Return from async NSOpenPanel - swift

Question

How can this function be re-written to return the variable "files"? I'm completely unfamiliar with async and completion handlers, so I wasn't able to use existing answers as a starting point. It's a simple function that returns the files from a user-selected directory.
func readFolder() {
let dialog = NSOpenPanel()
dialog.prompt = "Choose"
dialog.allowsMultipleSelection = false
dialog.canChooseDirectories = true
dialog.canCreateDirectories = true
dialog.canChooseFiles = false
dialog.showsResizeIndicator = true
dialog.showsHiddenFiles = false
dialog.begin {
(result) -> Void in if result == .OK {
let directory = dialog.url!
do {
var files = try FileManager.default.contentsOfDirectory(
at: directory,
includingPropertiesForKeys: nil
)
} catch {
NSLog(error.localizedDescription)
}
}
}
}

Answer 1

You're on the right track with your question when you mentioned "completion handlers." With async functions, you aren't going to actually return the value directly, but rather through a function that you provide as a completion handler. This takes some getting-used-to to restructure the way you think of some of your code, but is a great concept to become familiar with.
Code first, then explanation:
func doSomethingThatRequiresFiles() {
readFolder { result in
switch result {
case .success(let files):
print(files)
case .failure(let error):
print(error)
}
}
}
func readFolder(completion: #escaping (Result<[URL],Error>) -> Void) {
let dialog = NSOpenPanel()
dialog.prompt = "Choose"
dialog.allowsMultipleSelection = false
dialog.canChooseDirectories = true
dialog.canCreateDirectories = true
dialog.canChooseFiles = false
dialog.showsResizeIndicator = true
dialog.showsHiddenFiles = false
dialog.begin { (result) -> Void in
if result == .OK {
guard let directory = dialog.url else {
assertionFailure("Not a directory")
return
}
do {
let files = try FileManager.default.contentsOfDirectory(
at: directory,
includingPropertiesForKeys: nil
)
completion(.success(files))
} catch {
NSLog(error.localizedDescription)
completion(.failure(error))
}
} else {
//handle cancelled case
}
}
}
The first function (doSomethingThatRequiresFiles ) is an example of a place in your code where you want to deal with files. You'll see there's a switch statement that lets you handle either success or failure. You can see that the line print(files) is where you would put your code that needs to deal with the files somehow.
In the readFolder function, there's now a parameter (completion) gets a Result type -- it can either be an array of URLs or an Error. Read more about Result: https://www.hackingwithswift.com/articles/161/how-to-use-result-in-swift
And detail about why #escaping is used: https://www.donnywals.com/what-is-escaping-in-swift/
Inside the dialog.begin, you can see that completion gets called with either the list of files or the error.
Using async functions will be a familiar pattern when working with the filesystem (in particular if you have to deal with the iCloud APIs) and certainly with networking, where basically everything is asynchronous. It's also a good pattern to be familiar with in situations (like this one) where you're waiting for a UI interaction.