Share Extension to open containing app

Swift 4+ (tested on iOS 13)

@objc should be added to the declaration of openURL, that is,

@objc func openURL(_ url: URL) -> Bool {
    // Code below.
}

Without it one would see this compiler error:

Argument of '#selector' refers to instance method 'openURL' that is not exposed to Objective-C

Working solution in Swift 3.1 (tested in iOS10):

You need to create your own URL Scheme, then add this function to your ViewController and call it with openURL("myScheme://myIdentifier")

//  Function must be named exactly like this so a selector can be found by the compiler!
//  Anyway - it's another selector in another instance that would be "performed" instead.
func openURL(_ url: URL) -> Bool {
    var responder: UIResponder? = self
    while responder != nil {
        if let application = responder as? UIApplication {
            return application.perform(#selector(openURL(_:)), with: url) != nil
        }
        responder = responder?.next
    }
    return false
}

Edit: Notes for clarification: openURL is a method of UIApplication - since your ShareExtension is not derived from UIApplication I added my own openURL with the same definition as the one from UIApplication to keep the compiler happy (so that #selector(openURL(_:) can be found).

Then I go through the responders until I find one that is really derived from UIApplication and call openURL on that.

More stripped-down-example-code which copies files in a ShareExtension to a local directory, serializing filenames and calling openURL on another app:

//
//  ShareViewController.swift
//

import UIKit
import Social
import MobileCoreServices

class ShareViewController: UIViewController {

var docPath = ""

override func viewDidLoad() {
    super.viewDidLoad()

    let containerURL = FileManager().containerURL(forSecurityApplicationGroupIdentifier: "group.com.my-domain")!
    docPath = "\(containerURL.path)/share"
    
    //  Create directory if not exists
    do {
        try FileManager.default.createDirectory(atPath: docPath, withIntermediateDirectories: true, attributes: nil)
    } catch let error as NSError {
        print("Could not create the directory \(error)")
    } catch {
        fatalError()
    }

    //  removing previous stored files
    let files = try! FileManager.default.contentsOfDirectory(atPath: docPath)
    for file in files {
        try? FileManager.default.removeItem(at: URL(fileURLWithPath: "\(docPath)/\(file)"))
    }
}

override func viewDidAppear(_ animated: Bool) {

    let alertView = UIAlertController(title: "Export", message: " ", preferredStyle: .alert)
    
    self.present(alertView, animated: true, completion: {

        let group = DispatchGroup()
        
        NSLog("inputItems: \(self.extensionContext!.inputItems.count)")
        
            for item: Any in self.extensionContext!.inputItems {
                
            let inputItem = item as! NSExtensionItem
            
            for provider: Any in inputItem.attachments! {
                
                let itemProvider = provider as! NSItemProvider
                group.enter()
                itemProvider.loadItem(forTypeIdentifier: kUTTypeData as String, options: nil) { data, error in
                    if error == nil {
                        //  Note: "data" may be another type (e.g. Data or UIImage). Casting to URL may fail. Better use switch-statement for other types.
                        //  "screenshot-tool" from iOS11 will give you an UIImage here
                        let url = data as! URL
                        let path = "\(self.docPath)/\(url.pathComponents.last ?? "")"
                        print(">>> sharepath: \(String(describing: url.path))")

                        try? FileManager.default.copyItem(at: url, to: URL(fileURLWithPath: path))
                        
                    } else {
                        NSLog("\(error)")
                    }
                    group.leave()
                }
            }
        }
        
        group.notify(queue: DispatchQueue.main) {
            NSLog("done")
            
            let files = try! FileManager.default.contentsOfDirectory(atPath: self.docPath)
            
            NSLog("directory: \(files)")
            
            //  Serialize filenames, call openURL:
            do {
                let jsonData : Data = try JSONSerialization.data(
                    withJSONObject: [
                        "action" : "incoming-files"
                        ],
                    options: JSONSerialization.WritingOptions.init(rawValue: 0))
                let jsonString = (NSString(data: jsonData, encoding: String.Encoding.utf8.rawValue)! as String).addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)
                let result = self.openURL(URL(string: "myapp://com.myapp.share?\(jsonString!)")!)
            } catch {
                alertView.message = "Error: \(error.localizedDescription)"
            }
            self.dismiss(animated: false) {
                self.extensionContext!.completeRequest(returningItems: [], completionHandler: nil)
            }
        }
    })
}

//  Function must be named exactly like this so a selector can be found by the compiler!
//  Anyway - it's another selector in another instance that would be "performed" instead.
@objc func openURL(_ url: URL) -> Bool {
    var responder: UIResponder? = self
    while responder != nil {
        if let application = responder as? UIApplication {
            return application.perform(#selector(openURL(_:)), with: url) != nil
        }
        responder = responder?.next
    }
    return false
}
}

Technically you can't open containing app from share extension, but you can schedule local notification, and that's what I end up doing. Just before I call super.didSelectPost, I schedule local notification with some text, and if user wants to open containing app, they can, and if not - they can continue with their workflow. I even think its a better approach than automatically opening containing app and disrupting what they are doing.

Currently there's no way to do this. A share extension cannot open the containing app.

The intended approach for share extensions is that they handle all of the necessary work themselves. Extensions can share code with their containing apps by using custom frameworks, so in most cases that's no problem.

If you want to make data available to your app, you can set up an app group so that you have a shared directory. The extension can write data there, and the app can read it. That won't happen until the next time the user launches the app, though.