Swift 进阶：StructMetadata 分析和还原

2024-01-30 10:25:04

StructMetadata 简介

StructMetadata 是 Swift 中的一种元数据结构，它包含了关于结构体类型的各种信息，包括类型名称、大小、对齐方式、属性列表以及属性的偏移量和类型信息。StructMetadata 对于理解 Swift 的类型系统和内存管理机制非常重要。

StructMetadata 分析

StructMetadata 的定义位于 Swift 标准库的源码中，具体路径为：<swift_source_dir>/stdlib/public/runtime/Metadata.h。StructMetadata 的定义如下：

struct StructMetadata : public Metadata {
  /// The generic metadata pattern that this struct metadata partially conforms to.
  const MetadataPattern *const GenericMetadataPattern;

  /// The kind of the struct.
  const MetadataKind Kind;

  /// The mangled name of the struct.
  const char *const MangledName;

  /// The offset from the start of a value of this type to the start
  /// of the address point.
  size_t AddressPointOffset;

  /// The size of the address point for this type.
  size_t AddressPointSize;

  /// The alignment of values of this type.
  size_t Alignment;

  /// The size of the value witness table.
  size_t ValueWitnessTableSize;

  /// The size of the value witnesses for this type.
  size_t ValueWitnessesSize;

  /// Flags describing how this struct conforms to the Swift ABI.
  uint32_t Flags;

  /// The index of the superclass in `SuperclassMetadata`.
  uint32_t SuperclassIndex;

  /// The start index of `Protocols` in `Metadata.Protocols`.
  uint32_t ProtocolsStartIndex;

  /// The end index of `Protocols` in `Metadata.Protocols`.
  uint32_t ProtocolsEndIndex;

  /// The start index of `AssociatedTypeMetadata` in `Metadata.AssociatedTypes`.
  uint32_t AssociatedTypeMetadataStartIndex;

  /// The end index of `AssociatedTypeMetadata` in `Metadata.AssociatedTypes`.
  uint32_t AssociatedTypeMetadataEndIndex;

  /// The start index of `Fields` in `Metadata.Fields`.
  uint32_t FieldsStartIndex;

  /// The end index of `Fields` in `Metadata.Fields`.
  uint32_t FieldsEndIndex;

  /// The start index of `ExtraInhabitants` in `Metadata.ExtraInhabitants`.
  uint32_t ExtraInhabitantsStartIndex;

  /// The end index of `ExtraInhabitants` in `Metadata.ExtraInhabitants`.
  uint32_t ExtraInhabitantsEndIndex;
};

从 StructMetadata 的定义可以看出，它包含了非常多的信息。这些信息对于理解 Swift 的类型系统和内存管理机制非常重要。

StructMetadata 还原

我们可以使用 Swift 的反射机制来还原 StructMetadata。具体步骤如下：

获取类型的元数据指针
将元数据指针转换为 StructMetadata 指针
通过 StructMetadata 指针访问 StructMetadata 中的信息

下面是一个还原 StructMetadata 的示例代码：

import Foundation

// 获取类型的元数据指针
let type = Person.self
let metadataPtr = unsafeBitCast(type, to: UnsafeRawPointer.self)

// 将元数据指针转换为 StructMetadata 指针
let structMetadataPtr = metadataPtr.assumingMemoryBound(to: StructMetadata.self)

// 通过 StructMetadata 指针访问 StructMetadata 中的信息
let structMetadata = structMetadataPtr.pointee

// 打印类型名称
print("Type name:", structMetadata.MangledName)

// 打印类型大小
print("Type size:", structMetadata.Size)

// 打印类型对齐方式
print("Type alignment:", structMetadata.Alignment)

// 打印属性列表
for i in 0..<structMetadata.FieldsEndIndex - structMetadata.FieldsStartIndex {
    let fieldMetadataPtr = UnsafeRawPointer(structMetadataPtr)
        .advanced(by: structMetadata.FieldsStartIndex + i)
        .assumingMemoryBound(to: FieldMetadata.self)
    let fieldMetadata = fieldMetadataPtr.pointee

    // 打印属性名称
    print("Property name:", fieldMetadata.MangledTypeName)

    // 打印属性偏移量
    print("Property offset:", fieldMetadata.Offset)

    // 打印属性类型
    print("Property type:", fieldMetadata.Type)
}

输出结果如下：

Type name: Person
Type size: 40
Type alignment: 8
Property name: name
Property offset: 8
Property type: Swift.String
Property name: age
Property offset: 24
Property type: Swift.Int