How to save a string to a file on disk with write(to:)

Swift version: 5.1

Paul Hudson    @twostraws    May 28th 2019

All strings have a write(to:) method that lets you save the contents of the string to disk. You need to provide a filename to write to, plus two more parameters: whether the write should be atomic, and what string encoding to use. The second parameter should nearly always be true because it avoids concurrency problems. The third parameter should nearly always be String.Encoding.utf8, which is pretty much the standard for reading and writing text.

Be warned: writing a string to disk can throw an exception, so you need to catch any errors and warn the user.

Here's the code:

let str = "Super long string here"
let filename = getDocumentsDirectory().appendingPathComponent("output.txt")

do {
    try str.write(to: filename, atomically: true, encoding: String.Encoding.utf8)
} catch {
    // failed to write file – bad permissions, bad filename, missing permissions, or more likely it can't be converted to the encoding
}

That code uses a helper function called getDocumentsDirectory(), which finds the path to where you can save your app's files. Here it is:

func getDocumentsDirectory() -> URL {
    let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
    return paths[0]
}

Available from iOS 2.0

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