iOS8 Day-by-Day :: Day 2 :: Sharing Extension

Written by Sam Davies

Updated 9 Apr 2015: This post has been updated to Swift 1.2

This post is part of a daily series of posts introducing the most exciting new parts of iOS8 for developers – #iOS8DayByDay. To see the posts you’ve missed check out the index page, but have a read through the rest of this post first!

To enjoy each of the 39 posts all in one place, you can now also download the iOS8 Day-by-Day eBook free from our website! 


A huge new feature in iOS8 is the introduction of support for extensions. These are areas in which developers can enhance the operating system, including both 3rd-party apps and Apple-provided apps, with their own features and functionality. There are 6 extensions applicable to iOS:

  • Today screen widgets
  • Share extensions
  • Actions
  • Photo editing
  • Storage providers
  • Custom keyboards

We’ll cover some of these in later editions of iOS8 Day-by-Day, but today’s article is focused on sharing extensions.

Sharing extensions give you, the developer of an app, the ability to show an icon on the common share-sheet, and then to handle the sharing of the content the user has requested. This means that you can supplement the list of possibilities present in the OS (e.g. Twitter, Flickr, Sina Weibo).

A word of warning: this topic is not an easy one. From the mere nature of an extension it is pretty complex. The article will take you through some of the most common use cases, but be aware that you can pretty much build your own visual appearance. Apple has some excellent resources in this area, so it’s worth having a read through them if you get stuck with anything.

The sample app for today’s post is called “ShareAlike”, and demonstrates building a sharing extension which allows sharing an image and some text. The code is available on Github at

Creating a share extension

Extensions are compiled binaries which have to be contained within an app – it’s not possible to add an extension to the system without an accompanying app. Xcode has a template for adding an extension to an existing app – it’ll set up a new module within your project and provide you with the required files to get started.

The major part of a sharing extension is its visual appearance, and as such you’re provided with a subclass of SLComposeServiceViewController and a storyboard. The default appearance of SLComposeServiceViewController gives you a lot of sensible behaviour (including a character count, image display, text entry, post and cancel buttons) and fits in with the iOS UI. In this example we’re going to stick with this default behaviour.

In addition to the standard UIViewController methods, SLComposeServiceViewController has some methods and properties associated with the lifecycle of a share-sheet composition view:

  • presentationAnimationDidFinish() this provides a great hook to perform any heavy-lifting tasks. We’ll use this to extract the image to share.
  • contentText a string which represents what the user has typed into the composer
  • didSelectPost() called when the Post button is tapped. The point to kick off the upload task.
  • didSelectCancel() called when the Cancel button is tapped.
  • isContentValid() called every time the content in the compose view changes.
  • charactersRemaining is a number which appears on the compose sheet. When negative its appearance becomes red

Simply adding an extension to your app’s project will allow it to be selected when a user requests to share something. We’ll take a look at implementing some useful functionality soon, but first, let’s learn a little about how to build, run and debug.

Building, running and debugging

In theory, you should be able to select the scheme associated with the extension and hit run. You’ll then be asked which host app you’d like to debug it in, and you’ll be away. This experience can be more than a little flakey. The only apps it is possible to choose (as of Xcode 6b3) are your own development apps – not system apps. This seems a little strange – the perfect app for testing sharing would be the photos app.

However, the following approach seems to be reliable:

  1. Either use your extension’s host app, or add a new app to the same project which has easily available content for sharing. In the sample project, the ShareAlike host app has an image and a share button which will trigger a standard UI share sheet: 

    @IBAction func handleShareSampleTapped(sender: AnyObject) {   shareContent(text: "Highland Cow",                        image: sharingContentImageView.image) }  // Utility methods func shareContent(#text: String?, image: UIImage?) {   var itemsToShare = [AnyObject]()     if let text = text {     itemsToShare.append(text)   }   if let image = image {     itemsToShare.append(image)   }     let activityViewController = UIActivityViewController(activityItems: itemsToShare, applicationActivities: nil)   presentViewController(activityViewController, animated: true, completion: nil) }


  2. Develop your extension
  3. To debug and or test, run the scheme associated with your app. Then when you hit share then the debugger will automatically attach to the process associated with the extension.
  4. You can use the sharing extension from other apps on the simulator, but you won’t have debugging access. It’s good to get it to a working state and then test it within the Photos app.

Specifying what can be shared

In the same way that the settings associated with an app are contained within an Info.plist file, there is an equivalent file for the extension module. One of the things you can control is the name of the extension which appears under the icon in the share sheet.

The name is defined by the Bundle display name (CFBundleDisplayName):

Bundle Display Name

You can also use the Info.plist file to define under which circumstances your share extension is applicable – e.g. can it handle videos? There is a very powerful string-predicate language you can use here, but for 99% of cases, using a dictionary will suffice.

Inside the NSExtension dictionary, there is a NSExtensionAttributes dictionary, one of the values of which is NSExtenionActivationRule. This can be a boolean, a string or a dictionary. The following shows a dictionary with settings which will enable a single image to be shared, disables videos, files and URLs

Activation Rules

The different dictionary keys are pretty self-explanatory, and the result can be seen here; selecting one image to share shows ShareAlike as an option, whereas two images does not:

Share Single Image Share Two Images

Validating user input

Now that you’ve got an understanding of how to create extensions and control them, let’s take a look at implementing some custom behaviour. First up is how you can validate input provided by the user. One of the most common things you might want to do is to limit the number of characters in text string that the user enters, and SLComposeServiceViewController has create support for this.

As was mentioned in the in the overview of sharing extensions, there is a method called isContentValid() -> Bool which will get called every time that the content of the text field changes . Returning true states that you’re happy with the current content, and therefore the Post button should be enabled. The button will be disabled if you return false. The following implementation will perform a simple character limit:

let sc_maxCharactersAllowed = 25

override func isContentValid() -> Bool {
  if let currentMessage = contentText {
    let currentMessageLength = count(currentMessage)
    charactersRemaining = sc_maxCharactersAllowed - currentMessageLength

    if Int(charactersRemaining) < 0 {
      return false
  return true

The contentText property is a String which contains the current content of the compose view. Here you’re finding it’s length using count() (note that you can’t ask a String for its length. This approach is always O(N)). charactersRemaining is a numeric property, which is represented on the UI. You then determine whether you have reached the limit or not, and enable/disable the Post button appropriately.

Acceptable number of characters Too many characters

Uploading from within an extension

So far you’ve learnt about how to create an extension, how to configure it and how to control some of the user experience, but the main purpose of a share extension is to upload the content to some kind of network end-point. Let’s take a look at that.

Extensions are meant to be much more lightweight than full-blown apps, so you need to keep resource usage down. You can think of an extension as being invited by the host app to perform some simple operation – it would be rude to suddenly attempt to steal the limited system resources from your host. Therefore all uploading needs to happen as a background process (helpfully introduced in NSURLSession in iOS7). You’re probably thinking that this’ll be easy – you can just refer to the relevant chapter in iOS7 Day-by-Day and you’ll be done. Well, that’s not quite true.

There are a couple of complications with this. First of all, it’s not trivial to extract the content (i.e. image) you’ve been asked to share, and secondly, an extension doesn’t get provided any writable disc access. This might seem a little strange – why would an extension need disc access? Well, it’s all to do with the background network process – at the point it is called, it will cache the data on disc and then start the upload in the background. In order that this can happen, you need to create a shared container within your host app, and allow the extension to use it for caching. We’ll take a look at how to do this in a minute, but first, let’s take a look at how to get hold of the image.

Extracting an image to upload

There is a property on SLComposeServiceViewController called extensionContext, and this contains all the data that’s associated with this instance of the extension, including an array of NSInputItem objects, called inputItems. An NSInputItem has a collection of attachments, each of type NSItemProvider. Each of these attachments represents an item of media – such as an image, a video, a file or a link.

func imageFromExtensionItem(extensionItem: NSExtensionItem, callback: (image: UIImage?)->Void) {
  for attachment in extensionItem.attachments as! [NSItemProvider] {

This is a function which is going to extract a UIImage from an extension item. Notice that this function does not have a return type, but instead returns the result via a callback closure.

In order to determine whether an attachment contains a media of a particular type, you need to use the hasItemConformingToTypeIdentifier() method.

if(attachment.hasItemConformingToTypeIdentifier(kUTTypeImage as String)) {

The type identifier is a string and is part of the framework. In order to get it imported, you’ll need to import the MobileCoreServices module at the top of your Swift file:

import MobileCoreServices

There are many type identifiers defined, from incredibly general, to very specific, including the following:

  • kUTTypeImage
  • kUTTypeMovie
  • kUTTypeAudio
  • kUTTypeSpreadsheet
  • etc

Now that you’re sure you have an attachment which includes an image, you need to extract it. Since this could be an expensive process, it should be performed on a background queue to ensure that the UI doesn’t hang. This uses the loadItemForTypeIdentifier() method, which takes the same type as before, and a closure to which the result will be delivered:

// Marshal on to a background thread
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
  attachment.loadItemForTypeIdentifier(kUTTypeImage as String, options: nil) {
      (imageProvider, error) in

If you’re using objective-C then you can use the types of the block parameters to coerce the result into the type you expect (i.e. into UIImage), but this isn’t possible using Swift. Therefore, the imageProvider variable will be of type NSSecureCoding. You can then cast this to NSData, and create an image from this:

var image: UIImage? = nil
if let error = error {
  println("Item loading error: \(error.localizedDescription)")
image = imageProvider as? UIImage dispatch_async(dispatch_get_main_queue()) { callback(image: image) }

Notice that at the end of this, there is a callback() with the created image object.

This process of extracting the image from the attachments can be kicked off whilst the user is entering their text – after the view has finished animating into view. There is a perfect method to for this:

var attachedImage: UIImage?

override func presentationAnimationDidFinish() {
  // Only interested in the first item
  let extensionItem = extensionContext?.inputItems[0] as! NSExtensionItem
  // Extract an image (if one exists)
  imageFromExtensionItem(extensionItem) {
    image in
    if image {
      dispatch_async(dispatch_get_main_queue()) {
        self.attachedImage = image

This method asks for the image to be extracted, and in the callback it saves it off into the instance property attachedImage.

Performing a background upload

Once the user has completed their entry, and clicks the Post button, then the extension should upload the content to some web service somewhere. For the purposes of this example, the URL of the endpoint is contained within a property on the view controller:

let sc_uploadURL = ""

This is a URL for the Request Bin service, which gives you a temporary URL to allow you to test network operations. The above URL (and the one in the sample code) won’t work for you, but if you visit then you can get hold of your own URL for testing.

As mentioned previously, it’s important that extensions put very little strain on the limited system resources. Therefore, at the point the Post button is tapped, there is no time to perform a synchronous, foreground network operation. Luckily, NSURLSession provides a simple API for creating background network operations, and that’s what you’ll need here.

The method which gets called when the user taps post is didSelectPost(), and in its simplest form it must look like this:

override func didSelectPost() {
  // Perform upload

  // Inform the host that we're done, so it un-blocks its UI.
  extensionContext?.completeRequestReturningItems(nil, completionHandler: nil)

Setting up an NSURLSession is pretty standard:

let configName = "com.shinobicontrols.ShareAlike.BackgroundSessionConfig"
let sessionConfig = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier(configName)
// Extensions aren't allowed their own cache disk space. Need to share with application
sessionConfig.sharedContainerIdentifier = "group.ShareAlike"
let session = NSURLSession(configuration: sessionConfig)

The important part to note of the above code segment is the line which sets the sharedContainerIdentifier on the session configuration. This specifies the name of the container that NSURLSession can use as a cache (since extensions don’t have their own writable disc access). This container needs to be set up as part of the host application (i.e. ShareAlike in this demo), and can be done through Xcode:

  1. Go to the capabilities tab of the app’s target
  2. Enable App Groups
  3. Create a new app group, entitled something appropriate. It must start with group.. In the demo the group is called group.ShareAlike
  4. Let Xcode go through the process of creating this group for you.

App group for host app

Then you need to go to the extension’s target, and follow the same process. Note that you won’t need to create a new app group, but instead select the one that you created for your host application.

App group for extension

These app groups are registered against your developer ID, and the signing process ensures that only your apps are able to access these shared containers.

Xcode will have created an entitlements file for each of your projects, and this will contain the name of the shared container it has access to.

Now that you’ve got your session set up correctly, you need to create a URL request to perform:

// Prepare the URL Request
let request = urlRequestWithImage(attachedImage, text: contentText)

This calls a method which constructs a URL request which uses HTTP POST to send some JSON, which includes the string content, and some metadata properties about the image:

func urlRequestWithImage(image: UIImage?, text: String) -> NSURLRequest? {
  let url = NSURL.URLWithString(sc_uploadURL)
  let request = NSMutableURLRequest(URL: url)
  request.addValue("application/json", forHTTPHeaderField: "Content-Type")
  request.addValue("application/json", forHTTPHeaderField: "Accept")
  request.HTTPMethod = "POST"

  var jsonObject = NSMutableDictionary()
  jsonObject["text"] = text
  if let image = image {
    jsonObject["image_details"] = extractDetailsFromImage(image)

  // Create the JSON payload
  var jsonError: NSError?
  let jsonData = NSJSONSerialization.dataWithJSONObject(jsonObject, options: nil, error: &jsonError)
  if jsonData {
    request.HTTPBody = jsonData
  } else {
    if let error = jsonError {
      println("JSON Error: \(error.localizedDescription)")

  return request

This method doesn’t actually create a request which uploads the image, although it could be adapted to do so. Instead, it extracts some details about the image using the following method:

func extractDetailsFromImage(image: UIImage) -> NSDictionary {
  var resultDict = [String : AnyObject]()
  resultDict["height"] = image.size.height
  resultDict["width"] = image.size.width
  resultDict["orientation"] = image.imageOrientation.toRaw()
  resultDict["scale"] = image.scale
  resultDict["description"] = image.description
  return resultDict

Finally, you can ask the session to create a task associated with the request you’ve built, and then call resume() on it to kick it off in the background:

// Create the task, and kick it off
let task = session.dataTaskWithRequest(request!)

If you run through this process now, with your own URL in place, then you can expect to see results like this:

Request Bin Success


Sharing extensions are just one of the extensions available to developers in iOS8, and represents Apple opening up the operating system in a way they’ve been asked to do for a while. Interestingly, it’s done in a way that has prioritized security and privacy, arguably at a small cost of customisability.

Building sharing extensions is far from trivial, and along the route there are many things that can trip you up. However, if it’s applicable to you and your app then it’s definitely worth investing the time in. The API is good, and the gain to your users could be huge.

The code for this project is available on github at In order to get the uploading working, you will need to ensure that you’ve got a shared container configured correctly, although the project file should do most of that for you.

Look out for future articles in the iOS8 Day-by-Day series which will investigate some of the other new extensions available in iOS8.

Any questions / comments / complaints then drop me a tweet – I’m @iwantmyrealname.