How can I easily save the Window size and position state using Obj-C?

I tried all the solutions. It can only saves the position, not the size. So we should do that manually. This is how I do it on my GifCapture app https://github.com/onmyway133/GifCapture

class MainWindowController: NSWindowController, NSWindowDelegate {

  let key = "GifCaptureFrameKey"

  override func windowDidLoad() {
    super.windowDidLoad()

    NotificationCenter.default.addObserver(self, selector: #selector(windowWillClose(_:)), name: Notification.Name.NSWindowWillClose, object: nil)
  }

  override func awakeFromNib() {
    super.awakeFromNib()

    guard let data = UserDefaults.standard.data(forKey: key),
      let frame = NSKeyedUnarchiver.unarchiveObject(with: data) as? NSRect else {
        return
    }

    window?.setFrame(frame, display: true)
  }

  func windowWillClose(_ notification: Notification) {
    guard let frame = window?.frame else {
      return
    }

    let data = NSKeyedArchiver.archivedData(withRootObject: frame)
    UserDefaults.standard.set(data, forKey: key)
  }
}

Put a name that is unique to that window (e.g. "MainWindow" or "PrefsWindow") in the Autosave field under Attributes in Interface Builder. It will then have its location saved in your User Defaults automatically.

To set the Autosave name programmatically, use -setFrameAutosaveName:. You may want to do this if you have a document-based App or some other situation where it doesn't make sense to set the Autosave name in IB.

Link to documentation.

In Swift:

class MainWindowController : NSWindowController {
    override func windowDidLoad() {
        shouldCascadeWindows = false
        window?.setFrameAutosaveName("MainWindow")

        super.windowDidLoad()
    }

According to the doc, to save a window's position:

NSWindow *window = // the window in question
[[window windowController] setShouldCascadeWindows:NO];      // Tell the controller to not cascade its windows.
[window setFrameAutosaveName:[window representedFilename]];  // Specify the autosave name for the window.