主页 / coding / 问题 / 79376251
Accepted
Arthur Araujo
Asked: 2025-01-22 08:22:37 +0800 CST

是否有可能在 Delphi 12 中检索枚举值的属性？

  • 772

如果我有这样的枚举类型：

  TOAuthSubjectTypes = (
    [MappingAttr('public')]
    ostPublic,
    [MappingAttr('pairwise')]
    ostPairwise
  );

在特定时刻，我有一个保存的变量ostPairwise，是否可以检索值的属性ostPairwise？现在，我正在应用的解决方法是按枚举类型的顺序设置所有属性，然后通过GetAttributes枚举类型的序数值到达它们，但我想知道是否有更惯用的方法。

抱歉，如果重复了。我在其他语言标签上找到了很多答案，但没有与 Delphi 相关的答案。

delphi

2 个回答

  1. Best Answer

    不幸的是，根本就没有办法做你想做的事情，因为没有为单个枚举成员存储的 RTTI，只有枚举类型本身的 RTTI。

    根据对Delphi 2010 RTTI 的回答：探索枚举(对于 Delphi 12 仍然适用)：

    目前，枚举中元素关联的属性未存储在可执行文件中的 Win32 RTTI 数据中。RTTI 已经导致可执行文件的大小大幅增加，因此必须在某处划出一些界线。Delphi Win32 中的属性在类型、记录的字段以及类的字段、方法、它们的参数和属性上均受支持。

    由于与 Delphi for .NET 向后兼容，属性声明不会导致错误。

    因此，最好的解决方法是将属性放在枚举类型上，然后按枚举值对其进行索引。

    • 3

  2. 根据 Remy 的回答，似乎无法访问特定成员的属性。我实现类似结果的方法是将属性定义为：

      MappingAttr = class(TCustomAttribute)
  private
    FValue: String;
  public
    constructor Create(const AValue: string);
    property Value: string read FValue;
  end;

    还有两个通用函数，声明为

      ///  <summary>
  ///   Class to hold generic functions that deal with conversion between Strings
  ///  and Mapping Enums.
  ///  </summary>
  TEnumMapping = class
  public
  ///  <summary>
  ///  Takes an enumeration value and returuns the value of the <see cref="MappingAttr"/> for that entry
  ///  </summary>
  ///  <param name="Value"><i>T</i>: Type of the enum used.</param>
    class function EnumToString<T>(const Value: T): string; static;
  ///  <summary>
  ///  Takes a string value and returns the enumeration value (index) related to that string.
  ///  For it to work there are some assumptions that will lead to exceptions if ignored, (or worse, UB)
  ///
  ///  1. The Enum has the same amount of attributes and items. (For each enumerated name there is one and only one attribute AND it has a MappingAttr type)
  ///  2. The Enum has no custom ordinality.
  ///  3. The type of the attribute mapping the Enum is MappingAttr.
  ///  </summary>
  ///
  ///
    class function StringToEnum<T>(const Value: string): T; static;
  end;

    并实现为

    
class function TEnumMapping.EnumToString<T>(const Value: T): string;
var
  RTTICtx: TRttiContext;
  RTTIType: TRttiType;
  RTTIEnum: TRttiEnumerationType;
  GenericValue: TValue;
  OrdValue: Int64;
begin
  RTTICtx := TRTTIContext.Create;
  try
    RTTIType := RTTICtx.GetType(TypeInfo(T));
    // Raises an exception if the type is not an enumeration.
    if (RTTIType = nil) then
      raise ETypeHasNoRTI.Create('The type has no Runtime Information');

    if (RTTIType.TypeKind <> tkEnumeration) then
      raise Exception.CreateFmt('Type %s is not an enumeration', [RttiType.Name]);

    // Creates a TValue from Value and try casting it to Ordinal
    GenericValue := TValue.From<T>(Value);
    if not GenericValue.TryAsOrdinal(OrdValue) then
      raise EInvalidCast.Create('Could not cast generic value to ordinal');

    RTTIEnum := TRTTIEnumerationType(RTTIType);
    if (OrdValue > RTTIEnum.MaxValue) or (OrdValue < RTTIEnum.MinValue) then
      raise EEnumNameNotFound.CreateFmt('%d has no valid enum name for %s', [OrdValue, RTTIType.Name]);

    var LenStore: Integer := Length(RTTIEnum.GetAttributes);
    if (LenStore = 0) or (LenStore - 1 < OrdValue) then
      raise ERTTIMappingBadFormat.CreateFmt('RTTI Mapping in %s is badly formated.', [RTTIEnum.Name]);

    exit(MappingAttr(RTTIEnum.GetAttributes()[OrdValue]).Value);
  finally
    RTTICtx.Free;
  end;

  raise ERTTIMappingNotFound.Create('No [MappingAttr(...)] found');
end;

class function TEnumMapping.StringToEnum<T>(const Value: string): T;
var
  RTTICtx: TRTTIContext;
  RTTIType: TRTTIType;
begin
  RTTICtx := TRTTIContext.Create();
  try
    RTTIType := RTTICtx.GetType(TypeInfo(T));
    // Raises an exception if the type is not an enumeration.
    if (RTTIType = nil) or (RTTIType.TypeKind <> tkEnumeration) then
      raise Exception.CreateFmt('Type %s is not an enumeration', [RttiType.Name]);

    FillChar(Result, SizeOf(Result), 0); // Set a default value

    var ItemCount, AttrCount: Integer;
    ItemCount := Length(TRTTIEnumerationType(RTTIType).GetNames());
    AttrCount := Length(RTTIType.GetAttributes());

    if (AttrCount = 0) or (ItemCount <> AttrCount) then // No Attr or different amounts
      raise ERTTIMappingBadFormat.CreateFmt('RTTI Mapping in %s is badly formated.', [RTTIType.Name]);

    var Index: Integer := 0;
    var Walks: Integer := 0;
    var Found: Boolean := False;
    for var Attr: TCustomAttribute in RTTIType.GetAttributes do
    begin
      if Attr is MappingAttr then // just in case. Should always be MappingAttr.
      begin
        Inc(Index);
        Found := MappingAttr(Attr).Value = Value;
        if Found then // Will break if the attribute value matches the search.
          break;
      end;
    end;

    if not Found then
      raise ERTTIMappingNotFound.CreateFmt('No match for %s on %s', [Value, RTTIType.Name]);

    // Casts the address of result to a integer pointer, deref it, assigns Index.
    PInteger(@Result)^ := Index; // if not frend why frend shaped?
    Exit(Result);
  finally
    RTTICtx.Free();
  end;

  raise Exception.Create('Something went *very* wrong.');
end;

    这些都是匆忙编写的，但到目前为止还算有效。关键是将所有属性设置为枚举类型，而不是其值，因此，只要函数之前的假设成立，它就能够一致地检索，我猜。

    • 0

相关问题