DevAndArtist

import SwiftUI

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

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

DevAndArtist

import PlaygroundSupport
import Dispatch

let page = PlaygroundPage.current
page.needsIndefiniteExecution = true

struct Weak<T> {
  struct _ObjectBox {
    weak var object: AnyObject?
  }

DevAndArtist

// BUG: FB7635852

import SwiftUI

struct ContentView: View {
  let count: Int

  var body: some View {
    HStack
      .init(alignment: .top, spacing: 0) {

DevAndArtist

// SOME OBSERVATIONS:
// * In Xcode 11.3.1 `V_Struct_Representable_ObservedObject` won't update
//   which is clearly a bug. This bug is fixed in Xcode 11.4 beta 1.
//
// * `V_Struct` and `V_Struct_Representable` both never update because
//   they shouldn't as the framework internal raw-diffing likely won't
//   identify any change. (EXPECTED BEHAVIOR)
//
// * `V_Class` and `V_Class_ObservedObject` both produce a runtime crash,
//   which might be a bug as well. `View` conforming types shouldn't be

DevAndArtist

// This code uses 3 HIDDEN SwiftUI types, there is no guarantee 
// that it will pass app review, so please use it with caution.
//
//   * SwiftUI._PreferenceValue<Key>
//   * SwiftUI._DelayedPreferenceView<Key, Content>
//   * SwiftUI._PreferenceValue<Key>
//
// Feedback ID for an official support: (FB7577482)
//
// ⚠️ WARNING: 

DevAndArtist

// BUG: FB7569572

struct GeometrySizeKey: PreferenceKey {
  static func reduce(value: inout CGSize?, nextValue: () -> CGSize?) {
    value = nextValue()
  }
}

extension View {
  func readGeometrySize() -> some View {

DevAndArtist

struct PKey: PreferenceKey {
  static var defaultValue: Int?
  static func reduce(value: inout Int?, nextValue: () -> Int?) {
    print(#function, "value:", value as Any, "nextValue:", nextValue() as Any)
    value = nextValue()
  }
}

struct ContentView: View {
  let condition = true

DevAndArtist

import SwiftUI

class Base: CustomDebugStringConvertible {
  var debugDescription: String {
    "\(type(of: self)) \(ObjectIdentifier(self))"
  }

  init() {
    print("🟢 init\t", self)
  }

DevAndArtist

WWDC 2019 Labs Questions

Size changes propagation to child view controller

In A Look Inside Presentation Controllers - WWDC 2014 session the following pattern was introduced.

• A child view controller or a presented view controller can issue a request for size update by setting its preferredContentSize property.
• A parent view controller or a presentation controller will receive a callback on preferredContentSizeDidChangeForChildContentContainer method.
• At this point the receiver can decide if can allow the size request and layout the child view controller or the presented view controller.

DevAndArtist

import SwiftUI

struct ContentView: View {
  @State var flag = true

  var body: some View {
    let text = "\(flag ? "HStack" : "ZStack")"
    return VStack {
      Text(text)
      Button("change the stack") {