NSTextFiels check isFocus

viewcontroller.swift

import AppKit

// Credit: Vadim Shpakovski from AppKitAbusers

class ViewController: NSViewController {

  let textField1: CustomTextField = .init(string: "Field 1")
  let textField2: CustomTextField = .init(string: "Field 2")

  var observation1: NSKeyValueObservation?
  var observation2: NSKeyValueObservation?

  override func viewDidLoad() {
    super.viewDidLoad()

    let stackView = NSStackView(views: [textField1, textField2])
    stackView.translatesAutoresizingMaskIntoConstraints = false
    stackView.orientation = .vertical

    view.addSubview(stackView)
    NSLayoutConstraint.activate([
      stackView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
      stackView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
      stackView.widthAnchor.constraint(equalToConstant: 200),
    ])

    observation1 = textField1.observe(\.isFocused) { textField, _ in
      print(textField.stringValue, textField.isFocused ? "is focused" : "is not focused")
    }
    observation2 = textField2.observe(\.isFocused) { textField, _ in
      print(textField.stringValue, textField.isFocused ? "is focused" : "is not focused")
    }
  }
}

class CustomTextField: NSTextField {
  @objc dynamic var isFocused: Bool { currentEditor() != nil }

  override func becomeFirstResponder() -> Bool {
      let result = super.becomeFirstResponder()
      willChangeValue(for: \.isFocused)
      didChangeValue(for: \.isFocused)
      return result
  }
  override func textDidEndEditing(_ notification: Notification) {
    super.textDidEndEditing(notification)
    willChangeValue(for: \.isFocused)
    didChangeValue(for: \.isFocused)
  }
}