Introducing AirPlay 2

Session 509 WWDC 2017

AirPlay allows you to wirelessly stream content from your iOS device or Mac. Now with AirPlay 2, you can fill the home with multi-room audio. Learn how AirPlay 2 synchronizes playback and provides an even more reliable audio experience, and how to incorporate support for AirPlay 2 into your app.

Hello, good afternoon and welcome to session 509, Introducing AirPlay 2 Unlocking Multi-Room Audio.

My name is David Saracino, I'm a member of the AirPlay engineering team, and I'm very excited to be here this afternoon to talk to you about the many new features we've added to audio to AirPlay over the last year, and how you can adopt them in your app.

But before we get into that adoption and how you can build this into your app, let's first take a quick survey of AirPlay today, and where we're taking it with AirPlay 2.

So, with AirPlay today, you can wirelessly send your myriad screen, audio, or video content from just about any Apple device to an Apple TV or AirPlay speaker.

Over the past year, we've added so many features to the audio domain that that's where we're going to spend the majority of our talk.

And because we've added so many features to the audio domain we're introducing this as a new feature called AirPlay 2.

So, what is AirPlay 2?

Well with AirPlay 2, you can still wirelessly send the audio content from your app to an AirPlay speaker.

But additionally, with AirPlay 2, you can send that content to multiple AirPlay 2 speakers with very tight sync.

Additionally, in AirPlay 2, we've enhanced the audio buffering on the AirPlay 2 speakers so that your content can play with the high degree of robustness, reliability, and responsiveness.

And lastly, we're introducing multi-device control.

And this will allow multiple Apple devices in your home to interact with the audio that's being streamed throughout your house.

So, where is AirPlay 2 supported?

Well I'm happy to say if you have iOS, tvOS, or macOS app, you can take the steps that I'm about to outline today and build them into your app to take advantage of AirPlay 2.

And when you do, your app will be able to play on a very wide ecosystem of AirPlay 2 speakers, including the HomePod, latest generation Apple TV, and third-party AirPlay 2 speakers that will soon be coming to market.

So, that's the high-level overview of AirPlay and AirPlay 2.

Let's look at our agenda for the rest of the talk.

First, we're going to talk about basic AirPlay 2 adoption.

The steps that you need to do take in order to get AirPlay 2 inside your app.

Next, we're going to focus on some advanced playback scenarios that you may encounter as you adopt AirPlay 2.

And lastly, we're going to talk about the availability of AirPlay 2.

So, let's begin our talk about AirPlay 2 adoption.

So, in order to adopt AirPlay 2 in your app, there are basically four steps you need to take.

First, you should identify your app as presenting long-form audio.

I'll explain what long-form audio is in a minute.

Next, you should add an AirPlay picker inside your app.

And third, you should integrate with certain parts of the MediaPlayer framework.

And lastly, you should adopt one of the playback APIs that are specifically designed to take advantage of the enhanced buffering in AirPlay 2.

So, let's walk through each of these individually.

First, identifying yourself as presenting long-form audio content.

So, what is long-form audio content?

Well long-form audio is anything like, is content like music, podcasts, or audiobooks, as distinct from things like system sounds on a Mac.

Now, identifying yourself as presenting long-form audio content is very easy.

You simply set your application's AVAudioSession's route sharing policy to be long-form.

Now here's a code snippet of how you do that on iOS.

This is something that many of you iOS developers are likely familiar with setting your category and mode, and there's just a new parameter here which is routeSharingPolicy: .longform.

As I said, this has been around on iOS for a little while.

This class is actually, AVAudioSession is new altogether on macOS, and I'm not going to show a code snippet, but it's even simpler than what you have to do on iOS.

You simply set the route sharing policy to long-form.

And for more information on this new route sharing policy and what it can do for your app in regards to the rest of the system, for example allow the user to take a phone call while you're app is AirPlay, using AirPlay 2 to stream to speakers, I encourage you to take a look at this session from earlier in the week called What's New in Audio.

All right, so that's the first step you need to do to take advantage of AirPlay 2.

The second is to add an AirPlay picker inside your app.

This will allow users to route their content to AirPlay speakers without leaving the confines of your app.

And doing this is very easy.

You simply adopt a new API called AVKit's AVRoutePickerView.

You simply add this view to your view hierarchy.

And you may want to control when this is shown, for example only show it if there actually is an AirPlay 2 speaker or an AirPlay speaker available to be routed to.

And you can do that by adopting AVFoundation's AVRouteDetector which will tell you if there are speakers, if there are routes available.

These API, they're new in the latest OS releases and they're available on macOS, iOS, and tvOS.

And a note to all of you iOS developers who are currently using MPVolumeView, it will still work, but we are encouraging people to move to these API instead.

So, that's the second thing you need to do to get your app up and running with AirPlay 2.

The third thing you should do is to integrate your app with certain parts of the MediaPlayer framework.

Now this is rather large, so there's basically two things that I'm specifically focusing on today.

And these are things that will, these API will allow you to do things like display the currently playing album art on the lock screen or an Apple TV it's being AirPlayed to.

And additionally, they will allow you to receive playback commands and pause commands from other devices on the network or from other accessories like headphones.

And the two pieces of API that we need, we are asking you to adopt.

The first is MPRemoteCommandCenter and this will allow you to receive those remote commands, and the second is MPNowPlayingInfoCenter which will allow you to inform the system metadata about the currently playing track.

All right, so that's the third thing you need to do to get your app up and running with AirPlay 2.

The fourth this is you should adopt one of the playback APIs that are specifically designed to take advantage of enhanced buffering in AirPlay 2.

And to motivate this conversation, let's take a look a little bit deeper at AirPlay audio and the buffer levels beginning with how AirPlay works today.

So, AirPlay today is effectively a real-time stream of audio to which the speakers adds just a few seconds of buffering before playing out in real-time.

Now, this works very well to a single speaker, and it works pretty well with lots of content.

But, if we are able to restrict ourselves on only focusing on long-form content, like music, podcasts or audiobooks, we can probably do better.

So, let's talk about the enhanced buffering in AirPlay 2.

So, what am I talking about with enhanced buffering in AirPlay 2?

Well, as the name implies, we've added very large buffer capacity on the AirPlay 2 speakers.

Here I want you to think minutes, not seconds.

Additionally, we're able to stream the audio content from your app to the speaker faster-than-real-time.

And I think the benefits of this should be clear.

The large buffering on the AirPlay speakers will add a large degree of robustness to the AirPlay session.

You should be able to survive more typical network glitches, like walking, taking the trash out, or walking to the dead spot of the house, or even microwaving some popcorn.

Additionally, this should provide more responsive playback experience.

The real-time nature of existing AirPlay means that there's a fixed output latency that is related to the buffer level on the AirPlay speaker.

So, when you hit play, with AirPlay 2 it should play a lot faster.

When you hit skip, it should skip a lot faster.

This will add a great, this is a great user experience for your users which is why we're really excited to talk about the enhanced buffering.

And so in order to take advantage of the advanced buffering, you should adopt one of a few API.

The first, is AVPlayer and AVQueuePlayer.

The AVPlayer's set of playback API.

These have been around for quite a while and they're your easiest route to AirPlay 2 in the enhanced buffering.

The second API, set of API are new API that we're introducing today which are AVSampleBufferAudio Renderer and AVSampleBufferRender Synchronizer.

These will prevent, present more flexibility to apps that need it.

So, let's talk about these individually by first surveying AVPlayer and AVQueuePlayer.

So, as I've said these have been around for quite a while, since iOS 4 on the iOS platforms.

And because of that there's a ton of documentation on the developer website and a lot of sample code there.

So, if you want a lot of detail on these, I encourage you to look to the developer website, because I'm just going to give a very high-level overview of these API today.

So, what I'm going to do, is I'm going to walk through what it takes to build an app with this API.

So, here you have your Client App, and if you want to build an app with an AVPlayer or AVQueuePlayer, the first thing you're going to do is your going instantiate one of the objects.

Here I'm using an AVQueuePlayer but the steps are exactly the same if you use AVPlayer.

Next, you're going to take a URL that points to the content that you want to play, and that content can be local or it can be in the cloud, it can be remote.

You're going to take that URL, and you're going to wrap it in an AVAsset and you're going to wrap that AVAsset in an AVPlayerItem.

Next, you're going to give that AVPlayerItem to the AVQueuePlayer.

Now after you've handed it over, you're ready to initiate playback.

And you do so but simply setting the rate of 1 on the AVQueuePlayer, which in response will begin downloading the audio data from wherever it resides, and then playing it out the speakers.

So, this is the quick survey of AVPlayer and AVQueuePlayer, I should also note that this works really well for video content.

So, if you want to play some video content, you can do it pretty easily with AVPlayer and AVQueuePlayer.

So, just to reiterate, your easiest way to AirPlay 2 is using these API.

But we recognize that this won't work for everybody.

There's a certain class of audio playing apps that either require, that want to do their own IO possibly.

Possibly their own DRM or even preprocessing on the media data before it's rendered out.

They need more flexibility than the, than what these API provide.

And for these developers we're introducing these new classes, AVSampleBufferAudio Renderer and AVSampleBufferRender Synchronizer.

Now with these APIs when you use them to do playback, your app has additional responsibilities.

So, first of all your app is responsible for sourcing and parsing the content.

You need to go get it wherever it is, download it, read it off disc, whatever.

And then your app has to parse it and feed the raw audio data, the raw audio buffers to the API for rendering.

This is kind of similar to AudioQueue but it works better with the deeper buffers that we need for buffered AirPlay, for enhanced AirPlay.

Because these are new, we're going to spend the remainder of this session talking about these.

All right, so let's walk through a simple block diagram of how you might build an app using these API.

So, again use your Client App, and the first thing you're going to do when building an app with these is you're going to instantiate an AVSampleBufferRender Synchronizer and AVSampleBufferAudio Renderer.

Now of these classes, the AudioRenderer class is the one that is responsible for rendering the audio, and the Synchronizer class is the one that's responsible for establishing the media timeline.

Now, there's good reason, you may be asking yourself, why are they two classes, why didn't you roll it into one?

Well, I assure you there's a good reason.

We'll talk about that later.

But never the less, AudioRenderer it does the rendering of the audio.

The Synchronizer establishes the media timeline.

So, after you've instantiated these, you add the AudioRenderer to the Synchronizer.

This tells the AudioRenderer to follow the media timeline established by the Synchronizer.

Now, as you begin working with an AVSampleBufferAudio Renderer, one of the things it's going to do is it's going to tell you when it wants more media data.

And when it tells you that it wants more media data, in response, you should feed it.

So, give it some audio data.

And after you've done that, you can begin playback by setting the rate of 1 on the Synchronizer.

And after you've set the rate of 1 on the Synchronizer, audio data will begin flowing out of the AudioRenderer.

So, that's the basic high-level overview of how you build a playback engine, with AVSampleBufferAudio Renderer and AVSampleBufferRender Synchronizer.

Now I'd like to invite my colleague Adam Sonnanstine up on stage to give a demo of this.

[ Applause ]

Thank you, David.

I'm very happy to be here today to demonstrate AirPlay 2 and its enhanced reliability and robustness.

I'm going to do that using this sample application that we've developed.

And this application will be available as sample code shortly after the conference ends.

And we also have an Apple TV on stage so I can demonstrate AirPlaying from the phone to the Apple TV both with current AirPlay and with AirPlay 2.

So let's take a look at the app.

You can see it's just a simple sort of stripped down music player interface.

We have this nice AirPlay or sorry Control Center integration because I've adopted the MediaPlayer APIs that David mentioned before.

So, I'm just going to go ahead and start playing.

And so you can hear it, I'll start AirPlaying to my Apple TV.

[music] Now this app has not yet been optimized for AirPlay 2 so I just want to give you a base line of what the performance is with the current AirPlay.

And I'm going to do that, I want you to imagine that you are enjoying this music sitting on my couch in my living room, the music being streamed from my phone to my Apple TV and I decide that it's a good time to walk outside with the phone in my pocket to take out the trash, maybe to, walking right outside of my Wi-Fi range, and I'm going to simulate that by using this bag, which shields everything I put inside it from electromagnetic radiation going in and from coming out, and that includes Wi-Fi.

So, I'll take my phone, and here I am walking outside and you can hear that almost immediately the music cuts out.

You've probably experienced something like this before and if you're actually sitting in my living room, listening to music you're probably going to think I'm not a very good host.

So, let's go back and I'll go ahead and pause the music.

And we'll switch into Xcode here and I'll show you how you can do a couple of simple steps to opt your app, this app into AirPlay 2.

So, here we are in Xcode and I have just a very small snippet from my application here, the rest of it's implemented in other files.

And I just have a function that the rest of the app is using to grab an object that it can use to actually do the audio playback.

It's basically a factory function.

And this can return an object that conforms to a protocol that I've defined elsewhere in the app.

We don't need to look at the details, but it defines methods for things like play and pause and the sort of basic playback operations that a player should do.

And we have an existing implementation of this protocol which is being used in the demo that we just saw.

And what we're going to do right now is I'm going to create a brand-new implementation of this audio player, and I'm going to call it SampleBufferAudioPlayer because it's going to be built using AVSampleBufferAudio Renderer.

I'm just going to preemptively swap that in so that the next time we launch the app, it will use the new implementation instead of the old implementation.

So, as I mentioned, this class is going to be built on top of AVSampleBufferAudio Renderer, and we're also going to need to use AVSampleBufferRender Synchronizer.

These are the classes that David just introduced to you.

And I just want to reiterate what David said that AVPlayer would give you all of the benefits that I'm about to show you, but with a little bit less work on your part.

So, if you're already using AVPlayer, or you think you can use AVPlayer, we recommend that you do that.

For the rest of you, I want to show you how to use these new classes.

So, once I have my state here, I'm going to hook up my AudioRenderer with my RenderSynchronizer using the addRenderer method, and I'll do that as soon as my SampleBufferAudioPlayer is created.

And then we need to interact with the renderer, and we're going to do that by calling the requestMediaDataWhenReady method.

And what this is going to do is it's going to call a closure in my app so that I can feed the AudioRenderer some more audio data whenever the AudioRenderer decides that it's ready to receive more data.

And it will call this closure as often as it needs to to stay full of audio data.

Now within my closure, I'm actually going to loop so that I'm going to keep appending more data as long as the AudioRenderer is still ready for more media data.

Within my loop, the first thing I need to do is grab my next chunk of audio data and package it as a CMSampleBuffer.

Now this method here is my app's own logic for doing this, for grabbing its next little bit of audio data.

Your app will have your own version of this logic, maybe pulling the data off the network or decrypting it from disc.

If you want to see the details of my version, once again, check out the sample code.

And I have this set up to return an optional CMSampleBuffer and that's just so I can use a nil return value to signal that I've reached the end of data.

Once I have my sample buffer, I turn around and enqueue it into the AudioRenderer using the enqueue method.

And what this will do is it will hand the audio data off to the renderer so that it can schedule it to be played at the appropriate time.

As I mentioned, I'm using a nil sample buffer to signal that I don't have any more data.

And when that happens I need to explicitly tell the AudioRenderer to stop requesting more media data.

If I don't do this, and I exit my closure, that AudioRenderer is just going to invoke the closure right away, when it's ready for data again, but since I don't have anything more to give it, I don't want that to happen.

So, I need to tell it to stop.

And this function here is pretty much your entire interaction with the AudioRenderer for a simple use case like this.

Now, we need to do a couple things with the RenderSynchronizer.

I have a play method, and all that play really is, is setting the RenderSynchronizer's rate to 1, which will start playback.

I have a little bit of prep work I need to do there, which we won't look at the details, but it does end up calling this start enqueueing method so you can sort of get a feel for how things flow here.

And I have a little bit of UI updating to do every time I play or pause, so this method will dispatch back to the main queue in order to keep the UI up to date.

Similarly, I have a pause method, and the only difference here is that it sets the renderSynchronizer's rate to 0.

So, now you've seen all of the interaction that we're going to have with AVFoundation.

I have one more bit of code I need to drop in, just to make sure my app will build and work correctly, which we won't look at the details of, but I'll just drop it there.

And then, we will rebuild the application.

And I seem to have some compilation errors.

So, what I'm going to do is figure out what I did wrong.

Well, why don't we just take this and, ah I think I know what's wrong here.

Just drag that over here, and go back here and we'll try again.

All right, so we got our build ready.

And now we're relaunching the app.

So, we'll switch back to our side by side view.

And once the app finishes launching, I'm going to go ahead and restart the music playback.

Here we go.

And once the music starts playing [music] now we're playing using AirPlay 2 from the phone to the Apple TV.

So, now I want to run the same scenario that I did before using our bag again.

So, let's grab the phone.

And we'll put it in the bag.

Once again, here's me taking the phone outside, outside of Wi-Fi range.

I'll fold up the bag nice and tight.

And as you can see, where as in the first demo, the music cut out almost immediately, now that we're using AirPlay 2, the music can keep playing even through these sorts of minor Wi-Fi interruptions.

So, that's the power of AirPlay 2 using AVSampleBufferAudio Renderer.

Thank you very much and back to David.

[ Applause ]

Thank you, Adam.

So, now that we've shown, walked you through the steps to create a simple app with AVSampleBufferAudio Renderer and AVSampleBufferRender Synchronizer, let's move on to some more advanced playback scenarios that you may be, you may encounter when using these API.

So, what we're going to talk about in this section is audio buffer levels of AVSampleBufferAudio Renderer.

We're going to talk to about how to implement a seek, how to implement play queues, some of the support audio formats with AVSampleBufferAudio Renderer, and lastly, we're going to take a slight detour and talk about video synchronization.

So, let's jump into the next section and talk about the audio buffer levels of AVSampleBufferAudio Renderer.

So, what am I talking about?

I'm talking about the fact that the amount of audio data request by an AVSampleBufferAudio Renderer of your app will vary depending on the current route.

So, let's take a look at this pictorially.

Here I've drawn a media timeline and I'm going to drop in the playhead.

And when you're playing back locally, the AudioRenderer is only going to ask for a few seconds ahead of the playhead.

So, that is where the, that is where you should enqueue to, as much as it asks for.

And as you play, again you're just going to enqueue a few seconds ahead of the app.

But suppose your user suddenly decides to route to an AirPlay 2 speaker.

Now when that happens, the AVSampleBufferAudio Renderer is going to ask for up to multiple minutes ahead of the playhead.

And once again, when you hit play, you'll work multiple minutes ahead of the playhead.

Now the key here, is that the amount of data requested by AVSampleBufferAudio Renderer varies depending on where the audio is currently routed.

If it's routed locally, this will just be seconds, to Bluetooth seconds, to an AirPlay 1 speaker seconds.

But as soon as the user routes your audio content to an AirPlay 2 speaker, that AudioRenderer is going to get really hungry and it's going to ask for multiple minutes of audio data.

And the key here is that your app should be ready to handle these changes.

All right, next let's talk about seek.

So, what is a seek?

Well a seek very quick, very simply, is manually changing the location of the playhead.

So, again let's draw out our media timeline that we just saw a moment ago.

Put in the playhead, and talk about local play or talk about standard playback scenario.

User hits play, and then after it plays for a little bit the user decides you know what, I want to seek further into the track.

So, they pick up the playhead and they drag it.

So, they've asked for a seek.

So, how do we handle this with AVSampleBufferAudio Renderer?

Well it's actually really easy.

The first thing that you're going to do is you're going to stop.

You're going to stop playback of the AVSampleBufferRender Synchronizer and you're going to stop enqueueing media data into the AVSampleBufferAudio Renderer.

Next, you're going to issue a flush on the SampleBufferAudioRenderer which will clear out all media data that's already been enqueued.

And at that point, you're free to begin audio, re-enqueueing audio data at any media time, so you do it beginning at the seek to time.

And after you've enqueued media data the seek to time, kick off playback once again and there you go.

So, that's a seek.

So, now that I've shown you how seek works, let's take a look at how you would implement that in code, by extending Adam's app with a method called seek(toMediaTime.

So, how do we implement this?

Well it's pretty easy.

The first thing we're going to do is we're going to tell the renderSynchronizer to stop playback by setting the rate to 0.

And then we're going to tell the AudioRenderer to stop requesting media data.

Next, we're going flush the AudioRenderer to clear out all the old audio data we've enqueued.

And then we're going to call some app specific code, that tells your sample generating code that the next sample that it should prepare is at that seek to time.

Remember your app is responsible for producing the audio data when using this API, so you need to instruct it where the next audio sample should be generated from.

And after that happens, you can reinstall your closure on the AudioRenderer to tell it to call you back for more audio data.

And you can set a rate to 1 on the RenderSynchronizer.

Pretty simple.

So, that's seek.

Let's get into something more interesting which is play queues.

And what is a play queue?

Well here we have a screenshot from Adam's app.

And a play queue is very simple is, you know, you order a set of items.

I hit play on one and they all play out in order.

So, if we take these, this play queue and we lay it out on that media timeline, what we're going to see is Item 1 followed by Item 2, followed by Item 3.

That's just a very generic look of those items laid out on this timeline we've been showing.

But if we dig into the timelines especially we're going to see the timelines of each item, we're going to see something slightly different.

Let's suppose hypothetically that each item is 100 seconds in length.

That means that each item will go from 0 to 100, 0 to 100, 0 to 100.

Of course the AudioRenderer knows nothing about these individual items.

All the AudioRenderer has is a continuous media timeline.

So, as you enqueue audio data into the AVSampleBufferAudio Renderer, you may need to offset from the item's natural timeline to the continuous timeline of the AVSampleBufferAudio Renderer.

And again, let's take a look at this enqueuing animation that we've seen before.

Here are I'm playing back locally and I've just enqueued a few seconds ahead of the playhead and as you can see I'm going to queue each item as I go through it.

And if the user is routed to an AirPlay 2 speaker, and we're working well ahead of the playhead, same idea.

You're just going to enqueue ever so slightly ahead of the playhead.

All right.

So, that's the simple idea of play queues.

Let's get into something a little more interesting, editing.

And in this hypothetical example, let's suppose the user decides they don't want to listen to Item 2 anymore.

So, they remove it from the queue, and so items 3 and 4 shift over in place.

Now, when that happens, it's not a big deal.

The user will expect Item 1 to play, followed by Item 3, and followed by Item 4.

But that's a really relatively simple case of play queue editing, where I, made the edit before playback is engaged.

But since we're working well ahead of the playhead oftentimes when using the AVSampleBufferAudio Renderer, you could run into a situation like this, where the user is instantiating, or initiated playback.

You're still playing Item 1, but you've begin to enqueue media data from Item 2.

And then the user decides they don't want to listen to Item 2 anymore.

So, what the user expects is Item 2 disappears, Items 3 and 4 shift over.

And once again, what the user expects is that after Item 1, the currently playing track plays, Item 3 will begin to play.

But if we do nothing at this point, since we've already enqueued audio data from Item 2 into the AudioRenderer, well that's what we're going to hear, a blip of that, because we have the wrong audio data in the AudioRenderer.

So, what do we do here?

Well it's actually relatively simple to handle, because there's a command called Flush from Source Time.

And what this command Flush from Source Time does, what it means, is that given this time that I specify to you, I want you to throw away all of the media data on the timeline after it.

So, here I'm going to call Flush from Source Time with the source time pointing at the time in the continuous timeline that it's the transition from Item 1 to the next item.

So, I call that and it throws away all the media data.

Next thing that it does, is it resets the pointer to the last enqueued sample buffer to that source time, so then I'm free to enqueue media data from Item 3.

And of course, playback progresses and we're good to go.

The key point about this is, for the animation purposes in this talk, I showed it may look like playback was paused, but you can actually execute this command while playback is engaged, so it can be totally transparent to your users.

All right.

So, let's talk about the steps involved in executing the flush from source time.

There's basically three of them.

First is, stop enqueueing audio data in the renderer.

Note that I'm not saying stop playback, because again you don't have to do that.

Then you issue the flushfromSourceTime and after you've issued the flushfromSourceTime, well that flush from source is an asynchronous option, operation into which you pass a closure.

Well, you should wait for the callback to be called.

Now there are few gotchas with this, even though it's a relatively simple operation, and the first is that the flush may fail.

Now, why may it fail?

Well, suppose the source time the you specified is too close to or behind the playhead.

Well in those situations, we may not be able to get the audio data back from the audio hardware.

And in those cases well, rather than leaving you in an unknown state where you may play out stale data, we're just going to fail the operation and it's, and it's going to be as if it was never issued in the first place.

So, the second gotcha which is really just the other side of the same coin is that you need to wait for the callback.

So, you should wait for the callback which will tell you whether or not that flush failed, and if it did you need to take the appropriate action.

So, let's take a look at this in code, again by extended the sample app by calling a method called by implementing a method called FlushfromSourceTime, perform FlushfromSourceTime.

So, once again, in this method the first thing you're going to do is you're going to tell the AudioRenderer to stop requesting media data.

And then you're likely to call some app specific logic to really ensure that no media, additional media data is enqueued.

This is really important, because this is an asynchronous operation so it's naturally racy.

So, we want to make sure that you're not enqueueing any additional audio data while that FlushfromSourceTime is flight.

So, after you've made sure that you're no longer enqueueing audio data in the AudioRenderer, you can actually execute the FlushfromSourceTime.

Again, you'll pass in a closure because this is an asynchronous operation.

And then closure will tell you then whether or not the call to the closure will tell you whether or not the flush succeeded.

If it succeeded as I'm showing here, well, the first thing you're going to do is again you're going to tell your sample generating app, your app's sample generating code to begin preparing samples at that source time.

And then you can reinstall your callback closure and begin enqueueing again.

Of course, the flush may fail, in here it's really app specific logic to decide what you want to do in these situations.

Maybe you want to just advance to the next track.

Maybe you want to execute a full flush and glitch playback, it's really up to you.

So, that's play queues and Flush from Source Time.

Let's move on to the supported audio formats in AVSampleBufferAudio Renderer.

So, what audio formats does the AVSampleBufferAudio Renderer support?

Well good news, basically all platform supported audio formats are also supported by the AVSampleBufferAudio Renderer.

So, this includes LPCM, AAC, mp3, Apple Lossless, lots of sample rates, lots of bit depths.

And additionally, mixed formats may be enqueued.

If Item 1 is AAC at 44.1 you can then enqueue that and then MP3 at 48k and 16bit Apple Lossless at 48 kilohertz.

You can just enqueue the audio data from each of these back to back and we will take care of the format transitions for you.

So, those are the supported formats.

Let's talk about the preferred formats.

So, because we can play anything just give us what you got.

You have LPCM, give it to us, encoded give it to us.

But one caveat there is all things being equal, if you have a decision between encoded or PCM, we would prefer the encoded, but don't do any special logic to give us that.

Just give us what you have.

And we prefer interleaved channel formats and 1 to 2 seconds of audio per CMSampleBuffer.

All right, now I'd like to take a slight detour and talk about video synchronization.

Now, you may be wondering why we're talking about video in a AirPlay talk that's mainly about audio.

And it's because the classes that we're introducing today not only are they good for AirPlay 2 but they're also just really good playback APIs for general use cases.

And you may want to use them to play video.

Now, once again, if you can use AVPlayer, please use that.

But if you can't use AVPlayer and you want to play video, here you go.

So, let's refer, jump back to a block diagram that we showed earlier in the presentation.

We've got the Client App, the AudioRenderer and the Synchronizer.

Well, if I simply move those out of the way, I can make room for a new renderer class and here I'm going to add an AVSampleBufferDisplayLayer.

Now for those of you who are unfamiliar with AVSampleBufferDisplayLayer, it's a class similar to the new AVSampleBufferAudio Renderer, except this one is geared towards video.

It's been around for quite a while and there's some documentation and sample code on the developer website.

But new in these releases is the idea that you can have its timeline controlled by a RenderSynchronizer.

You can add it to a RenderSynchronizer just as you add the AudioRenderer to your RenderSynchronizer.

And this is really cool.

Because then you can add the audio data to the AudioRenderer.

The video data to the video renderer, set a rate of 1 on the Synchronizer, and because they're tied to the same timeline, that will come out in sync.

Now a couple words of caution, the video will not be AirPlayed, and additionally when you're using long-form you can only add one AudioRenderer to a RenderSynchronizer.

But never the less, I think you can see the power in this and flexibility in this architecture and I'm sure you'll come up with some really cool use case, applications and we're excited to find out what you can do.

All right.

Lastly, let's talk about the availability of AirPlay 2.

So, I'm happy to say that the APIs that I've discussed today plus the advanced buffering are all available in the beta that you guys have access to today.

And you simply set the AirPlay 2 toggle in the developer pane, flip that on.

And then you can use an updated Apple TV as your AirPlay 2 receiver and send to it.

In an upcoming beta, we're going to enable multi-room audio.

And lastly, this will be available to customers in an upcoming release.

Let's go through the summary.

We've covered a lot today.

So, AirPlay 2 introduces many new features for audio.

Long-form audio applications can enable them with the steps that I've outlined today.

And AirPlay 2 adoption can begin with the beta you guys all have in your hands.

Additional information, this website will have the presentation that we've gone over today, and there will be sample code available soon.

A couple sessions that we've talked about earlier.

There's the What's New in Audio session, that's the one that will discuss long-form in more depth and also Introducing MusicKit.

That's one that you should check out.

Thank you very much and I hope you have a great rest of the week.

[ Applause ]

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