swiftui_alignment_keys.swift

import SwiftUI

// T.Type -> AlignmentID.Type -> Any.Type -> UInt

extension HorizontalAlignment {
  enum TestH: AlignmentID {
    static func defaultValue(in context: ViewDimensions) -> CGFloat {
      0
    }
  }
  static let testH = HorizontalAlignment(TestH.self)
}

extension VerticalAlignment {
  enum TestV: AlignmentID {
    static func defaultValue(in context: ViewDimensions) -> CGFloat {
      0
    }
  }
  static let testV = VerticalAlignment(TestV.self)
}

var testType = HorizontalAlignment.TestH.self
var alignmentIDType: AlignmentID.Type = testType
let anyType: Any.Type = alignmentIDType

MemoryLayout<HorizontalAlignment.TestH.Type>.size // 0
MemoryLayout<AlignmentID.Type>.size               // 16
MemoryLayout<Any.Type>.size                       // 8

let ob_1 = ObjectIdentifier(testType)
let ob_2 = ObjectIdentifier(alignmentIDType)
let ob_3 = ObjectIdentifier(anyType)

let testH = HorizontalAlignment.testH

dump(testH)

// Obtain `bits: UInt` from `AlignmentKey` type.
let key = Mirror(reflecting: testH).children.first!.value
let bits = Mirror(reflecting: key).children.first!.value as! UInt

print(String(bits, radix: 16))

let i_1 = UInt(bitPattern: ob_1)
let i_2 = UInt(bitPattern: ob_2)
let i_3 = UInt(bitPattern: ob_3)
let i_4 = unsafeBitCast(anyType, to: UInt.self)

i_1 == i_2 && i_2 == i_3 && i_3 == i_4 // true

print(String(i_1, radix: 16))

// UInt -> Any.Type -> AlignmentID.Type

let e_1 = unsafeBitCast(i_1, to: Any.Type.self)
let e_2 = unsafeBitCast(bits, to: Any.Type.self)

anyType == e_1
anyType == e_2

let a_1 = e_1 as! AlignmentID.Type
a_1 == testType        // true
a_1 == alignmentIDType // true

let a_2 = e_2 as! AlignmentID.Type
a_2 == testType        // true
a_2 == alignmentIDType // true


func extract(_ guide: HorizontalAlignment) -> AlignmentID.Type {
  let key = Mirror(reflecting: guide).children.first!.value
  let bits = Mirror(reflecting: key).children.first!.value as! UInt
  let existential = unsafeBitCast(bits, to: Any.Type.self)
  return existential as! AlignmentID.Type
}

func extract(_ guide: VerticalAlignment) -> AlignmentID.Type {
  let key = Mirror(reflecting: guide).children.first!.value
  var bits = Mirror(reflecting: key).children.first!.value as! UInt
  // align the value as VerticalAlignment key is intentionally misaligned
  bits -= 1
  let existential = unsafeBitCast(bits, to: Any.Type.self)
  return existential as! AlignmentID.Type
}

print(extract(.leading))                   // Leading
print(extract(HorizontalAlignment.center)) // Center
print(extract(.trailing))                  // Trailing
print(extract(HorizontalAlignment.testH))  // TestH
print(extract(.top))                       // Top
print(extract(VerticalAlignment.center))   // Center
print(extract(.bottom))                    // Bottom
print(extract(VerticalAlignment.testV))    // TestV
print(extract(.firstTextBaseline))         // FirstTextBaseline
print(extract(.lastTextBaseline))          // LastTextBaseline