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The iPhone and iPad are awesome devices with beautiful displays. Sometimes though, we might want to show parts of our app on a bigger, external screen.

Today we'll look at how we can do just that on iOS, let's dive in.

Before we begin let's break down what exactly we're talking about here:

UIKit exposes any external display we connect to our iPhone or iPad with an adapter as a UIScreen. The same is also true when we turn-on Airplay Mirroring to an Apple TV.

Now, let's write some code.

We'll eventually use Notifications to learn when a screen is connected/disconnected, but many users may launch our app with a screen already connected, so let's start by checking for that.

We'll create a couple instance properties to hold our the UIWindow and UIViewController we'll be displaying externally:

var externalDisplayWindow: UIWindow?
var externalDisplayVC: StatusBoardViewController?

Next, we'll make a function we can call when our app first launches, and when a new display is connected:

func checkForExternalDisplay() {
  guard let screen = UIScreen.screens.last
  else { return }

We'll create a new UIWindow and assign it to the external UIScreen:

  if externalDisplayWindow == nil {
    externalDisplayWindow = UIWindow(
      frame: screen.bounds
    )

    externalDisplayWindow?.screen = screen
  }

Next, we'll need a view controller. We'll create one and set it as the rootViewController just like usual:

  if externalDisplayVC == nil {
    externalDisplayVC = StatusBoardViewController()
    externalDisplayWindow?.rootViewController = externalDisplayVC

Last but not least, we'll show the window by setting its isHidden property to false.

    externalDisplayWindow?.isHidden = false
  }
}

If we connect a display now, and launch our app, we'll see our content displayed externally, neat!

Let's finish up by wiring up those notifications we mentioned earlier. First we'll start observing them:

NotificationCenter.default
  .addObserver(
    forName: NSNotification.Name.UIScreenDidConnect,
    object: nil,
    queue: nil,
    using: displayConnected
  )

NotificationCenter.default
  .addObserver(
    forName: NSNotification.Name.UIScreenDidDisconnect,
    object: nil,
    queue: nil,
    using: displayDisconnected
)

Then we'll write those two functions to handle each notification:

func displayConnected(notification: Notification) {
  checkForExternalDisplay()
}

func displayDisconnected(notification: Notification) {
  externalDisplayWindow?.isHidden = true

  externalDisplayVC = nil
  externalDisplayWindow = nil
}

Now our app will start/stop showing on the external display automatically when the user connects it. Very cool.

There's plenty more to dive into when working with external displays and UIScreen. We'll go further in future Bites.

That's all for today. Have an idea or request for a Bite? Send it along to hello@littlebitesofcocoa.com!