Check out example codes for "save file with % swift". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

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
}

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