Swift 3: cannot write data to plist file - ios

Question

I am trying to use a file called Data.plist to store some simple unstructured data, and I placed this file at the root folder of my app. To make it simple to read/write to this file, I created the following DataManager struct. It can read Data.plist file with no problem, but it cannot write data to the file. I am not sure where the problem is, could anyone spot where might be wrong?
struct DataManager {
static var shared = DataManager()
var dataFilePath: String? {
return Bundle.main.path(forResource: "Data", ofType: "plist")
}
var dict: NSMutableDictionary? {
guard let filePath = self.dataFilePath else { return nil }
return NSMutableDictionary(contentsOfFile: filePath)
}
let fileManager = FileManager.default
fileprivate init() {
guard let path = dataFilePath else { return }
guard fileManager.fileExists(atPath: path) else {
fileManager.createFile(atPath: path, contents: nil, attributes: nil) // create the file
print("created Data.plist file successfully")
return
}
}
func save(_ value: Any, for key: String) -> Bool {
guard let dict = dict else { return false }
dict.setObject(value, forKey: key as NSCopying)
dict.write(toFile: dataFilePath!, atomically: true)
// confirm
let resultDict = NSMutableDictionary(contentsOfFile: dataFilePath!)
print("saving, dict: \(resultDict)") // I can see this is working
return true
}
func delete(key: String) -> Bool {
guard let dict = dict else { return false }
dict.removeObject(forKey: key)
return true
}
func retrieve(for key: String) -> Any? {
guard let dict = dict else { return false }
return dict.object(forKey: key)
}
}

Answer 1

You cannot modify the files inside your app bundle. So all the files that you get with Bundle.main.path(forResource:ofType:) are readable but not writable.
If you want to modify this file you will need to copy it inside your app's document directory first.
let initialFileURL = URL(fileURLWithPath: Bundle.main.path(forResource: "Data", ofType: "plist")!)
let documentDirectoryURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).last!
let writableFileURL = documentDirectoryURL.appendingPathComponent("Data.plist", isDirectory: false)
do {
try FileManager.default.copyItem(at: initialFileURL, to: writableFileURL)
} catch {
print("Copying file failed with error : \(error)")
}
// You can modify the file at writableFileURL