iOS14でTextFieldにdidBecomeFirstResponderを設定する

CustomTextField.swift

struct CustomTextField: UIViewRepresentable {
  @Binding var text: String
  var font: UIFont = .init()
  var textAlignment: NSTextAlignment = .left

  var isFirstResponder: Bool

  init(text: Binding<String>, isFirstResponder: Bool = false) {
    self._text = text
    self.isFirstResponder = isFirstResponder
  }

  func makeUIView(context: UIViewRepresentableContext<CustomTextField>) -> UITextField {
    let textField = UITextField(frame: .zero)
    textField.font = font
    textField.textAlignment = textAlignment
    textField.delegate = context.coordinator
    textField.setContentCompressionResistancePriority(.defaultLow, for: .horizontal)

    return textField
  }

  func updateUIView(_ uiView: UITextField, context: UIViewRepresentableContext<CustomTextField>) {
    uiView.text = text

    if isFirstResponder && !context.coordinator.didBecomeFirstResponder {
      uiView.becomeFirstResponder()
      context.coordinator.didBecomeFirstResponder = true
    }
  }
}

extension CustomTextField {
  func makeCoordinator() -> CustomTextField.Coordinator {
    Coordinator(text: $text)
  }

  class Coordinator: NSObject, UITextFieldDelegate {
    @Binding var text: String
    var didBecomeFirstResponder = false

    init(text: Binding<String>) {
      _text = text
    }

    func textFieldDidChangeSelection(_ textField: UITextField) {
      text = textField.text ?? ""
    }
  }
}

extension CustomTextField {
  public func font(_ font: UIFont) -> CustomTextField {
    var view = self
    view.font = font
    return view
  }

  public func textAlignment(_ alignment: NSTextAlignment) -> CustomTextField {
    var view = self
    view.textAlignment = alignment
    return view
  }
}

Content.swift

struct ContentView : View {
  @State var text: String = ""

  var body: some View {
    CustomTextField(text: $text, isFirstResponder: true)
      .borderStyle(.roundedRect)
      .frame(width: 200, height: 40)
      .background(Color.red)
  }
}

iOS15以降は@FocusStateを使う

参考にしたのはこちら