ViewController.swift

// https://stackoverflow.com/questions/42179567/what-does-startaccessingsecurityscopedresource-actually-do/49196035?noredirect=1#comment85430898_49196035
// - Create ViewController with 3 Buttons
// - Enable App Sandbox User Selected File Read/Write
// - Click Button1, select folder /tmp/
// - Restart app, press Button3 -> crash
// - Restart app, press Button2 and Button3 -> works

import Cocoa

class ViewController: NSViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

    }

    @IBAction func pressButton(_ sender: AnyObject) {
        let panel = NSOpenPanel()
        panel.canChooseDirectories = true
        panel.canChooseFiles = false
        panel.prompt = "Select /tmp"
        guard panel.runModal() == NSApplication.ModalResponse.OK else { return }
        guard let url = panel.url else { return }

        NSLog("%@", "\(url.absoluteString)")
        let data = try! url.bookmarkData(options: .withSecurityScope, includingResourceValuesForKeys: nil, relativeTo: nil)
        UserDefaults.standard.set(data, forKey: "bookmark")

        NSLog("bookmark stored")
    }

    @IBAction func pressButton2(_ sender: AnyObject) {
        let data = UserDefaults.standard.data(forKey: "bookmark")!
        var bookmarkDataIsStale: ObjCBool = false

        let url = try! (NSURL(resolvingBookmarkData: data, options: [.withoutUI, .withSecurityScope], relativeTo: nil, bookmarkDataIsStale: &bookmarkDataIsStale) as URL)
        if bookmarkDataIsStale.boolValue {
            NSLog("WARNING stale security bookmark")
            return
        }
        if !url.startAccessingSecurityScopedResource() {
            NSLog("startAccessingSecurityScopedResource FAILED")
        }

        NSLog("startAccessing done")
    }

    @IBAction func pressButton3(_ sender: AnyObject) {
        let text = "just a test"
        let fileURL = URL(fileURLWithPath: "/tmp/file.txt")
        try! text.write(to: fileURL, atomically: false, encoding: .utf8)
        let text2 = try! String(contentsOf: fileURL, encoding: .utf8)
        NSLog("READ: \(text2)")
    }

}