Swift 4 alternative if let - swift

Question

I have this code, it checks if a file exists in the document directory, if it does it used this to set up the data for the controller, if it doesn't it uses a file supplier in the bundle, and if also that fails it set up an empty view controller.
Is there a way to conditionally set the url variable so that I can write the parse(fromFile:) method once instead of in an if else? The most intuitive way would appear to be if let url = something || let url = somethingElse then perform in the parse(fromFile: url) in the brackets, but it appears that it's not possible with swift.
if let url = try? FileManager().url(for: .documentDirectory,
in: .userDomainMask,
appropriateFor: nil,
create: true).appendingPathComponent("database.json") {
parse(fromFile: url)
} else if let url = Bundle.main.url(forResource: "database", withExtension: "json") {
parse(fromFile: url)
} else {
setUpWithNothing()
}

Answer 1

I would recommend to split the code into several steps:
let documentsUrl = try! FileManager.default.url(
for: .documentDirectory,
in: .userDomainMask,
appropriateFor: nil,
create: true
)
The documents directory should always exists therefore I am not too worried about using try!. However, if you worry, you should solve that special case separately, e.g. using:
guard let documentUrl = try? ... else {
setUpWithNothing()
return
}
Then:
let fileUrl = documentUrl.appendingPathComponent("database.json")
This call never fails, it doesn't check the existence of the file. You need to check existence explicitly:
let fileExists = FileManager.default.fileExists(atPath: fileUrl.path)
let defaultUrl = Bundle.main.url(forResource: "database", withExtension: "json")
let databaseUrl = fileExists ? fileUrl : defaultUrl!
Again, you shouldn't worry about the existence of the file in your bundle, if you know it's there and simply call:
parse(fromFile: databaseUrl)
However, if you want to be extra careful, just remove the ! and use:
if let databaseUrl = fileExists ? fileUrl : defaultUrl {
parse(fromFile: databaseUrl)
} else {
setUpWithNothing()
}

Answer 2

You can use the nil-coalescing operator (??):
If the expression with FileManager returns nil (by throwing, thanks to the try?), then Bundle.main.url(...) will be used instead.
If that is also nil, then the conditional binding will fail entirely, and the else block will be run setUpWithNothing().
if let url = (try? FileManager.default.url(for: .documentDirectory,
in: .userDomainMask,
appropriateFor: nil,
create: true
).appendingPathComponent("database.json"))
?? Bundle.main.url(forResource: "database", withExtension: "json") {
parse(fromFile: url)
} else {
setUpWithNothing()
}

Answer 3

Since you're doing the same thing in the first two cases and something different in the second, this is a good candidate for it's own function:
func databaseFileURL() -> URL? {
if let url = try? FileManager().url(for: .documentDirectory, in: .userDomainMask,
appropriateFor: nil, create: true) {
return url.appendingPathComponent("database.json")
}
else {
return Bundle.main.url(forResource: "database", withExtension: "json")
}
}
and then you can just do
if let url = databaseFileURL() {
parse(fromFile: url)
} else {
setUpWithNothing()
}
It nicely separates concerns and makes it cleaner and clearer at the point of use. And if you need to do anything else with the file you can just call the function rather than copy-pasting the logic.