I want to share some files I have locally in my app using Share Sheet functionality on iPhone. I display the file in a UIWebView and when the user clicks the sh         
        
Swift 4.2 and Swift 5
If you already have a file in a directory and want to share it, just add it's URL into activityItems:
let fileURL = NSURL(fileURLWithPath: "The path where the file you want to share is located")
// Create the Array which includes the files you want to share
var filesToShare = [Any]()
// Add the path of the file to the Array
filesToShare.append(fileURL)
// Make the activityViewContoller which shows the share-view
let activityViewController = UIActivityViewController(activityItems: filesToShare, applicationActivities: nil)
// Show the share-view
self.present(activityViewController, animated: true, completion: nil)
If you need to make the file:
I'm using this extension to make files from Data (read the comments in the code for explanation how it works):
As in the typedef's answer, get the current documents directory:
/// Get the current directory
///
/// - Returns: the Current directory in NSURL
func getDocumentsDirectory() -> NSString {
    let paths = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)
    let documentsDirectory = paths[0]
    return documentsDirectory as NSString
}
Extension for Data:
extension Data {
    /// Data into file
    ///
    /// - Parameters:
    ///   - fileName: the Name of the file you want to write
    /// - Returns: Returns the URL where the new file is located in NSURL
    func dataToFile(fileName: String) -> NSURL? {
        // Make a constant from the data
        let data = self
        // Make the file path (with the filename) where the file will be loacated after it is created
        let filePath = getDocumentsDirectory().appendingPathComponent(fileName)
        do {
            // Write the file from data into the filepath (if there will be an error, the code jumps to the catch block below)
            try data.write(to: URL(fileURLWithPath: filePath))
            // Returns the URL where the new file is located in NSURL
            return NSURL(fileURLWithPath: filePath)
        } catch {
            // Prints the localized description of the error from the do block
            print("Error writing the file: \(error.localizedDescription)")
        }
        // Returns nil if there was an error in the do-catch -block
        return nil
    }
}
Examples how to use:
Share image-files:
// Your image
let yourImage = UIImage()
in png-file
// Convert the image into png image data
let pngImageData = yourImage.pngData()
// Write the png image into a filepath and return the filepath in NSURL
let pngImageURL = pngImageData?.dataToFile(fileName: "nameOfYourImageFile.png")
// Create the Array which includes the files you want to share
var filesToShare = [Any]()
// Add the path of png image to the Array
filesToShare.append(pngImageURL!)
// Make the activityViewContoller which shows the share-view
let activityViewController = UIActivityViewController(activityItems: filesToShare, applicationActivities: nil)
// Show the share-view
self.present(activityViewController, animated: true, completion: nil)
in jpg-file
// Convert the image into jpeg image data. compressionQuality is the quality-compression ratio in % (from 0.0 (0%) to 1.0 (100%)); 1 is the best quality but have bigger filesize
let jpgImageData = yourImage.jpegData(compressionQuality: 1.0)
// Write the jpg image into a filepath and return the filepath in NSURL
let jpgImageURL = jpgImageData?.dataToFile(fileName: "nameOfYourImageFile.jpg")
// Create the Array which includes the files you want to share
var filesToShare = [Any]()
// Add the path of jpg image to the Array
filesToShare.append(jpgImageURL!)
// Make the activityViewContoller which shows the share-view
let activityViewController = UIActivityViewController(activityItems: filesToShare, applicationActivities: nil)
// Show the share-view
self.present(activityViewController, animated: true, completion: nil)
Share text-files:
// Your String including the text you want share in a file
let text = "yourText"
// Convert the String into Data
let textData = text.data(using: .utf8)
// Write the text into a filepath and return the filepath in NSURL
// Specify the file type you want the file be by changing the end of the filename (.txt, .json, .pdf...)
let textURL = textData?.dataToFile(fileName: "nameOfYourFile.txt")
// Create the Array which includes the files you want to share
var filesToShare = [Any]()
// Add the path of the text file to the Array
filesToShare.append(textURL!)
// Make the activityViewContoller which shows the share-view
let activityViewController = UIActivityViewController(activityItems: filesToShare, applicationActivities: nil)
// Show the share-view
self.present(activityViewController, animated: true, completion: nil)
Other files:
You can make a file from anything which is in Data format and as far as I know, almost everything in Swift can be converted into Data like String, Int, Double, Any...:
// the Data you want to share as a file
let data = Data()
// Write the data into a filepath and return the filepath in NSURL
// Change the file-extension to specify the filetype (.txt, .json, .pdf, .png, .jpg, .tiff...)
let fileURL = data.dataToFile(fileName: "nameOfYourFile.extension")
// Create the Array which includes the files you want to share
var filesToShare = [Any]()
// Add the path of the file to the Array
filesToShare.append(fileURL!)
// Make the activityViewContoller which shows the share-view
let activityViewController = UIActivityViewController(activityItems: filesToShare, applicationActivities: nil)
// Show the share-view
self.present(activityViewController, animated: true, completion: nil)
I want to share my solution of UIActivityViewController and sharing text as a image file. This solution works for sharing via Mail and even Save to Dropbox.
@IBAction func shareCsv(sender: AnyObject) {
    //Your CSV text
    let str = self.descriptionText.text!
    filename = getDocumentsDirectory().stringByAppendingPathComponent("file.png")
    do {
        try str.writeToFile(filename!, atomically: true, encoding: NSUTF8StringEncoding)
        let fileURL = NSURL(fileURLWithPath: filename!)
        let objectsToShare = [fileURL]
        let activityVC = UIActivityViewController(activityItems: objectsToShare, applicationActivities: nil)
        self.presentViewController(activityVC, animated: true, completion: nil)
    } catch {
        print("cannot write file")
        // failed to write file – bad permissions, bad filename, missing permissions, or more likely it can't be converted to the encoding
    }
}
func getDocumentsDirectory() -> NSString {
    let paths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)
    let documentsDirectory = paths[0]
    return documentsDirectory
}
Here's the Swift 3 version:
let dictToSave: [String: Any] = [
    "someKey": "someValue"
]
let jsonData = try JSONSerialization.data(withJSONObject: dictToSave, options: .prettyPrinted)
let filename = "\(self.getDocumentsDirectory())/filename.extension"
let fileURL = URL(fileURLWithPath: filename)
try jsonData.write(to: fileURL, options: .atomic)
let vc = UIActivityViewController(activityItems: [fileURL], applicationActivities: [])
self.present(vc, animated: true)
func getDocumentsDirectory() -> String {
    let paths = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)
    let documentsDirectory = paths[0]
    return documentsDirectory
}