swift底层探索 08-根据Mirror原理还原TargetStructMetadata结构

在swift源码中有这样一段代码，可以看到所有的类型都是有元类型的,类、枚举、结构体都被已经被定义了,下面就通过mirror来还原TargetStructMetadata结构。

1、Mirror

struct Teacher{
    var age = 18
    var name = "henry"
    var height = 1.8
    var hobby = "woman"
}

let t = Teacher()
let mirror = Mirror(reflecting: t)

for pro in mirror.children{
   print("\(pro.label):\(pro.value)")
}

为什么Mirror可以获取到结构体的属性名称？

1.1 Mirror源码

public struct Mirror {
   //初始化方法
  public init(reflecting subject: Any) {
    if case let customized as CustomReflectable = subject {
      self = customized.customMirror
    } else {
      self = Mirror(internalReflecting: subject)
    }
  }
}

1.2 Mirror初始化

extension Mirror {
  internal init(internalReflecting subject: Any,
              subjectType: Any.Type? = nil,
              customAncestor: Mirror? = nil)
{
// type(of:)获取到subject的真实类型
    let subjectType = subjectType ?? _getNormalizedType(subject, type: type(of: subject))
// 获取属性大小
    let childCount = _getChildCount(subject, type: subjectType)
// 遍历，将属性存储到字典中
    let children = (0 ..< childCount).lazy.map({
      getChild(of: subject, type: subjectType, index: $0)
    })
    self.children = Children(children)
// 设置父类反射
    self._makeSuperclassMirror = {
      guard let subjectClass = subjectType as? AnyClass,
            let superclass = _getSuperclass(subjectClass) else {
        return nil
      }
      if let customAncestor = customAncestor {
        if superclass == customAncestor.subjectType {
          return customAncestor
        }
        if customAncestor._defaultDescendantRepresentation == .suppressed {
          return customAncestor
        }
      }
      return Mirror(internalReflecting: subject,
                    subjectType: superclass,
                    customAncestor: customAncestor)
    }
 // 获取并解析显示的样式，并设置Mirror的其他属性
    let rawDisplayStyle = _getDisplayStyle(subject)
    switch UnicodeScalar(Int(rawDisplayStyle)) {
    case "c": self.displayStyle = .class
    case "e": self.displayStyle = .enum
    case "s": self.displayStyle = .struct
    case "t": self.displayStyle = .tuple
    case "\0": self.displayStyle = nil
    default: preconditionFailure("Unknown raw display style '\(rawDisplayStyle)'")
    }

    self.subjectType = subjectType
    self._defaultDescendantRepresentation = .generated
}

type(of:) 获取到subject的真实类型

2、TargetStructMetadata结构

源码中继承关系比较复杂，因为设计到继承和一些模板类，所以只放出一张结构图:

转换为代码形式：

struct StructMetaData{
    var kind : Int32
    var description : UnsafePointer
}

struct StructDescriptor {
    let flags: Int32
    let parent: Int32
    var name: RelativePointer
    var AccessFunctionPtr: RelativePointer
    var Fields: RelativePointer
    var NumFields: Int32
    var FieldOffsetVectorOffset: Int32
}

struct FieldDescriptor {
    var MangledTypeName: RelativePointer
    var Superclass: RelativePointer
    var kind: UInt16
    var fieldRecordSize: Int16
    var numFields: Int32
    var fields: FieldRecord //连续的存储空间
}

struct FieldRecord {
    var Flags: Int32
    var MangledTypeName: RelativePointer
    var FieldName: RelativePointer
}


struct RelativePointer {
    var offset:Int32
    
    mutating public func get() -> UnsafePointer {
        let offSet = self.offset
        
        return withUnsafePointer(to: &self){ ptr in
            UnsafeRawPointer(ptr).advanced(by: numericCast(offSet)).assumingMemoryBound(to: T.self)
        }
    }
}

3、还原结构

根据源码要先获取元类指针,2种方式均可。swift指针使用起来有点难度（摊牌了我是个菜鸡。。。）swift底层探索 06 - 指针简单使用

withUnsafeMutablePointer

var type = Teacher.self
//指向元类指针的指针
let a = withUnsafeMutablePointer(to: &type) { ptr in
    UnsafeRawPointer(ptr).bindMemory(to: UnsafeMutablePointer.self, capacity: 1)
}
// 获取元类指针
let metaPtr = a.pointee

注意：a是：指向元类指针的指针

unsafeBitCast按位强转

let metaPtr = unsafeBitCast(Teacher.self as Any.Type, to: UnsafePointer.self)

unsafeBitCast需要两个参数的内存大小相同。必须使用：Teacher.self as Any.Type进行转换，因为根据测试发现Teacher.self获取内存大小为0（这个地方是真的坑），如下图：

Teacher.self as Any.Type 动作

根据sil查看发现 Teacher.self as Any.Type在底层的操作；

sil官方文档
创建 T 的元类型。

3.1 获取结构体名

let structName = metaPtr.pointee.description.pointee.name.get()
let structNameStr = String(cString: structName)
print("获取结构体名:\(structNameStr)")

输出：

3.2 获取内部属性个数

let cnt = metaPtr.pointee.description.pointee.NumFields
print("获取结构体属性个数:\(cnt)")

输出：

3.3 获取所有属性

let fieldsPtr = b.pointee.description.pointee.Fields.get()
for i in 0..

 
 输出： 
 
 
  
   
     
    
   
   
  
  
  si : 其实就是Int 
  ss : 其实就是String 
  sd : 其实就是Double 
  
 demo下载-github