Optimizing Drawing and Scrolling on OS X

Session 215 WWDC 2013

Creating a responsive app requires fast drawing and smooth scrolling. Discover techniques to optimize drawing, find out best practices for handling layers, and learn how to combine those techniques with new features to achieve smooth scrolling.

Hello, welcome to Optimizing Drawing and Scrolling on Mac OS X [applause].

My name is Corbin Dunn and I'm an AppKit Software Engineer.

I'm going to be giving this talk with my colleague, Raleigh Ledet and let's just jump right into it.

So what we're going to be talking about today, we have four major subjects.

We're going to be talking about Optimizing AppKit Drawing, Layer-Backed View Drawing and utilizing Core Animation in your views and how to make it fast.

Raleigh is going to come on stage during the second half and talk about Responsive Scrolling and what you can do to opt into that and make it fast on Mavericks.

And then finally, Raleigh is going to talk about Magnification in NSScrollView.

So let's talk about Optimizing AppKit Drawing and the best practices that you can do in your application.

So you're probably already doing this inside your drawRect.

Inside of your drawRect implementation, you're looking at the dirtyRect and just filling and pointing your model objects inside the area that's actually dirty that you really need to draw.

The interesting thing about this is you're probably already doing setNeedsDisplayInRect on just that small little rect that's really dirty that you need to redraw.

You're hopefully not doing setNeedsDisplay:YES an entire view and then validating everything which is not good for performance.

Of course, if you have a big red view, you do a setNeedsDisplayInRect on this little orange rect, you do another setNeedsDisplayInRect on another rect and the same run will pass.

What's going to happen is your dirtyRect is going to be the union of those two Rects.

So what you can do as a developer to do better drawing is inside of your drawRect, utilize getRectsBeingDrawn:count which is some older API that we've had for a while.

You can enumerate all the dirtyRects and then just fill in those Rects that are actually really dirty.

Or, what you can also do here is just pull in your model objects that exist inside those dirtyRects instead of pulling in everything inside of your view bounds or the visibleRect or the dirtyRect.

This is much more performant.

Another thing you can do is you can use needsToDrawRect which is some API where you can say, "Hey, I already have this rect, I need to draw it or fill in my model.

Do I really need to draw it?"

If you do, call needsToDrawRect, it'll say yes or no and then you can actually do your logic based on that answer.

What types of things should you be doing inside of your drawRect, we should only be doing drawing inside of your drawRect.

You don't want to be doing network calls, you don't want to be doing image allocation or loading, you probably don't want to be doing file access, you definitely don't want to be doing layout or adding and removing subviews which are kind of be a recursive loop.

If you add a view inside drawing, it's going to need to draw again and that's not good.

Now, other things that you can do that are performant, well, hiding a view may be actually better and faster than adding the view.

So, you can utilize setHidden to make the views be hidden.

An exception to this, which I'll discuss a little bit later is if you're using layer-backed views and I'll discuss the details on why in a bit.

So let's talk a little bit more about images and utilizing loading of images and what you should do.

It's really good to cache images so if you're using -imageNamed to load an image, you probably want to retain it into an ivar.

It's quite possible that AppKit may not retain that image and it may be cached, the cache may go away so if you want to draw it again and again, retain it.

Now, if you have a big image that you want to load, it's probably a good idea to not load it inside drawRect and to asynchronously load it in the background thread.

So you can use an NSOperationQueue, add an operation with block to do some work on the background.

You can actually do your heavy lifting such as initWithContentsOfURL to allocate your image.

You can then kind of pre-cache and warm the image by calling CGImageForProposedRect context: hints and pass in the actual size which you're going to be drawing and this will kind of warm the image up to get be to get to be ready to be drawing.

Then once it's warmed up, you can kick it back over to the main queue and add another operation that actually updates your image property of your view and actually marks the area need to be redrawn which will then be happening on the main thread.

As I mentioned before, you only want to do drawing and don't do any layout in the background and inside of drawRect so where should you do layout?

You should probably do layout in the layout method or utilize viewWillDraw, that's the location to add and remove subviews, mark areas as being dirty and whatnot.

Definitely, do not do inside drawRect because it's bad for performance.

Of course, if your view is opaque, and you want to override isOpaque on [inaudible] view and say yes.

If your view isn't, then you have to so say, no.

But if it opaque, then you tell AppKit, that we can do more performing operations by knowing this information.

Another thing that you can do to make faster drawing is override wantsDefaultClipping.

By default, wantsDefaultClipping says yes, meaning your view is going to clip to whatever bounce it is actually drawing to.

Now, if you can strain all your drawing to get Rects being drawn, you can override -wantsDefaultClipping and say no and we won't do that clipping for you and it might be faster for drawing.

There are some methods in the AppKit which are a little bit heavyweight and called frequently and it's very good to avoid overriding these methods if possible.

The class methods are the GState methods.

So if you're overriding or calling GState, allocateGState, releaseGState and in particular, setupGState and renewGState, we'd frequently see people overriding these methods, in particular, renewGstate to do things to know when their view's global position within the window changes.

So for example, the views frame hasn't really changed but its position inside the window globally does change and some people are using this as a hook to know when that happens.

Instead, it is much better to use notifications to find out when your views position changes.

Use the NSViewFrameDidChangeNotification or the NSViewBalanceDidChange Notification.

Raleigh is going to come up in a little bit and talk about this a little bit more with the context of ScrollView.

So, that was discussing typical AppKit drawing and how to do some performant things for a traditional drawing.

Let's talk about layer-backed view drawing and some of the best practices for Core Animation and what you can do.

So last year, we gave a talk on layer-backed views which I highly recommend going, digging up, and watching.

It discusses a lot of things that are great to get performant animations using Core Animation.

I'm going to cover a couple of the properties here very quickly to just reiterate how important they are to get good fast animations.

I'm going to talk about the layerContentsRedrawPolicy and also updateLayer and wantsUpdateLayer.

So in Lion, we introduced some new API called the layerContentsRedrawPolicy.

It has a whole set of different values.

The one that's the most important is that NSViewLayerContentsRedraw OnSetNeedsDisplay.

So when you set this property on your view, what it means is you as a developer using a layer-backed view, whenever your content changes on the view or your frame changes, you are responsible for calling setNeedsDisplay.

We do not automatically call a setNeedsDisplay on the view when the frame changes.

And what this does is it allows us to utilize Core Animation animations to smoothly animate your view.

So prefer to use this property if you can.

The frame change default is the default value and so we require you to set the OnSetNeedsDisplay version of it to opt in and please do so and should get some fast animations doing this.

So how do we do drawing when we have a layer-backed view?

But since we added updateLayer in Mac OS 10.8, here is the typical flow path that we do for drawing.

You have a Core Animation layer and it needs to draw so it was dirty in some way.

The next thing that happens is it ask NSView, hey, do you want to use updateLayer and depending on what the answer to wantsUpdateLayer is, we do two different code pass.

So let's say you say no to wantsUpdateLayer, which is more of a traditional drawing in drawRect-based copath.

What happens is Core Animation creates a CGContextRef the size of our view.

That can be thought of as an image the entire size of your view then uses a delegate method drawLayer:inContext in order for AppKit to call your drawRect implementation.

Then whatever you draw and setup your views bounds is captured into a layered contents as an image.

Now, you may want to use wantsUpdateLayer and I'll describe why in just a moment.

The way that works is we use a different delegate method called displayLayer and then we call instead of AppKit updateLayer which is a method that you can override, you can set whatever properties you want in the layer including the layer contents to provide your representation of the view.

So let's take a look at doing that.

So here is an example using wantsUpdateLayer.

You override wantsUpdateLayer, you say yes.

Now, instead of getting a drawRect, you get an update layer call and you can set layer properties to represent how your view should be represented.

So for here, we don't have to create the backing store, we don't have to create that image that back to review instead, you can just set properties such as the background color, the border color, the border width and you can represent your user interface this way without having to use memory for the actual backing store.

This is a very efficient way to update layers without using a lot of memory.

Speaking of properties that people can set on Core Animation layers, you probably want to avoid properties that are expensive so if you're using the cornerRadius property, the mask, filters, backgroundFilters, those are all expensive properties which might make your layer rendering a little slow.

So if possible, try to do your UI in another way that can avoid these properties in order to get better performance.

As I mentioned before, if your view is opaque and you're saying yes from isOpaque, that value is directly assigned to a layer.

And so, opaque layers are much faster to composite together so prefer to say yes from isOpaque and realize that property is transferred over to the layer.

Now, let's say that you are drawing a big view inside of your drawRect of document view.

So you're drawing this big electric bug picture here.

What you're probably doing is you're pipe clipping so you have just a tiny little area that you want the user to see so you have it inside of this NSClipView.

The ClipView in itself is inside of the NSScrollView so you can scroll around to whatever portion of that view you want to see.

So how do we do this when you're using layer-backed views in AppKit?

We have a special layer called the Tile Layer that we use in AppKit to take your big view and just chop it up into lots of little individual tiles.

Now, your view still has just the visible area because a clip use clipping to what you just see.

So what we can do is we can make intelligent decisions here where only the tiles that intersect in that visible area are things that are going to actually be drawn.

Everything outside of it doesn't have to be drawn.

Add a little asterisk next to this because Raleigh is going to cover some details where we might do something differently.

So why is this important to know?

Well, all those tiles inside of your visible area, each one is going to get its individual drawRect in order to fill in its contents.

So if you're doing the things that I recommended at the start, we're properly watching for the dirtyRect and getRectsbeingdrawn, you'll only fill in your model area and only draw in the areas which you are drawing to an actual individual tile so it's very important that you do that and it's important to be aware off that we might be calling this more than once for one particular visible area.

Of course, we're actually much more intelligent than the picture I was showing before.

If your view is very skinny and tall, our tiles might be really wide and not very tall and so we're creating pow sizes that dynamically change to be the most efficient for your application and your view size.

It's important to realize this because our actual implementation and how we do the tiling may change over time to make it more efficient.

So another thing you can do to get performant drawing in a Core Animation layer-backed view is to try and reduce your layer count.

So let's take it a typical layer-backed view and see how the hierarchy works.

So you have your top most view here and you do setWantsLayer, yes.

Implicitly, all your subviews are going to get their own backing layers so your big parent one has a layer, all the subviews each have their own backing layer.

What could be the problem with this?

Well, as I mentioned before, if you're using drawRect, each of those views might have their own backing store, again, their own little image contents and they might be compositing on top of each other which might be a waste of memory.

You also might have a high composition cost where if you're not animating that view around, why have a bunch of layers there when they're not really needed.

And unlike just regular AppKit, hidden layers actually might have a composition cost wherein if they're in the layer tree, this have to be processed.

So if you a layer-backed view, it's probably more performant to actually remove the view from the hierarchy as opposed to hiding it.

And what I mean here having a couple you are hiding is fine, but if you're actually hiding hundreds of views, that's probably not a good idea and you probably want to remove them from the hierarchy instead.

So, another way to reduce your subview count or sorry, your sublayer count is to use some new Mac OS 10.9 API called CanDrawSubviewsIntoLayer and the setter, setCanDrawSubviewsIntoLayer.

Let's take a look at how this works and why you would want to use it.

So in the same example here, you have setWantsLayer:YES on your top most view.

In addition, you do setCanDrawSubviewsIntoLayer: YES.

And what's going to happen is just that top most subview is going to get the actual layer.

All those other subviews will no longer have their individual layers instead, they're all drawn with their drawRect implementation into the parent layer.

The interesting thing here is even though the parent layer parent view has a layer, if it says yes to wantsUpdateLayer, it's not going to get an update layer call, that view and its layer and all the children must utilize drawRect to do their drawing.

But this is a great way to reduce your layer count.

So, one interesting thing here is that what if you did want to view a subview that has its own layer because you want to animate that button around, you can opt in one individual subview really easily by just doing setWantsLayer:YES, and that view which normally would have been drawn to parent layer will now get its own layer and you can animate it around smoothly.

So why would you want to reduce these layers?

Let's get a direct example where it might be more applicable for what you can do.

So here's a view base table view and you want to reduce a lot of these subviews into a single layer.

You might have made your layer or you might have made your ScrollView layer-backed so that you can actually get fast mode fast smooth scrolling and also, you can do cool row animations.

But the row animations themselves are just on the individual row views.

And so what you can do is you can collapse these row views, the subviews layers into one single layer.

So what you can do for each of those table row views do setCanDrawSubviewsintheLayer: YES for each of them and all those individual subviews, the image, the text and whatnot will be drawn instead of having individual layers into just one layer of the row view.

One interesting caveat to note here is that for text fonts moving to work such as the title of that word there, it must be drawn into an opaque area so the row view or something else that was drawn in that layer must have filled with some opaque color in order for fonts moving to work just something to be aware of.

So that was just discussing layer-backed views in Core Animation, I'm now going to bring up Raleigh Ledet to talk about Responsive Scrolling.

So you've seen the demos already for responsive scrolling and for an overview, I want to give you another demo of that so let's take an example.

So, I'm going to go ahead and turn off responsive scrolling globally real quick and I'm going to run my little test app here.

And this test app does lots of horrible things during drawRect so your drawRect performance is very poor and you can see that when we try and scroll the scrolling performance is painful in this app.

So we're going to go ahead and turn responsive scrolling back on and we'll go ahead and run the same app and I'll just [inaudible] so you can see it, there we go.

And now, when we do scrolling, we have nice smooth 60 frames per second scrolling and [applause].

Thank you.

And, you know, that's that was the point of responsive scrolling, we want 60 frames per second buttery smooth scrolling, I could just keep listing bullet points of describing this all day long.

We're excited about it.

And I'm going to give you a quick brief overview of how this works and you've already seen this but say you have your document view which is obviously inside of a clip view and only a small portion of it is visible and traditionally, that's all that was drawn.

When we get responsive scrolling, we're going to ask your document view to draw portions that aren't visible and we call that the overdraw.

And now that once we have this overdraw, on a background thread, we can go ahead and change what the user sees on screen very, very quickly to any portion that we have that's already drawn in the overdraw.

And in a nutshell, that's all that we're really doing with responsive scrolling.

Under the hood, there's a lot going on and I'm going to cover some of those details.

Particular, I'm going to talk about the overdraw model and exactly how it works.

The event model, there's lots of big changes going on there.

Some API that we have to help you adapt to the changes that we have and what you need to do in your application so that you can make sure that your application adopts in to responsive scrolling.

So let's kick off with overdraw.

The main thing about overdraw is still main thread driven.

Your drawrect calls, they're always going to be called in the main thread so you don't have to worry about doing any additional locking that your app wasn't already doing, you can access the view hierarchy and you're data model just like you always did.

Of course, drawRect is now going to be called with nonvisible Rects.

So as Corbin mentioned earlier, it's really important that you respect those dirtyRects and you only do drawing and you do it just in the areas that we're asking you to draw.

When your app is idle, that's when one figures a good time for us to go ahead and ask you to generate some overdraw.

We're only going to ask you though for a little portion of overdraw that way your drawRect can be fast and if the user tries to interact with the application while you're in the middle of generating overdraw, that overdraw drawing will be quick and the user will not see any lag between trying to interact with your application while you're generating overdraw.

So last just for a little bit, that will get drawn, we have some overdraw, that's great.

The application is still idle so we'll say, "Hey, great, let's draw a little bit more and we'll draw some more and this is going to go around all the access that you have for your scroll view."

This one is only doing vertically so we'll just go up and down.

So we have a little bit more.

If your app is still idle, we just continue this process until AppKit has decided that we have enough overdraw to be responsive for what the user is likely to do.

Now, we don't want to draw your entire document, that would be a huge backing store and that would take a lot of memory and it would even take a lot of power just to have you draw that whole document view.

So we don't want to do that.

AppKit plays a very careful balancing act between how much overdraw we have so that it's responsive for the user and not using too much memory and not using too much power to be able to accomplish this.

And we even go a step further and where possible.

We'll make sure that those backing stores that we're creating to hold the overdraw are purgeable by the Kernel.

So if there's memory pressure on the system, that memory can be freed by the Kernel without even waking your application and when your application becomes active again, we'll notice that and we'll ask for those areas to be redrawn.

And this is great and in general, you won't have to do anything but sometimes it's not quite enough.

You need to be able to watch what's going on with overdraw and react to that.

One example is if you are adding your own subviews that you only want the subviews for your documents that are visible to be there.

This is a common technique.

Table view does this for a view-based table views.

Now, you need to make sure with overdraw that those subviews exist in the overdraw area is well so that they are always going to be ready for when the user scrolls to those.

So we have new API that you can adopt and you can play around in the overdraw world then make sure that your content is going to be available.

And let's give you an example of that.

In your document view, you would override PrepareContentInRect and we'll go ahead and pass in the rect during idle when whenever we want to prepare some new content.

And the rect that actually gets passed in is not just a little sliver that we're going to eventually ask you to draw.

It includes the entire overdraw area that we want currently which is always going to at least include your visible rect and in this case, the new little section of purple here.

Then you go ahead and you prepare you content as needed.

In this example, this is what we're going to add our subview to the view hierarchy because this is of course called on the main thread always.

So, adding a few to the hierarchy at this point is perfectly safe.

Now, when you override and PrepareContentInRect, you're not just reacting to the rects that we're provided in you to do overdraw.

You can actually be an active participant in deciding how much overdraw is being used.

And in this example of the subviews there, we don't want to clip half of it off in the overdraw, it's just a little bit.

So when we tell super how much overdraw you've prepared, we want to extend it just a little bit and cover the edge of that subview and that will be a little bit more efficient.

So you can be an active participant in this.

After you return from this, little bit later, we'll go ahead ask that new section to be drawn and we'll have that in the overdraw ready to be scrolled to for the user at a moment's notice.

Of course, if you have a still idle, we'll go ahead and start asking for more overdraw and as you've seen before, we'll just continue this process.

Continue to call, PrepareContentInRect and to just further drive the point home.

Now, the rect thats getting passed includes that overdraw that we drew earlier, the visible rect and the new section of overdraw that we want so you can make sure the whole area is properly prepared.

Again, when I've talked about being an active participant under very specialized circumstances you might know, when is the better time to end overdraw?

AppKit plays is very careful balancing act, there are some situations out there where you know that pulling in a certain amount of content for you makes more sense than AppKit would otherwise want to do.

And in that case, if the rect you return as super as the same as the previous time we called PrepareContentInRect, then we'll of course we won't ask for that area to be drawn and we're going to stop asking for overdraw on idle unless the overdraw gets blown away for some reason and then we'll come back on idle and rebuild it from scratch where you can terminate it again through the same mechanism that once you return the same rect twice in a row, we go ahead and stop asking for it.

Of course, now that you have content that isn't even visible, if that content becomes dirty, please setNeedsDisplayInRect and just those portions that have been dirtied and we will go ahead and have those redrawn when appropriate and that way when we scroll to that section, it's always the most up to date content for the user.

There's some special circumstances where you need to tell AppKit that you should totally get rid of any of all that prepared overdraw that we have.

And you can set the PreparedContentInRect in this case to the visible rect.

For example, the user might have changed something, choose a different group of items which are going to totally change the content that you're showing in the scroll view and the original prepared content there is completely not even appropriate anymore.

So, you want to go ahead and tell AppKit to just drop it all and we'll drop down to the visible rect and the next pass to the run loop.

We'll draw the content for the visible rect and that's what the user will see, and during idle, we'll build it back up with your new content that's appropriate.

So, all of this with overdraws are done on the main thread, drawRect is being called your nonvisibleRect, make sure your drawRects are only doing drawing and they're as fast as possible.

In general, let AppKit balance the amount of overdraw along with memory empower usage and purgeable memory and handle all of that complicated matter.

Only if you have specialized needs then you can go ahead and cut that short a little bit perhaps.

In other circumstances like when you're adding your own views and then make sure that the prepared content is there in the view hierarchy, then go ahead and use the prepared content rect API that we have.

So that's overdraw.

Let's move switch gears and talk about the event model.

A lot of exciting things here as already been pointed out.

But let's back off a little bit and talk about the traditional way that scrolling is handled.

You got a scroll wheel event comes in through your scrolling device.

It gets put into an event queue and your main run loop is running on the main thread of your application handling event sources and other run loop sources, pulls the scroll wheel event out of the queue, we hitTest that scroll wheel event, they get passed to your document your some subview of your document view.

And then we call scroll wheel on that, that goes up the responder chain finally gets to scroll wheel and in that scroll view, then that scroll view moves your content, withdraw the little section of content that became available and we'd let the run loop go back to handling event sources and the next scroll wheel comes in and we do the whole process all over again for every single scroll wheel event that comes in.

Well with Responsive scrolling, we break that cycle and once that first event comes in as soon as it gets to in a scroll view, and the scroll view sets up concurrent tracking thread and it will now handle the events on the background thread.

The scroll wheel events from the track pad are now going to be routed to this private event queue so you won't be able to see them at all and they will be processed by the scroll wheel and now your main thread is allowed to just run as it normally would processing other events and other run loop sources.

Let's dig in a little bit more with the concurrent tracking thread and inspect a little bit further on exactly what its doing.

It's pulling events from the event queue and it figures out where we want to move the scrolling.

And so we can change that do the user very quickly on a background thread, anywhere that we have the overdraw.

So let's say it lands right there, course your main thread run loop is still running, it's still doing its thing respecting timers that are firing.

But if you would ask your view hierarchy at this point what the visible rect is, it's still going to say is that blue square up there.

Though what the users sees on screen is the red dashed area.

So, that's an important thing to realize as what the users sees on screen can be different than what your main thread is reporting.

Now, the concurrent tracking thread whenever it updates the screen, it goes ahead and it issues a synchronization request on the main thread.

This gets run in the main thread, it talks it in a scroll view and it actually does the scroll too and all of your view properties are going to be updated and their visible rect will be the most up to date version of the visible rect that we possible can have.

And normally and traditionally, this would cause drawRects to occur but hopefully here, since we already have overdraw, this is just updating the view frames changes, this can happen very, very, quickly.

And if and once the synchronization occurs, it'll match with what we have on the screen and every everything will be in sync.

And if this happens in exact same display refresh that the concurrent tracking thread move things then everything appears to be in sync.

The other thing that the synchronization request does is if your app if your main thread is otherwise idle, it'll ask for some prefetch.

So we'll do some more overdraw.

In this case, the concurrent tracking thread and the synchronizer know what direction the user is scrolling in.

So we ask for prefetch in that direction.

So unlike idle which is trying to get a general case, this is more specific and we can get drawing ahead of where the user is going but it does bring up an interesting situation.

What happens when the user tries to scroll to area that you haven't been able to catch up to yet.

Well, in that case, we have to back off and respect what the main thread can keep up with.

We don't want to show blank content that could be drawing and confusing to the user.

So we end up what's we run out of overdraw, we have to slow down to whatever the main thread can do and if the main thread catches back up, because maybe it was just a long processing of some timer information, once the main thread can catch back up, if the user is still scrolling, we can get back into responsive scrolling.

But it's a situation we want to avoid.

So, my point here is it's not a silver bullet.

Responsive scrolling works really great to make sure that it's responsive right away when the users starts scrolling and that there are any little hiccups that your main thread might have in responding to timers or network request or anything like that won't interfere with user's experience but at some point, if your app can't keep up and the user is scrolling fast and they're scrolling far, we'll have to drop down to whatever your main thread can handle.

So quick overview, event tracking is now done concurrently.

Once we get that first scroll wheel event, you won't see the other ones until the gesture is fully completed.

What's on the screen may not match what the main thread is reporting as the visibleRect and it's not a silver bullet.

So make sure you drawRects are as fast as possible and you're only doing drawing in those drawRects.

Let's go ahead and move on to some API and how you can play with this brand new world.

Overriding scroll wheel is obviously isn't going to work the way that it used to because scroll will use to see every single scroll wheel event in your document view or in your subclass and that's not the case anymore.

A better way of watching scrolling changes is to watch the clip views bound changes.

And the clip view of course is the contentView of the scrollView and you'll need to tell the clip view to pout to post its bounds change notifications so setPostBoundsChanged Notifications: YES, and then once the clip view is posting its bounds change notifications, you can ask to observe that on the notification center with the NSViewBoundsDidChange Notification on the clip view.

Now, this is better in general then overriding scroll wheel because now you'll be informed anytime the scroll wheel the scroll view scrolls which could be in response to a scroll wheel event, it might be in response to the user moving the scroll bar, it might be in response to keyboard access or it could be even you've programatically moved the clip view bounds.

And this way, you'll catch all of those and you can respond to them appropriately in your application all in one central case all in one central place without having to have codes sprinkled all over your application to handle that.

So this is the way that we've been suggesting that you watch for your scroll view bounds changes instead of overriding scroll wheel.

However, there's a case where you really want to know that the user initiated scrolling.

Well, until now, we've implemented some new live scroll notifications and you can get exactly that information.

You can find out when the user starts to do a live scroll, you'll get a whole bunch of Dids as the user is scrolling every time we move the content.

And then when the last scrolling completely ends, you'll get a DidEndLiveScroll.

So this worked great with the gesture capable scrolling devices.

And what we do here with these devices is as the user scrolls multiple times consecutively, you will only see one bracket of will start and did end.

So even though the user scrolled three or four, five times in a row will coalesce those all into one will start, a whole bunch of dids and did end.

Not only that, we will actually extend this across the animations.

So if the user goes into rubber banding for example and there are no more events coming in, there's that little bit of a rubber band animation going on.

We don't call dead-end until the animation completes.

And every frame of the animation we issued dids.

So you've got a will start, some dids, the animation starts, you'll get a few more dids.

When it ends, you'll get a in a scroll view did end live scroll and the scroll is completely over at that point.

We take it a step further and says this is user-initiated scrolling, we can we know when they're tracking the scroll bar.

Now, this is checking the scroll bar still driven on the main thread in this case but we know when it starts, we know every frame that happens in between and we know when it stops.

So we'll go ahead and report those notifications in that case.

We'll even do this for keyboard things, page up and page down for example.

Those are animations generally.

Well, we know when the animation starts and we knows when we know when it ends.

And then all of these cases, the these notifications are being issued on the main thread.

So that's an important thing to point out.

Your notifications these live scroll notifications are always issued on the main thread.

But this device is a little interesting.

If you look at the devices that actually have a physical scroll wheel or something that has like a scroll ball like the Mighty Mouse, it does we don't know when it starts, we don't know when it ends.

All we know is that a scroll wheel event happened.

Another one might happen soon, it might not.

So in that case, we issued just a did live scroll notification and of course, that's going to happen on the main thread just like all the others.

So, if you're looking for this and you're looking for user-initiated scrolling, it's important to realize that some devices won't be bracketed with the will start and a did end.

If you have some floating subviews in your content, like let's look at this table view here.

It's a floating group row.

What's going on here during scrolling is that your generally, you have a subview in your content, it scrolls and you change the frame of this view.

So to the user, it appears that it's floating above your content.

It hasn't moved.

Though in reality, it has moved within your view hierarchy or for moving the content on the background thread that can get out of sync.

And it's a lot of code that you shouldn't have to write for yourself anyway.

So in a scroll view, we have this new API addFloatingSubviews for access, you tell us which access you want this view to be floating on and we'll go ahead and put in a special place in the view hierarchy and scrolling will be handled automatically in sync.

So if you're scrolling ob the access is not floating on, it'll scroll that content in sync with the clip views content and when it's floating, it's just handled automatically for you without you having to update those frames.

So, it's less code for you to write, a lot easier to do, and this is what table view does automatically for responsive scrolling on 10.9.

So that was a whole there's a whole lot there going on with responsive scrolling.

In order to adopt in to responses scrolling, you have to be linked on 10.8 or later.

Your window that your scroll view was in, its alpha must be 1, it has to be completely opaque and your document must not be an OpenGL context.

These are the absolute basics that we need to start adopting your scroll views into responsive scrolling.

We have a few more.

But from that this point on, we try to make it as automatic as possible, we want you to have to do as a little work as possible.

But as you know, scrolling is the cooperation of a scroll view, the clip view, and your document view.

If anyone of these views for whatever reason can adopt into responsive scrolling, then as a collection, these three views won't do responsive scrolling.

I'm going to talk about some more things that you need to do to adopt in the responses going in the next few slides.

But from this point on, you can use this explicit API isCompatibleWithResponsive Scrolling to bypass all those checks.

When you return yes, we won't even check and we'll just adopt you in the responsive scrolling from this point on.

Obviously, you can override scrollWheel or lockFocus, scrollWheel in particular as we've covered, there's a huge behavior change here.

So if you notice that, your scroll view subclass, your clip view subclass, your document view is overriding scroll wheel then we're going to adopt you in to responsive scrolling.

So please as I've pointed out earlier, use other API and try and just remove your scroll wheel override all together.

Or if you absolutely have to have it and you're OK with just peaking at the one event because you need to decide if it's a scroll or not period, before you have been passed it to super, then go ahead and override scrollWheel and in the class that you override scrollWheel, override is compatible with responsive scrolling.

Remember, it's a class method and return yes.

lockFocus turns out with the way that we do overdraw just isn't applicable.

So, please stop overriding lockFocus.

But, you can like I said, with the previous slide, this is something that you can override with the isCompatibleWithResponsive Scrolling and you can find out what happens in your application.

From this point on, we have two models that we've been talking about, the traditional drawing model and the layer-backed model that we have and Corbin covered the various cases.

Responses scrolling support both.

If you're using traditional drawing then copiesOnScroll which is a property actually on the clip view, but there's also on the scroll view and it brought it over to the clip view and there's little check you can check in interface builder.

That should be yes and isOpaque must return yes for the document view.

The isOpaque one is really important because as Corbin covered earlier, in order to get fonts moving right, your checks needs to be on an opaque needs to have opaque pixels in order for fonts moving to work correctly.

And if your isOpaque is returning no which is the default case, then we're not sure what's going on there and we won't be able to adopt you in to responsive scrolling.

However, if you know that your view has some transparent parts but those transparent parts don't have any text, then go ahead and override the isCompatibleWithResponsive Scrolling and you can have a non-opaque document view.

Or you can just go layer-backed.

We don't even check the copiesOnScroll and isOpaque if you layer-back.

But when you go layer-back, you need to make sure that your layer-back at least at the end of scroll view layer, in a scroll view level, or any of the scroll views ancestors.

You can do that by calling setWantsLayer on the scroll view or you have a scroll view subclass, you can just return yes from wantsLayer.

An interface builder in the Core Animation section, there's a nice little check box and that check box is the same thing as saying setWantsLayer and so you would do that on a scroll view or its ancestor.

And also, as Corbin mentioned, I want to reiterate the canDrawSubviewsIntoLayer.

There are different performance characteristics when you go layer-back, when you have you have to composite all those different layers together.

So, depending on your application and the amount of memory that all these layers use and what and exactly what you're animating, you can decide at what is the appropriate level that you want to collapse these all down into one layer and the Corbin's example that he gave was great, you might want to do it for a view-based table view on the row view for example.

And that way, you can animate each individual row smoothly and quickly, but this will be a lot less layers for us to have in the view hierarchy to composite together for each frame.

We have some support in Xcode for helping you debug some of this stuff.

When you're running application at this new menu which has some neat some neat options, in particular Show Responsive Scrolling Status.

If your scroll view is not opted into responsive scrolling, it'll look like this and well, we don't want that.

What we want is this.

We want green.

So, if everything turns out everything is laid out appropriately, and we can opt in your view, all of the views into the responsive scrolling, your scroll view or subclass, your NSClipView subclass, and the document view, it'll be green and this is a way of being able to provide that feedback to you through the debugger that your scroll view has completely opted in to responsive scrolling.

So in summary, we really want to make this as automatic as possible.

You can explicitly opt in but do that as a last resort please, but there are cases where that's the only way to do it.

Think carefully about layer-backing if you're not already layer backed, if you want to go layer-backed, there's some difference performance characteristics.

So, but for responsive scrolling, we'll support both layer-backed and traditional drawing.

And you can use Xcode to verify.

So that comes out to be handy.

So, thats responsive scrolling and we'll go ahead and move on to magnification.

Specifically, in magnification as relates to NSScrollView.

And 10.8, we introduced magnification support natively into NSScrollView and all you have to do is set the allowsMagnification property which you can also do an interface builder.

There's a nice little check box and you could check that.

And your scroll view will response to the pitch gesture and we'll do zooming and magnification for you automatically.

When it comes to responsiveness, if your scroll view is opted into responsive scrolling, magnification is still main thread driven at the moment.

But you'll likely to have overdraw and we can use that.

So during the magnification gesture, we use that overdraw and we scale your existing content.

So if you have content like this and you zoom into it, during the overdraw, it will scale that content and it will look kind of like this.

But when the gesture completes, we'll go ahead and redraw the visible rect when the gesture ends and we'll get the nice crisp content back in there.

But during that whole gesture, it was nice and responsive to the user, we weren't even calling drawRect, so that was great.

If you're going the opposite direction for example and you get to here and we run out of overdraw, we don't want to draw blank content on the sides there.

So in this case, if we run out of overdraw, we have to pause and wait for new drawing to come in since this is all main thread driven and so the user will see a lag in the responsiveness of your magnification here.

So make sure you drawRects are as fast as possible but they'll pop in.

During the middle of the gesture, if new content comes in, that content will be drawn at the appropriate scale because your drawRect will be called.

Likewise, if you dirty any content in the middle of a magnification gesture, we will redraw that content at the appropriate scale factors but that will require a drawRect right in the middle of your gestures.

So you want to try and prevent that.

So when it comes to magnification, your drawRect speed is crucial to getting responsive feedback to the user.

We do have some live magnification notifications, these were introduced with 10.8 when we introduced magnifications NSScrollView, the WillStartLiveMagnification Notification and DidEndLiveMagnifynNotification.

These are great that you can perhaps stop doing some things in the main thread, turn off some timers or pause some things so that you can devote as much as your resources as possible to being responsive to what the user is the doing with the magnification gesture.

If you're overriding clip view, one of the reasons main reasons people override clip view is to center the content in your clip view when your content is smaller than the size of your clip view.

And to do that, you override constrainScrollPoint.

And with magnification, if you zoom pass the minimum amount here, when the user removes their fingers, we want that to animate to the center, but this is what happens.

In constrainScrollPoint, you're given a point and all you have is whatever the current bounds are of the clip view.

You don't know where we're going to, what the new size of the clip view is going to be.

So you can't give us an appropriately constrained scroll point and your content isn't centered like you want it to be.

So we've deprecated constrainScrollPoint and we've replaced it with constrainBoundsRect.

We'll go ahead and pass in a complete rect so that you have what we want the proposed new size to be and you can constrain that hopefully, all you're changing is the origin.

But now, as the user pinches and go and pass the minimum allowed size so that we need to bounce back, when they let go, we'll animate to the appropriate centered position.

So, for those of you that are overriding clip views, please add constrainBoundsRect and override that in your application.

So in conclusion, we covered a lot in this session.

We talked about optimizing your AppKit drawing and layer-backed drawing and optimizing that and some of the performance characteristics with layer-backed drawing.

These are still very important with responsive scrolling as you've seen, it can get over a lot of rough edges and that the main thread might be doing and be responsive to the user and that is a tremendous advantage for your application as you've seen in that demo application, it can make a huge difference.

However, it's not a silver bullet, so make sure your drawing is as fast as possible at all times.

So all everything that we said at the beginning of this talk is very important.

And we've also covered magnification and how we've made some changes there to make it a little bit more responsive even though it's still main thread driven.

For more information, there's you can see our App Frameworks Evangelist Jake and we have the documentation in particular the Core Animation Programming Guide has got some really nice information there for when you're making your views layer-backed and doing animations.

The Developer Forums and I didn't get a chance to put on the slide but make sure you read the release notes, we cover a lot of details and everything I've talked here in the release notes as it relates to responsive scrolling.

The Best Practices for Cocoa Animation which actually just occurred right before this one, but if you haven't seen that one yet, please go back and watch the video, it's another great session that you that relates to responsive scrolling and animating your views.

And that's it for responsive scrolling.

Thank you guys for coming out.

I hope you enjoy the rest of the show.

[Applause]

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