当前状态的字符串表示。

DumpObjectVisitor

类如下所示：


public class DumpObjectVisitor    : IPropertyBagVisitor    , IPropertyVisitor{    private const int k_InitialIndent = 0;    private readonly StringBuilder m_Builder = new StringBuilder();    private int m_IndentLevel = k_InitialIndent;    public void Reset()    {        m_Builder.Clear();        m_IndentLevel = k_InitialIndent;    }    public string GetDump()    {        return m_Builder.ToString();    }}

获取属性

在

DumpObjectVisitor

类中，覆盖

IPropertyBagVisitor.Visit

方法以循环访问容器对象的属性。在对象转储访问者中，显示值并将访问委托给属性。

要使用

this

对属性调用

Accept

方法，请实现

IPropertyVisitor

接口。此接口可用于在访问属性时指定访问行为，类似于

PropertyVisitor

类的

VisitProperty

方法。

在

DumpObjectVisitor

类中，添加覆盖

IPropertyBagVisitor.Visit

和

IPropertyVisitor.Visit

方法。


void IPropertyBagVisitor.Visit<TContainer>(IPropertyBag<TContainer> propertyBag, ref TContainer container){    foreach (var property in propertyBag.GetProperties(ref container))    {        property.Accept(this, ref container);    }}    void IPropertyVisitor.Visit<TContainer, TValue>(Property<TContainer, TValue> property, ref TContainer container){    var value = property.GetValue(ref container);    // Code goes here.}

与

PropertyVisitor

基类一起使用的

IVisitPropertyAdapter

适配器需要访问访问者的内部状态，因此不能在该类之外使用它们。但是，可以定义具有必要信息的特定域的适配器。在

DumpObjectVisito

类中，更新

IPropertyVisitor

的实现以首先使用适配器：


// Create the following methods to encapsulate the formatting of the message and display the value.public readonly struct PrintContext{    private StringBuilder Builder { get; }    private string Prefix { get; }    public string PropertyName { get; }    public void Print<T>(T value)    {        Builder.AppendLine($"{Prefix}- {PropertyName} = {{{TypeUtility.GetTypeDisplayName(value?.GetType() ?? typeof(T))}}} {value}");    }        public void Print(Type type, string value)    {        Builder.AppendLine($"{Prefix}- {PropertyName} = {{{TypeUtility.GetTypeDisplayName(type)}}} {value}");    }    public PrintContext(StringBuilder builder, string prefix, string propertyName)    {        Builder = builder;        Prefix = prefix;        PropertyName = propertyName;    }}public interface IPrintValue{}public interface IPrintValue<in T> : IPrintValue{    void PrintValue(in PrintContext context, T value);}public class DumpObjectVisitor    : IPropertyBagVisitor    , IPropertyVisitor    , IPrintValue<Vector2>    , IPrintValue<Color>{    public IPrintValue Adapter { get; set; }        public DumpObjectVisitor()    {        // For simplicity        Adapter = this;    }    void IPropertyVisitor.Visit<TContainer, TValue>(Property<TContainer, TValue> property, ref TContainer container)    {        // Here, we need to manually extract the value.        var value = property.GetValue(ref container);            var propertyName = GetPropertyName(property);            // We can still use adapters, but we must manually dispatch the calls.         if (Adapter is IPrintValue<TValue> adapter)        {            var context = new PrintContext(m_Builder, Indent, propertyName);            adapter.PrintValue(context, value);            return;        }                // Fallback behaviour here     }        void IPrintValue<Vector2>.PrintValue(in PrintContext context, Vector2 value)    {        context.Print(value);    }    void IPrintValue<Color>.PrintValue(in PrintContext context, Color value)    {        const string format = "F3";        var formatProvider = CultureInfo.InvariantCulture.NumberFormat;        context.Print(typeof(Color), $"RGBA({value.r.ToString(format, formatProvider)}, {value.g.ToString(format, formatProvider)}, {value.b.ToString(format, formatProvider)}, {value.a.ToString(format, formatProvider)})");    }}

完成的代码如下所示：


public readonly struct PrintContext{    // A context struct to hold information about how to print the property    private StringBuilder Builder { get; }    private string Prefix { get; }    public string PropertyName { get; }    // Method to print the value of type T with its associated property name    public void Print<T>(T value)    {        Builder.AppendLine($"{Prefix}- {PropertyName} = {{{TypeUtility.GetTypeDisplayName(value?.GetType() ?? typeof(T))}}} {value}");    }    // Method to print the value with a specified type and its associated property name    public void Print(Type type, string value)    {        Builder.AppendLine($"{Prefix}- {PropertyName} = {{{TypeUtility.GetTypeDisplayName(type)}}} {value}");    }    // Constructor to initialize the PrintContext    public PrintContext(StringBuilder builder, string prefix, string propertyName)    {        Builder = builder;        Prefix = prefix;        PropertyName = propertyName;    }}// Generic interface IPrintValue that acts as a marker interface for all print value adapterspublic interface IPrintValue{}// Generic interface IPrintValue<T> to define how to print values of type T// This interface is used as an adapter for specific types (Vector2 and Color in this case)public interface IPrintValue<in T> : IPrintValue{    void PrintValue(in PrintContext context, T value);}// DumpObjectVisitor class that implements various interfaces for property visiting and value printingprivate class DumpObjectVisitor : IPropertyBagVisitor, IPropertyVisitor, IPrintValue<Vector2>, IPrintValue<Color>{    // (Other members are omitted for brevity)    public IPrintValue Adapter { get; set; }    public DumpObjectVisitor()    {        // The Adapter property is set to this instance of DumpObjectVisitor        // This means the current DumpObjectVisitor can be used as a print value adapter for Vector2 and Color.        Adapter = this;    }    // This method is called when visiting a property bag (a collection of properties)    void IPropertyBagVisitor.Visit<TContainer>(IPropertyBag<TContainer> propertyBag, ref TContainer container)    {        foreach (var property in propertyBag.GetProperties(ref container))        {            // Call the Visit method of IPropertyVisitor to handle individual properties            property.Accept(this, ref container);        }    }    // This method is called when visiting each individual property of an object.    // It tries to find a suitable adapter (IPrintValue<T>) for the property value type (TValue) and uses it to print the value.    // If no suitable adapter is found, it falls back to displaying the value using its type name.    void IPropertyVisitor.Visit<TContainer, TValue>(Property<TContainer, TValue> property, ref TContainer container)    {        // Here, we need to manually extract the value.        var value = property.GetValue(ref container);        var propertyName = GetPropertyName(property);        // We can still use adapters, but we must manually dispatch the calls.        // Try to find an adapter for the current property value type (TValue).        if (Adapter is IPrintValue<TValue> adapter)        {            // If an adapter is found, create a print context and call the PrintValue method of the adapter.            var context = new PrintContext(m_Builder, Indent, propertyName);            adapter.PrintValue(context, value);            return;        }        // Fallback behavior here - if no adapter is found, handle printing based on type information.        var type = value?.GetType() ?? property.DeclaredValueType();        var typeName = TypeUtility.GetTypeDisplayName(type);        if (TypeTraits.IsContainer(type))            m_Builder.AppendLine($"{Indent}- {propertyName} {{{typeName}}}");        else            m_Builder.AppendLine($"{Indent}- {propertyName} = {{{typeName}}} {value}");        // Recursively visit child properties (if any).        ++m_IndentLevel;        if (null != value)            PropertyContainer.Accept(this, ref value);        --m_IndentLevel;    }    // Method from IPrintValue<Vector2> used to print Vector2 values    void IPrintValue<Vector2>.PrintValue(in PrintContext context, Vector2 value)    {        // Simply use the Print method of PrintContext to print the Vector2 value.        context.Print(value);    }    // Method from IPrintValue<Color> used to print Color values    void IPrintValue<Color>.PrintValue(in PrintContext context, Color value)    {        const string format = "F3";        var formatProvider = CultureInfo.InvariantCulture.NumberFormat;                // Format and print the Color value in RGBA format.        context.Print(typeof(Color), $"RGBA({value.r.ToString(format, formatProvider)}, {value.g.ToString(format, formatProvider)}, {value.b.ToString(format, formatProvider)}, {value.a.ToString(format, formatProvider)})");    }}

打印子属性的当前状态

对数据运行访问程序时，默认情况下，它会直接在给定的对象上开始访问。对于任何属性访问程序，要开始访问对象的子属性，请将

PropertyPath

传递给

PropertyContainer.Accept

方法。

更新

DebugUtilities

方法以采取可选的

PropertyPath

：


public static class DebugUtilities{    private static readonly DumpObjectVisitor s_Visitor = new();    public static void PrintObjectDump<T>(T value, PropertyPath path = default)    {        s_Visitor.Reset();        if (path.IsEmpty)            PropertyContainer.Accept(s_Visitor, ref value);        else            PropertyContainer.Accept(s_Visitor, ref value, path);        Debug.Log(s_Visitor.GetDump());    }}

使用
```
Data
```
对象调用
```
PrintObjectDump
```
方法。这将获得所需输出。

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