Open with file in swift

For loading into your own app it depends on which class you're using to display content on the exact code you'd use but for opening in another app you'd normally use a share button. Here is example code that will work if you wire up the @IBAction and @IBOutlet to the same bar button in your UI (and place a file at the fileURL that you specify):

import UIKit
class ViewController: UIViewController {

    // UIDocumentInteractionController instance is a class property
    var docController:UIDocumentInteractionController!
    @IBOutlet weak var shareButton: UIBarButtonItem!
    // called when bar button item is pressed
    @IBAction func shareDoc(sender: AnyObject) {
        // present UIDocumentInteractionController
        if let barButton = sender as? UIBarButtonItem {
            docController.presentOptionsMenuFromBarButtonItem(barButton, animated: true)
        }
        else {
            print("Wrong button type, check that it is a UIBarButton")
        }

    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // retrieve URL to file in main bundle
        if let fileURL = NSBundle.mainBundle().URLForResource("MyImage", withExtension: "jpg") {
            // Instantiate the interaction controller
            self.docController = UIDocumentInteractionController(URL: fileURL)
        }
        else {
            shareButton.enabled = false
            print("File missing! Button has been disabled")
        }
    }

}

Notes

A UIDocumentInteractionController is used to enable the sharing of documents between your app and other apps installed on a user's device. It is simple to set up as long as you remember three rules:

1. Always make the UIDocumentInteractionController instance a class (type) property. If you only retain a reference to the controller for the life of the method that is triggered by the button press your app will crash.

2. Configure the UIDocumentInteractionController before the button calling the method is pressed so that there is not a wait in which the app is waiting for the popover to appear. This is important because while the presentation of the controller happens asynchronously, the instantiation does not. And you may find that there is a noticeable delay to open the popover if you throw all the code for instantiation and presentation inside a single method called on the press of a button. (When testing you might see a delay anyway because the share button is likely going to be pressed almost straightaway but in real world use there should be more time for the controller to prepare itself and so the possibility of lag is less likely.)

3. The third rule is that you must test this on a real device not in the simulator.

More can be found in my blogpost on the subject.

Edit: Using a UIActivityViewController

Code for using UIActivityViewController instead of UIDocumentInteractionController

import UIKit
class ViewController: UIViewController {

    // UIDocumentInteractionController instance is a class property
    var activityController: UIActivityViewController!
    @IBOutlet weak var shareButton: UIBarButtonItem!
    // called when bar button item is pressed
    @IBAction func shareStuff(sender: AnyObject) {
        if let barButton = sender as? UIBarButtonItem {
            self.presentViewController(activityController, animated: true, completion: nil)
            let presCon = activityController.popoverPresentationController
            presCon?.barButtonItem = barButton
        }
        else {
            print("not a bar button!")
        }

    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // retrieve URL to file in main bundle
       if let img = UIImage(named:"MyImage.jpg") {
        // Instantiate the interaction controller
        activityController = UIActivityViewController(activityItems: [img], applicationActivities: nil)
        }
        else {
            shareButton.enabled = false
            print("file missing!")
        }
    }

}

You can also add custom activities to the UIActivityViewController and here is code for adding an "Open In..." button to a UIActivityViewController so that you can switch to a UIDocumentInteractionController from a UIActivityViewController.