一、存储属性
- 定义：存储属性是一个作为特定类和结构体实例一部分的常量或变量。存储属性要么是变量存储属性（var 引入）或者是常量存储属性（let 引入）。枚举
enum不可定义
/// 存储属性
let kp: Int
var pk: String
init(_ kp: Int, _ pk: String) {
self.kp = kp
self.pk = pk
}
var origin: Point = Point()
var size: Size = Size()
/// 计算属性、
/// 必须var申明
/// 属性的类型不可忽略
/// 要修改属性的值，必须实现setter，否则只要getter
var pointer: Point {
get {
let pointx = origin.x + size.width
let pointy = origin.y + size.height
return Point(x: pointx, y: pointy)
}
/// 修改值
set(newPointer) {
origin.x = newPointer.x - size.width
origin.y = newPointer.y - size.height
}
}
/// 类型属性
static var allWidth: Double = 0
占用空间
MemoryLayout<Circle>.stride打印为8。说计算属性没用占内存
二、计算属性
- 定义：不直接存储值，而是通过
getset方法来取值或者赋值，同时还可以对其他属性进行操作。本质是方法函数
三、类型属性
- 定义：定义在类上的属性，用static来修饰属性。
四、延迟属性
- 定义： 在第一次使用时初始化。
lazy
五、属性观察者
- 定义：用来观察属性值的变化，
willset当属性被调用这个值相同，而didset属性已经变化后调用。类似于setget。只有在完全初始化的实例分配新值的时候才会调用
属性在的位置
struct TargetClassMetadata {
/// The address point of instances of this type. 此类型实例的地址点
var InstanceAddressPoint: Int32
/// The required size of instances of this type. 此类型实例的所需大小。
/// 'InstanceAddressPoint' bytes go before the address point; 字节位于地址点之前
/// 'InstanceSize - InstanceAddressPoint' bytes go after it. 字节位于其后面
var InstanceSize: Int32
/// The alignment mask of the address point of instances of this type. 此类型实例的地址点的对齐掩码
var InstanceAlignMask: Int32
/// Reserved for runtime use. 保留供运行时使用
var Reserved: Int32
/// The total size of the class object, including prefix and suffix
/// extents. 类对象的总大小，包括前缀和后缀范围。
var ClassSize: Int32
/// The offset of the address point within the class object. 类对象内地址点的偏移量。
var ClassAddressPoint: Int32
/// An out-of-line Swift-specific description of the type, or null
/// if this is an artificial subclass. We currently provide no
/// supported mechanism for making a non-artificial subclass
/// dynamically.
var typeDescriptor: UnsafeMutableRawPointer /// -> v_table
////// A function for destroying instance variables, used to clean up after an
/// early return from a constructor. If null, no clean up will be performed
/// and all ivars must be trivial.
var iVarDescroyer: UnsafeRawPointer
}
struct Metadata {
/// The kind. Only valid for non-class metadata; getKind() must be used to get
/// kind 仅对非类元数据有效；必须使用getKind（）来获取值。
/// the kind value.
var kind: Int
var superClass: Any.Type
var cacheData: (Int, Int)
var data: Int
var classFlags: Int32
/// start TargetClassMetadata
var instanceAddressPoint: UInt32
var instanceSize: UInt32
var instanceAligmentMask: UInt16
var reserver: UInt16
var classSize: UInt32
var classAdressPoint: UInt32
var typeDescriptor: UnsafeMutableRawPointer /// -> v_table TargetTypeContextDescriptor
var iVarDescroyer: UnsafeRawPointer
/// end TargetClassMetadata
}
struct TargetTypeContextDescriptor {
/// The name of the type. 类型的名称。
var name: UInt32
/// 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() 这里的函数类型是一个替代。您应该使用getAccessFunction（）
/// to wrap the function pointer in an accessor that uses the proper calling 将函数指针包装在使用正确调用的访问器中
/// convention for a given number of arguments. 给定数量的参数的约定。
var accessFunctionPointer: Int32
/// A pointer to the field descriptor for the type, if any. 指向类型的字段描述符（如果有）的指针。 属性信息
var FieldDescriptor: Int32
}
/// TargetClassDescriptor 包含 TargetTypeContextDescriptor
struct TargetClassDescriptor {
var flags: UInt32
var parent: UInt32
var name: UInt32
var accessFunctionPointer: Int32
var FieldDescriptor: Int32
// var targetTypeContextDescriptor: TargetTypeContextDescriptor
/// The type of the superclass, expressed as a mangled type name that can 超类的类型，表示为可以
/// refer to the generic arguments of the subclass type. 请参阅子类类型的泛型参数。
var SuperclassType: Int32
/// If this descriptor does not have a resilient superclass, this is the 如果此描述符没有弹性超类，则为 此类元数据对象的负大小（大写）。
/// negative size of metadata objects of this class (in words).
var MetadataNegativeSizeInWords: UInt32
/// If this descriptor does not have a resilient superclass, this is the 如果此描述符没有弹性超类，则为此类元数据对象的正大小（大写）。
/// positive size of metadata objects of this class (in words).
var MetadataPositiveSizeInWords: UInt32
/// The number of additional members added by this class to the class 此类添加到该类的其他成员数,元数据。默认情况下，此数据对运行时是不透明的，而不是在其他成员中暴露；真的只是数据的nummediatemembers*sizeof（void*）字节。
/// metadata. This data is opaque by default to the runtime, other than
/// as exposed in other members; it's really just
/// NumImmediateMembers * sizeof(void*) bytes of data.
///这些字节是在地址点之前还是之后添加的 取决于areImmediateMembersNegative（）
/// Whether those bytes are added before or after the address point
/// depends on areImmediateMembersNegative().
var NumImmediateMembers: UInt32 // ABI: could be uint16_t?
/// The number of stored properties in the class, not including its 类中存储的属性数，不包括其超级类。如果存在场偏移向量，则这是其长度。
/// superclasses. If there is a field offset vector, this is its length.
var NumFields: UInt32
\
/// The offset of the field offset vector for this class's stored 此类存储的字段的字段偏移向量的偏移量
/// properties in its metadata, in words. 0 means there is no field offset 属性在其元数据中，以文字表示。0表示没有字段偏移
/// vector.
///
/// If this class has a resilient superclass, this offset is relative to 如果该类具有弹性超类，则该偏移量相对于弹性超类元数据的大小。否则就是
/// the size of the resilient superclass metadata. Otherwise, it is
/// absolute.
var FieldOffsetVectorOffset: UInt32
var offset: UInt32
var size: UInt32
// V-Table
}
\
// Field descriptors contain a collection of field records for a single
// class, struct or enum declaration.
//字段描述符包含单个字段的字段记录集合
//类、结构或枚举声明。
struct FieldDescriptor {
var MangledTypeName: Int32
var Superclass: Int32
var kind: Int32
var FieldRecordSize: Int32
var NumFields: Int32 /// 代表有多少属性，
var FieldRecords: [FieldRecord] /// 每个属性的信息
// const FieldRecord *getFieldRecordBuffer() const {
// return reinterpret_cast<const FieldRecord *>(this + 1);
// }
}
struct FieldRecord{
var Flags: Int32
var MangledTypeName: Int32
var FieldName: Int32
}
