How to observe shared app groups file changed - ios

Question

I have file which is shared between my app and extensions:
Write to file from extension:
func writeToFile()
{
let file = "file.txt"
let text = "data" //just a text
let dir = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: "group.com.ml.test.apps")!
let fileURL = dir.appendingPathComponent(file)
do {
try text.write(to: fileURL, atomically: false, encoding: .utf8)
}
catch {/* error handling here */}
}
Read from the app:
func readFromFile()
{
let file = "file.txt"
let dir = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: "group.com.ml.test.apps")!
let fileURL = dir.appendingPathComponent(file)
do {
let text2 = try String(contentsOf: fileURL, encoding: .utf8)
NSLog("Dan: \(text2)")
}
catch {/* error handling here */}
}
My question is how can I observe changes to this file. In case the extension writes to it and changes data so the app will get notification change and read the file.

Answer 1

Here is a simple demo of approach based on usage NSFileCoordinator/NSFilePresenter pattern.
Tested with Xcode 11.4 / iOS 13.4
Application part. Here is a ViewController plays a file presenter role, for simplicity (if one controller can manage many files, then it is better to create explicit presenters per-file)
class ViewController: UIViewController, NSFilePresenter {
var presentedItemURL: URL?
var presentedItemOperationQueue: OperationQueue = OperationQueue.main
#IBOutlet weak var userNameField: UILabel!
func presentedItemDidChange() { // posted on changed existed file only
readFromFile()
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
// register for presentedItemDidChange work
NSFileCoordinator.addFilePresenter(self)
}
override func viewWillDisappear(_ animated: Bool) {
super.viewWillDisappear(animated)
// unregister - required !!
NSFileCoordinator.removeFilePresenter(self)
}
override func viewDidLoad() {
super.viewDidLoad()
let file = "file.txt"
let dir = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: "group.com.test.apps")!
presentedItemURL = dir.appendingPathComponent(file)
readFromFile() // read previously stored data
}
private func readFromFile()
{
let coordinator = NSFileCoordinator(filePresenter: self)
coordinator.coordinate(readingItemAt: presentedItemURL!, options: [], error: nil) { url in
if let text2 = try? String(contentsOf: url, encoding: .utf8) {
userNameField.text = text2 // demo label in view for test
} else {
userNameField.text = "<no text>"
//just initial creation of file needed to observe following changes
coordinator.coordinate(writingItemAt: presentedItemURL!, options: .forReplacing, error: nil) { url in
do {
try "".write(to: url, atomically: false, encoding: .utf8)
}
catch { print("writing failed") }
}
}
}
}
}
Extension part (simple Today extension for demo having one button)
class TodayViewController: UIViewController, NCWidgetProviding, NSFilePresenter {
var presentedItemURL: URL?
var presentedItemOperationQueue: OperationQueue = OperationQueue.main
override func viewDidLoad() {
super.viewDidLoad()
let file = "file.txt"
let dir = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: "group.com.test.apps")!
presentedItemURL = dir.appendingPathComponent(file)
}
#IBAction func post(_ sender: Any) { // action on button in extension
writeToFile()
}
func writeToFile()
{
let text = "new data" //just a text
let coordinator = NSFileCoordinator(filePresenter: self)
coordinator.coordinate(writingItemAt: presentedItemURL!, options: .forReplacing, error: nil) { url in
do {
try text.write(to: url, atomically: false, encoding: .utf8)
}
catch { print("writing failed") }
}
}
func widgetPerformUpdate(completionHandler: (#escaping (NCUpdateResult) -> Void)) {
completionHandler(NCUpdateResult.newData)
}
}