Swift-进阶 07：Mirror源码解析

<!--1、定义c方法-->
//.h声明
int cjl_add(int a, int b);
//.c中实现
int cjl_add(int a, int b){
    return a+b;
}
<!--2、桥接文件中引入头文件-->
#import "test.h"
<!--3、swift使用-->
var value = cjl_add(10, 20)
print(value)
<!--4、打印结果-->
30

<!--1、swift中针对cjl_add方法的声明-->
//通过@_silgen_name声明
@_silgen_name("cjl_add")
func swift_cjl_add(_ a: Int32, _ b: Int32) -> Int32
<!--2、使用-->
var value = swift_cjl_add(20, 30)
print(value)
<!--3、打印结果-->
50

public init(reflecting subject: Any) {
    //判断 subject 是否符合 CustomReflectable动态类型
    if case let customized as CustomReflectable = subject {
      //如果符合，则由 customMirror 确定属性
      self = customized.customMirror
    } else {
      //如果不符合，则由语言生成
      self = Mirror(internalReflecting: subject)
    }
}

extension Mirror {
  // Mirror的初始化器中检索需要的信息
  /*
  - subject 将要被反射的值
  - subjectType 将要被反射的subject值的类型，通常是值的运行时类型
  - 
  */ 
  internal init(internalReflecting subject: Any,
              subjectType: Any.Type? = nil,
              customAncestor: Mirror? = nil)
  {
    //根据_getNormalizedType获取传入的subject的真正类型，其中type(of: subject)获取的动态类型
    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函数时C++的_getChild 函数的简单封装，将标签名字中包含的C字符串转换为Swift字符串
      getChild(of: subject, type: subjectType, index: $0)
    })
    // 赋值给Mirror的属性children
    self.children = Children(children)
    // 设置父类反射
    self._makeSuperclassMirror = {//按需求构建父类的Mirror的闭包
      // 获取传入对象的类
      guard let subjectClass = subjectType as? AnyClass,
            // 获取父类
            let superclass = _getSuperclass(subjectClass) else {
        return nil//非类的类型、没有父类的类的Mirror，会获取到nil
      }
      // 调用者可以用一个可作为父类的Mirror直接返回Mirror实例来指定自定义的祖先的表现
      // Handle custom ancestors. If we've hit the custom ancestor's subject type,
      // or descendants are suppressed, return it. Otherwise continue reflecting.
      if let customAncestor = customAncestor {
        if superclass == customAncestor.subjectType {
          return customAncestor
        }
        if customAncestor._defaultDescendantRepresentation == .suppressed {
          return customAncestor
        }
      }
      // 给相同值返回一个将 superclass作为 subjectType的新的Mirror
      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
  }
  // 快速查找
  internal static func quickLookObject(_ subject: Any) -> _PlaygroundQuickLook? {
#if _runtime(_ObjC)
    let object = _getQuickLookObject(subject)
    return object.flatMap(_getClassPlaygroundQuickLook)
#else
    return nil
#endif
  }
}
👇
<!--superclassMirror属性的底层返回-->
public var superclassMirror: Mirror? {
    return _makeSuperclassMirror()
}
nternal let _makeSuperclassMirror: () -> Mirror?

@_silgen_name("swift_reflectionMirror_normalizedType")
internal func _getNormalizedType<T>(_: T, type: Any.Type) -> Any.Type
👇
SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_API
const Metadata *swift_reflectionMirror_normalizedType(OpaqueValue *value,
                                                      const Metadata *type,
                                                      const Metadata *T) {
  return call(value, T, type, [](ReflectionMirrorImpl *impl) { return impl->type; });
}
👇
/*
- passedValue 实际需要传入的swift的值的指针
- T 该值的静态类型
- passedType 被显式传入且会用在反射过程中的类型
- f 传递被查找到的会被调用的实现的对象引用
- 返回值：返回f参数调用时的返回值
*/ 
template<typename F>
auto call(OpaqueValue *passedValue, const Metadata *T, const Metadata *passedType,
          const F &f) -> decltype(f(nullptr))
{
  // 获取type
  const Metadata *type;
  OpaqueValue *value;
  std::tie(type, value) = unwrapExistential(T, passedValue);
  // 判断传入type是否为空，如果不为空，则直接赋值给type
  if (passedType != nullptr) {
    type = passedType;
  }
  // 使用 ReflectionMirrorImpl 子类的实例去结束调用f，然后会调用这个实例上的方法去真正的工作完成
  auto call = [&](ReflectionMirrorImpl *impl) {
    // 返回的type是传入非type
    impl->type = type;
    impl->value = value;
    auto result = f(impl);
    return result;
  };
  .....
  switch (type->getKind()) {
    case MetadataKind::Tuple: {//元组
        ......
    }
    case MetadataKind::Struct: {//结构体
       ......
    }
    case MetadataKind::Enum://枚举
    case MetadataKind::Optional: {//可选
     ......
    }
    ......
}
👇
<!--unwrapExistential实现-->
static std::tuple<const Metadata *, OpaqueValue *>
unwrapExistential(const Metadata *T, OpaqueValue *Value) {
  // If the value is an existential container, look through it to reflect the
  // contained value.如果该值是一个存在的容器，请查看它以反映包含的值。
  // TODO: Should look through existential metatypes too, but it doesn't
  // really matter yet since we don't have any special mirror behavior for
  // concrete metatypes yet.
  while (T->getKind() == MetadataKind::Existential) {
    auto *existential
      = static_cast<const ExistentialTypeMetadata *>(T);
    // Unwrap the existential container.打开存在容器
    T = existential->getDynamicType(Value);
    Value = existential->projectValue(Value);
    // Existential containers can end up nested in some cases due to generic
    // abstraction barriers.  Repeat in case we have a nested existential.
  }
  return std::make_tuple(T, Value);
}
👇
<!--getDynamicType的实现-->
template<> const Metadata *
ExistentialTypeMetadata::getDynamicType(const OpaqueValue *container) const {
// 根据 获取此存在类型使用的表示形式 判断
  switch (getRepresentation()) {
  case ExistentialTypeRepresentation::Class: {
    auto classContainer =
      reinterpret_cast<const ClassExistentialContainer*>(container);
    void *obj = classContainer->Value;
    return swift_getObjectType(reinterpret_cast<HeapObject*>(obj));
  }
  case ExistentialTypeRepresentation::Opaque: {
    auto opaqueContainer =
      reinterpret_cast<const OpaqueExistentialContainer*>(container);
    return opaqueContainer->Type;
  }
  case ExistentialTypeRepresentation::Error: {
    const SwiftError *errorBox
      = *reinterpret_cast<const SwiftError * const *>(container);
    return errorBox->getType();
  }
  }
  swift_runtime_unreachable(
      "Unhandled ExistentialTypeRepresentation in switch.");
}

class CJLTeacher{var age = 18}
var t = CJLTeacher()
let mirror = Mirror(reflecting: t)

// Abstract base class for reflection implementations.
struct ReflectionMirrorImpl {
  const Metadata *type;
  OpaqueValue *value;
  // 显示的样式
  virtual char displayStyle() = 0;
  // 属性个数
  virtual intptr_t count() = 0;
  // 获取偏移值
  virtual intptr_t childOffset(intptr_t index) = 0;
  // 获取元数据
  virtual const FieldType childMetadata(intptr_t index,
                                        const char **outName,
                                        void (**outFreeFunc)(const char *)) = 0;
  //获取属性
  virtual AnyReturn subscript(intptr_t index, const char **outName,
                              void (**outFreeFunc)(const char *)) = 0;
  //获取枚举的case名字
  virtual const char *enumCaseName() { return nullptr; }
#if SWIFT_OBJC_INTEROP
// 快速查找
  virtual id quickLookObject() { return nil; }
#endif
  // For class types, traverse through superclasses when providing field
  // information. The base implementations call through to their local-only
  // counterparts.
  // 递归查找父类的属性
  virtual intptr_t recursiveCount() {
    return count();
  }
  // 递归查找父类属性的偏移值
  virtual intptr_t recursiveChildOffset(intptr_t index) {
    return childOffset(index);
  }
  // 递归获取父类的元数据
  virtual const FieldType recursiveChildMetadata(intptr_t index,
                                                 const char **outName,
                                                 void (**outFreeFunc)(const char *))
  {
    return childMetadata(index, outName, outFreeFunc);
  }
// 析构函数
  virtual ~ReflectionMirrorImpl() {}
};

// Implementation for structs.
// ReflectionMirrorImpl 的子类 StructImpl 结构体反射
struct StructImpl : ReflectionMirrorImpl {
  bool isReflectable() {//是否支持反射
    const auto *Struct = static_cast<const StructMetadata *>(type);
    const auto &Description = Struct->getDescription();
    return Description->isReflectable();
  }
  // 用 s 的显式样式来表明这是一个结构体
  char displayStyle() {
    return 's';
  }
  intptr_t count() {
    if (!isReflectable()) {
      return 0;
    }
// 首先也是找到metadata，然后通过metadata找到desc，然后找到fields，即 NumFields 记录属性的count
    auto *Struct = static_cast<const StructMetadata *>(type);
    return Struct->getDescription()->NumFields;//属性的count
  }
  intptr_t childOffset(intptr_t i) {
    auto *Struct = static_cast<const StructMetadata *>(type);
    // 边界检查
    if (i < 0 || (size_t)i > Struct->getDescription()->NumFields)
      swift::crash("Swift mirror subscript bounds check failure");
    // Load the offset from its respective vector.
    // 获取偏移值
    return Struct->getFieldOffsets()[i];
  }
  const FieldType childMetadata(intptr_t i, const char **outName,
                                void (**outFreeFunc)(const char *)) {
    StringRef name;
    FieldType fieldInfo;
    //通过getFieldAt获取属性的名称
    std::tie(name, fieldInfo) = getFieldAt(type, i);
    assert(!fieldInfo.isIndirect() && "indirect struct fields not implemented");
    *outName = name.data();
    *outFreeFunc = nullptr;
    return fieldInfo;
  }
// subscript 用来获取当前属性的名称和值
  AnyReturn subscript(intptr_t i, const char **outName,
                      void (**outFreeFunc)(const char *)) {
    // 获取metaadata
    auto fieldInfo = childMetadata(i, outName, outFreeFunc);
    auto *bytes = reinterpret_cast<char*>(value);
    // 获取属性的偏移值
    auto fieldOffset = childOffset(i);
    // 计算字段存储的指针
    auto *fieldData = reinterpret_cast<OpaqueValue *>(bytes + fieldOffset);
    return copyFieldContents(fieldData, fieldInfo);
  }
};

using StructMetadata = TargetStructMetadata<InProcess>;

/// The common structure of metadata for structs and enums.
template <typename Runtime>
struct TargetValueMetadata : public TargetMetadata<Runtime> {
  using StoredPointer = typename Runtime::StoredPointer;
  TargetValueMetadata(MetadataKind Kind,
                      const TargetTypeContextDescriptor<Runtime> *description)
      : TargetMetadata<Runtime>(Kind), Description(description) {}
    //用于记录元数据的描述
  /// An out-of-line description of the type.
  TargetSignedPointer<Runtime, const TargetValueTypeDescriptor<Runtime> * __ptrauth_swift_type_descriptor> Description;
    ......
}

template <typename Runtime>
class TargetStructDescriptor final
    : public TargetValueTypeDescriptor<Runtime>,
      public TrailingGenericContextObjects<TargetStructDescriptor<Runtime>,
                            TargetTypeGenericContextDescriptorHeader,
                            /*additional trailing objects*/
                            TargetForeignMetadataInitialization<Runtime>,
                            TargetSingletonMetadataInitialization<Runtime>> {
    ......
    /// The number of stored properties in the struct.
  /// If there is a field offset vector, this is its length.
  uint32_t NumFields;//记录属性的count
  /// The offset of the field offset vector for this struct's stored
  /// properties in its metadata, if any. 0 means there is no field offset
  /// vector.
  uint32_t FieldOffsetVectorOffset;//记录属性在metadata中便宜向量的偏移量
    ......
}

template <typename Runtime>
class TargetTypeContextDescriptor
    : public TargetContextDescriptor<Runtime> {
public:
  /// The name of the type. 类型的名称
  TargetRelativeDirectPointer<Runtime, const char, /*nullable*/ false> Name;
  /// A pointer to the metadata access function for this type.
  /// 指向此类型的元数据访问函数的指针
  /// The function type here is a stand-in. You should use getAccessFunction()
  /// to wrap the function pointer in an accessor that uses the proper calling
  /// convention for a given number of arguments.
  TargetRelativeDirectPointer<Runtime, MetadataResponse(...),
                              /*Nullable*/ true> AccessFunctionPtr;
  /// A pointer to the field descriptor for the type, if any.指向类型的字段描述符的指针
  TargetRelativeDirectPointer<Runtime, const reflection::FieldDescriptor,
                              /*nullable*/ true> Fields;
    ......
}

/// Base class for all context descriptors.
template<typename Runtime>
struct TargetContextDescriptor {
  /// Flags describing the context, including its kind and format version.
  ContextDescriptorFlags Flags;
  /// The parent context, or null if this is a top-level context.
  TargetRelativeContextPointer<Runtime> Parent;
  ......
}

template<typename T, bool Nullable, typename Offset>
class RelativeDirectPointerImpl {
private:
  /// The relative offset of the function's entry point from *this.
  Offset RelativeOffset;
    ......
public:
  using ValueTy = T;//是一个值
  using PointerTy = T*;//是一个指针
}
    //get方法 - 用于获取属性
  PointerTy get() const & {
    // Check for null.检查是否为空
    if (Nullable && RelativeOffset == 0)
      return nullptr;
    // The value is addressed relative to `this`. 值是相对于“this”寻址的
    uintptr_t absolute = detail::applyRelativeOffset(this, RelativeOffset);
    return reinterpret_cast<PointerTy>(absolute);
  }
    ......
}
👇
<!--applyRelativeOffset的实现-->
template<typename BasePtrTy, typename Offset>
static inline uintptr_t applyRelativeOffset(BasePtrTy *basePtr, Offset offset) {
  static_assert(std::is_integral<Offset>::value &&
                std::is_signed<Offset>::value,
                "offset type should be signed integer");
// 指针地址
  auto base = reinterpret_cast<uintptr_t>(basePtr);
  // We want to do wrapping arithmetic, but with a sign-extended
  // offset. To do this in C, we need to do signed promotion to get
  // the sign extension, but we need to perform arithmetic on unsigned values,
  // since signed overflow is undefined behavior.
  auto extendOffset = (uintptr_t)(intptr_t)offset;
  return base + extendOffset;//指针地址+存放的offset（偏移地址） -- 内存平移获取值
}

class FieldDescriptor {
  const FieldRecord *getFieldRecordBuffer() const {
    return reinterpret_cast<const FieldRecord *>(this + 1);
  }
public:
  const RelativeDirectPointer<const char> MangledTypeName;
  const RelativeDirectPointer<const char> Superclass;
    ......
  const FieldDescriptorKind Kind;
  const uint16_t FieldRecordSize;
  const uint32_t NumFields;
    ......
  // 获取所有属性，每个属性用FieldRecord封装
   llvm::ArrayRef<FieldRecord> getFields() const {
    return {getFieldRecordBuffer(), NumFields};
  }
  ......
} 

class FieldRecord {
  const FieldRecordFlags Flags;
public:
  const RelativeDirectPointer<const char> MangledTypeName;
  const RelativeDirectPointer<const char> FieldName;

/// metadata元数据
struct StructMetadata {
//    （取自类 - TargetMetadata：kind）
    //(继承关系：TargetStructMetadata -> TargetValueMetadata -> TargetMetadata)
    var kind: Int
//    （取自结构体 -  TargetValueMetadata：Description）
    var desc: UnsafeMutablePointer<StructMetadataDesc>
}
/// metada的描述信息
struct StructMetadataDesc {
//    （取自底层结构体 - TargetContextDescriptor：flags + parent）
    var flags: Int32
    var parent: Int32
//    （取自底层类 - TargetTypeContextDescriptor：name + AccessFunctionPtr + Fields）
    //type的名称
    //相对指针位置的存储
    var name: RelativeDirectPointer<CChar>
    //补充完整
    var AccessFunctionPtr: RelativeDirectPointer<UnsafeRawPointer>
    //是通过Fields的getFiledName获取属性名称
    var Fields: RelativeDirectPointer<FieldDescriptor>
//    (取自底层类 - TargetClassDescriptor：NumFields + FieldOffsetVectorOffset)
    //属性的count
    var NumFields: Int32
    var FieldOffsetVectorOffset: Int32
}
/// 属性的描述信息
//（取自底层类 - FieldDescriptor）
struct FieldDescriptor {
    var MangledTypeName: RelativeDirectPointer<CChar>
    var Superclass: RelativeDirectPointer<CChar>
    var Kind: UInt16
    var FieldRecordSize: Int16
    var NumFields: Int32
    //每个属性都是FieldRecord，记录在这个结构体中
    var fields: FieldRecord//数组中是一个连续的存储空间
}
/// 属性封装类
//（取自底层类 - FieldRecord）
struct FieldRecord{
    var Flags: Int32
    var MangledTypeName: RelativeDirectPointer<CChar>
    var FieldName: RelativeDirectPointer<CChar>
}
/// 记录offset偏移值
struct RelativeDirectPointer<T>{
    var offset: Int32
    //模拟RelativeDirectPointerImpl类中的get方法 this+offset指针
    mutating func get() -> UnsafeMutablePointer<T>{
        let offset = self.offset
        return withUnsafePointer(to: &self) { p in
            /*
             获得self，变为raw，然后+offset
             - UnsafeRawPointer(p) 表示this
             - advanced(by: numericCast(offset) 表示移动的步长，即offset
             - assumingMemoryBound(to: T.self) 表示假定类型是T，即自己制定的类型
             - UnsafeMutablePointer(mutating:) 表示返回的指针类型
            */
            return UnsafeMutablePointer(mutating: UnsafeRawPointer(p).advanced(by: numericCast(offset)).assumingMemoryBound(to: T.self))
        }
    }
}

struct CJLTeacher {
    var age = 18
    var name = "CJL"
}

/将t1绑定到StructMetadata（unsafeBitCast-按位强转，非常危险，没有任何校验、没有任何修饰）
//unsafeBitCast - 所有的内存按位转换
//1、先获取指向metadata的指针
let ptr = unsafeBitCast(CJLTeacher.self as Any.Type, to: UnsafeMutablePointer<StructMetadata>.self)
//2、然后获取字符串的地址
/*
 ptr.pointee 表示StructMetadata
 ptr.pointee.desc.pointee 表示StructMetadataDesc
 ptr.pointee.desc.pointee.name 表示RelativeDirectPointer<T>
 */
let namePtr = ptr.pointee.desc.pointee.name.get()
print(String(cString: namePtr))
//读取内存值
print(ptr.pointee.desc.pointee.NumFields)

//获取首地址
let filedDescriptorPtr = ptr.pointee.desc.pointee.Fields.get()
//指针移动来获取访问属性的地址
let recordPtr = withUnsafePointer(to: &filedDescriptorPtr.pointee.fields) {
    /*
     - UnsafeRawPointer + assumingMemoryBound -- 类型指针
     - advanced 类型指针只需要移动 下标即可
     */
    return UnsafeMutablePointer(mutating: UnsafeRawPointer($0).assumingMemoryBound(to: FieldRecord.self).advanced(by: 0))
}
//输出age属性
print(String(cString: recordPtr.pointee.FieldName.get()))