I tried to make it similar to the swapchain using
renderPassDescriptor.colorAttachments[0].loadAction,
but it wasn't possible because it fetched a drawable that wasn't the previous drawable.
Because drawable's texture cannot swapped, i think swap chain cannot implemented.
Can metal implement a swap chain?
In Metal, swapchains are implemented by retrieving a new MTLDrawable from the CAMetalLayer on each frame, and retrieving the MTLTexture from that drawable.
The number of available drawables is very limited (typically 3), so you need to manage these carefully in your render loop.
Apple's documentation is not great on this topic, but you can find more information here. This document is written in terms of including the swapchain logic in a custom view, but you don't have to do so. You can write the swapchain as part of a separate renderer class, etc.
To clarify the purpose of this question: I know HOW to create complicated views with both subviews and using drawRect. I'm trying to fully understand the when's and why's to use one over the other.
I also understand that it doesn't make sense to optimize that much ahead of time, and do something the more difficult way before doing any profiling. Consider that I'm comfortable with both methods, and now really want a deeper understanding.
A lot of my confusion comes from learning how to make table view scroll performance really smooth and fast. Of course the original source of this method is from the author behind twitter for iPhone (formerly tweetie). Basically it says that to make table scrolling buttery smooth, the secret is to NOT use subviews, but instead do all the drawing in one custom uiview. Essentially it seems that using lots of subviews slows rendering down because they have lots of overhead, and are constantly re-composited over their parent views.
To be fair, this was written when the 3GS was pretty brand spankin new, and iDevices have gotten much faster since then. Still this method is regularly suggested on the interwebs and elsewhere for high performance tables. In fact it's a suggested method in Apple's Table Sample Code, has been suggested in several WWDC videos (Practical Drawing for iOS Developers), and many iOS programming books.
There are even awesome looking tools to design graphics and generate Core Graphics code for them.
So at first I'm lead to believe "there’s a reason why Core Graphics exists. It’s FAST!"
But as soon as I think I get the idea "Favor Core Graphics when possible", I start seeing that drawRect is often responsible for poor responsiveness in an app, is extremely expensive memory wise, and really taxes the CPU. Basically, that I should "Avoid overriding drawRect" (WWDC 2012 iOS App Performance: Graphics and Animations)
So I guess, like everything, it's complicated. Maybe you can help myself and others understand the When's and Why's for using drawRect?
I see a couple obvious situations to use Core Graphics:
You have dynamic data (Apple's Stock Chart example)
You have a flexible UI element that can't be executed with a simple resizable image
You are creating a dynamic graphic, that once rendered is used in multiple places
I see situations to avoid Core Graphics:
Properties of your view need to be animated separately
You have a relatively small view hierarchy, so any perceived extra effort using CG isn't worth the gain
You want to update pieces of the view without redrawing the whole thing
The layout of your subviews needs to update when the parent view size changes
So bestow your knowledge. In what situations do you reach for drawRect/Core Graphics (that could also be accomplished with subviews)? What factors lead you to that decision? How/Why is drawing in one custom view recommended for buttery smooth table cell scrolling, yet Apple advises drawRect against for performance reasons in general? What about simple background images (when do you create them with CG vs using a resizable png image)?
A deep understanding of this subject may not be needed to make worthwhile apps, but I don't love choosing between techniques without being able to explain why. My brain gets mad at me.
Question Update
Thanks for the information everyone. Some clarifying questions here:
If you are drawing something with core graphics, but can accomplish the same thing with UIImageViews and a pre-rendered png, should you always go that route?
A similar question: Especially with badass tools like this, when should you consider drawing interface elements in core graphics? (Probably when the display of your element is variable. e.g. a button with 20 different color variations. Any other cases?)
Given my understanding in my answer below, could the same performance gains for a table cell possibly be gained by effectively capturing a snapshot bitmap of your cell after your complex UIView render's itself, and displaying that while scrolling and hiding your complex view? Obviously some pieces would have to be worked out. Just an interesting thought I had.
Stick to UIKit and subviews whenever you can. You can be more productive, and take advantage of all the OO mechanisms that should things easier to maintain. Use Core Graphics when you can't get the performance you need out of UIKit, or you know trying to hack together drawing effects in UIKit would be more complicated.
The general workflow should be to build the tableviews with subviews. Use Instruments to measure the frame rate on the oldest hardware your app will support. If you can't get 60fps, drop down to CoreGraphics. When you've done this for a while, you get a sense for when UIKit is probably a waste of time.
So, why is Core Graphics fast?
CoreGraphics isn't really fast. If it's being used all the time, you're probably going slow. It's a rich drawing API, which requires its work be done on the CPU, as opposed to a lot of UIKit work that is offloaded to the GPU. If you had to animate a ball moving across the screen, it would be a terrible idea to call setNeedsDisplay on a view 60 times per second. So, if you have sub-components of your view that need to be individually animated, each component should be a separate layer.
The other problem is that when you don't do custom drawing with drawRect, UIKit can optimize stock views so drawRect is a no-op, or it can take shortcuts with compositing. When you override drawRect, UIKit has to take the slow path because it has no idea what you're doing.
These two problems can be outweighed by benefits in the case of table view cells. After drawRect is called when a view first appears on screen, the contents are cached, and the scrolling is a simple translation performed by the GPU. Because you're dealing with a single view, rather than a complex hierarchy, UIKit's drawRect optimizations become less important. So the bottleneck becomes how much you can optimize your Core Graphics drawing.
Whenever you can, use UIKit. Do the simplest implementation that works. Profile. When there's an incentive, optimize.
The difference is that UIView and CALayer essentially deal in fixed images. These images are uploaded to the graphics card (if you know OpenGL, think of an image as a texture, and a UIView/CALayer as a polygon showing such a texture). Once an image is on the GPU, it can be drawn very quickly, and even several times, and (with a slight performance penalty) even with varying levels of alpha transparency on top of other images.
CoreGraphics/Quartz is an API for generating images. It takes a pixel buffer (again, think OpenGL texture) and changes individual pixels inside it. This all happens in RAM and on the CPU, and only once Quartz is done, does the image get "flushed" back to the GPU. This round-trip of getting an image from the GPU, changing it, then uploading the whole image (or at least a comparatively large chunk of it) back to the GPU is rather slow. Also, the actual drawing that Quartz does, while really fast for what you are doing, is way slower than what the GPU does.
That's obvious, considering the GPU is mostly moving around unchanged pixels in big chunks. Quartz does random-access of pixels and shares the CPU with networking, audio etc. Also, if you have several elements that you draw using Quartz at the same time, you have to re-draw all of them when one changes, then upload the whole chunk, while if you change one image and then let UIViews or CALayers paste it onto your other images, you can get away with uploading much smaller amounts of data to the GPU.
When you don't implement -drawRect:, most views can just be optimized away. They don't contain any pixels, so can't draw anything. Other views, like UIImageView, only draw a UIImage (which, again, is essentially a reference to a texture, which has probably already been loaded onto the GPU). So if you draw the same UIImage 5 times using a UIImageView, it is only uploaded to the GPU once, and then drawn to the display in 5 different locations, saving us time and CPU.
When you implement -drawRect:, this causes a new image to be created. You then draw into that on the CPU using Quartz. If you draw a UIImage in your drawRect, it likely downloads the image from the GPU, copies it into the image you're drawing to, and once you're done, uploads this second copy of the image back to the graphics card. So you're using twice the GPU memory on the device.
So the fastest way to draw is usually to keep static content separated from changing content (in separate UIViews/UIView subclasses/CALayers). Load static content as a UIImage and draw it using a UIImageView and put content generated dynamically at runtime in a drawRect. If you have content that gets drawn repeatedly, but by itself doesn't change (I.e. 3 icons that get shown in the same slot to indicate some status) use UIImageView as well.
One caveat: There is such a thing as having too many UIViews. Particularly transparent areas take a bigger toll on the GPU to draw, because they need to be mixed with other pixels behind them when displayed. This is why you can mark a UIView as "opaque", to indicate to the GPU that it can just obliterate everything behind that image.
If you have content that is generated dynamically at runtime but stays the same for the duration of the application's lifetime (e.g. a label containing the user name) it may actually make sense to just draw the whole thing once using Quartz, with the text, the button border etc., as part of the background. But that's usually an optimization that's not needed unless the Instruments app tells you differently.
I'm going to try and keep a summary of what I'm extrapolating from other's answers here, and ask clarifying questions in an update to the original question. But I encourage others to keep answers coming and vote up those who have provided good information.
General Approach
It's quite clear that the general approach, as Ben Sandofsky mentioned in his answer, should be "Whenever you can, use UIKit. Do the simplest implementation that works. Profile. When there's an incentive, optimize."
The Why
There are two main possible bottlenecks in an iDevice, the CPU and GPU
CPU is responsible for the initial drawing/rendering of a view
GPU is responsible for a majority of animation (Core Animation), layer effects, compositing, etc.
UIView has a lot of optimizations, caching, etc, built in for handling complex view hierarchies
When overriding drawRect you miss out on a lot of the benefits UIView's provide, and it's generally slower than letting UIView handle the rendering.
Drawing cells contents in one flat UIView can greatly improve your FPS on scrolling tables.
Like I said above, CPU and GPU are two possible bottlenecks. Since they generally handle different things, you have to pay attention to which bottleneck you are running up against. In the case of scrolling tables, it's not that Core Graphics is drawing faster, and that's why it can greatly improve your FPS.
In fact, Core Graphics may very well be slower than a nested UIView hierarchy for the initial render. However, it seems the typical reason for choppy scrolling is you are bottlenecking the GPU, so you need to address that.
Why overriding drawRect (using core graphics) can help table scrolling:
From what I understand, the GPU is not responsible for the initial rendering of the views, but is instead handed textures, or bitmaps, sometimes with some layer properties, after they have been rendered. It is then responsible for compositing the bitmaps, rendering all those layer affects, and the majority of animation (Core Animation).
In the case of table view cells, the GPU can be bottlenecked with complex view hierarchies, because instead of animating one bitmap, it is animating the parent view, and doing subview layout calculations, rendering layer effects, and compositing all the subviews. So instead of animating one bitmap, it is responsible for the relationship of bunch of bitmaps, and how they interact, for the same pixel area.
So in summary, the reason drawing your cell in one view with core graphics can speed up your table scrolling is NOT because it's drawing faster, but because it is reducing the load on the GPU, which is the bottleneck giving you trouble in that particular scenario.
I am a game developer, and I was asking the same questions when my friend told me that my UIImageView based view hierarchy was going to slow down my game and make it terrible. I then proceeded to research everything I could find about whether to use UIViews, CoreGraphics, OpenGL or something 3rd party like Cocos2D. The consistent answer I got from friends, teachers, and Apple engineers at WWDC was that there won't be much of a difference in the end because at some level they are all doing the same thing. Higher-level options like UIViews rely on the lower level options like CoreGraphics and OpenGL, just they are wrapped in code to make it easier for you to use.
Don't use CoreGraphics if you are just going to end up re-writing the UIView. However, you can gain some speed from using CoreGraphics, as long as you do all your drawing in one view, but is it really worth it? The answer I have found is usually no. When I first started my game, I was working with the iPhone 3G. As my game grew in complexity, I began to see some lag, but with the newer devices it was completely unnoticeable. Now I have plenty of action going on, and the only lag seems to be a drop in 1-3 fps when playing in the most complex level on an iPhone 4.
Still I decided to use Instruments to find the functions that were taking up the most time. I found that the problems were not related to my use of UIViews. Instead, it was repeatedly calling CGRectMake for certain collision sensing calculations and loading image and audio files separately for certain classes that use the same images, rather than having them draw from one central storage class.
So in the end, you might be able to achieve a slight gain from using CoreGraphics, but usually it will not be worth it or may not have any effect at all. The only time I use CoreGraphics is when drawing geometric shapes rather than text and images.
All you ios architects out there, please help me choose architecture/technology for the following iphone/ipad app.
The app itself is a financial app, but we want more of a game look-and-feel of the app, so we probably don't want to use the builtin looks of the cocoa widgets. The elements on the screen will probably be some kind of blob-shaped images.
The app will essentially have five "blob"-shaped areas, spread out evenly across the screen. One of the blobs will be centered and larger than the other ones. Within each blob there will be clickable areas which will pop up "details" and menu-action blobs. These blobs are also graphics objects and must not take over the whole screen. The blobs should animate nicely when popping up. The graphics elements will have a couple of lines of text, which are generated, so the overlaying text itself cannot be part of the static background-image.
The main user interaction will be swiping within the center blob, displaying summaries of the items that are conceptually contained within the blobs underlying data store. Now and then, the user will drag and drop the item to one of the other blobs. While dragging, the item should be traced by a line and when dropping on the other blob, the item should be animated to look like it's being "sucked into" the blob.
Now, what kind of technique would you suggest for this app? Is Cocoa suitable in this scenario? Should I use a game framework like Cocos2D? All kinds of suggestions including example code snippets are most welcome.
I realize that this question might not be as straightforward and to the point as questions generally are on SO, but I hope your answers will come to use by more people than me. Thanks!
EDIT (MY SOLUTION):
I eventually ended up doing everything in UIKit, which was a lot easier than I expected.
Briefly described I used UIButtons with Custom style and an image background, which gave me full control over the visual appearance of the "items". I also found it very useful to manipulate the underlying CALayer of many of my other UIViews. It is often easier than drawing things from scratch using Core Graphics programming.
Another thing that was useful were the UIGestureRecognizer:s. I found them useful for both handling "real" gestures like swiping, longpress etc, but also for handling normal "tap" for UIView classes that aren't subclasses of UIControl. Two examples are UIImage, UILabel and UIView itself. That way I could handle taps for these simple classes. I could for example use a normal UIView, modify it's CALayer to change the look of it completely and still handle taps. Using this technique, I didn't have to subclass any views at all in my app.
The animations were pretty easy too, even though I had to use a non-public method to use "suck" animation, so my app will never pass App Store moderation. It was just a prototype anyway so I don't care.
When this app will be for real, I will probably implement it in HTML5/JavaScript wrapped by Phonegap. The reason for this is mainly reuse of existing mobile web services and also for code reuse across platforms. It will probably also be easier to hook into the existing security solution when using a webapp.
Cocos2d is great if you need to move elements around really fast as it is a layer on top of OpenGLES. I think from what you have said the UIKit will be fine, you get nice animation support, you can do some nice things with UIScrollViews to handle moving elements around etc.
If you need more detailed graphics support and lots of moving elements, particle effects etc then by all means go for Cocos2D but be aware that in Cocos2d the application works more on a scheduled update method, i.e. you get notified every 1/60th of a second to move stuff draw stuff etc, whereas with normal UIKit approach it is more event drive, i.e. I click a button and show a view etc.
I know that sharing a single context between threads is bad news. I know that I can safely create and use a context with an offscreen framebuffer on a secondary thread when nothing is happening with GL on the main thread.
I haven't yet been able to find a definitive answer to the question of whether I can safely create two contexts on two different threads (say, a main thread drawing to the screen, and a secondary thread doing offscreen drawing work) and have them both making GL function calls simultaneously.
In other words, as long as the contexts are different, can two threads "share" the C API and thus the GPU? Or is that inherently something that is unshareable? Or is this implementation-specific?
Asking specifically for OpenGL ES on iOS, but it's probably a general GL question.
Yes, you need to use one context for each thread you want to use OpenGL with, also you can share objects between the contexts. This is the way to go :)
Option 1: If you don't use the context by two threads simultaneously, one context is enough.
Option 2: If you need to use OpenGL by several threads simultaneously, you need more than one context. Then, if the contexts share their Sharegroup, they share their OpenGL content like textures. This way you can load textures or do heavy framebuffer processing on a background thread.
Have a look at the last section about Sharegroups here: http://developer.apple.com/library/ios/#documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/WorkingwithOpenGLESContexts/WorkingwithOpenGLESContexts.html
Option 3: GLKit provides some built-in background processing, for example asynchronous texture loading via GLKTextureLoaders - textureWithContentsOfFile. I don't know all the options, but it definitely simplifies some of uses cases of asynchronous OpenGL.
My iPhone development is stepping up a notch and I'm looking at the UI. We're thinking of having a few nice interface-y features - things like dragging and dropping images onto one another from a gallery list, or similar.
How far does the basic iPhone interface stretch? Do most people create their own interfaces and code, and if so what's the base there? CoreGraphics? OpenGL?
I don't want to reinvent the wheel, but neither do I want to take an overcomplicated option if someone's done the work already.
There are several tiers to Cocoa-based interfaces. Generally, I recommend working at the highest level of abstraction that meets your needs for presentation and performance.
The base UIKit elements that you can place using Interface Builder or code are designed to handle the most common cases within an application's interface. These provide some degree of customization, depending on their type, but what you see is generally all you get. On the iPhone, Apple even tries to maintain a certain look and feel for these stock elements by rejecting applications during review that use them in ways that contradict the Human Interface Guidelines.
The next level down are custom UIViews. These can be made to look like anything through the use of Quartz drawing within the -drawRect: method. You can do your own touch handling by overriding methods like –touchesBegan:withEvent: or by using the new UIGestureRecognizers. Given the level of customization you can do here, this is where most people stop when tweaking their interfaces.
You can go a little lower than this by working with Core Animation layers and animations. You don't gain a lot, performance-wise, by using CALayers instead of UIViews on the iPhone, but they can be useful if you want to craft visual items that use the same code on Mac and iPhone. Custom animations may be required if you want to do something more than animate a view between two states linearly. You can even do some limited 3-D work using Core Animation.
Finally, there is OpenGL ES for display of full 3-D scenes and for really high performance graphic display. This is about as close to the metal as you're going to get when dealing with the iPhone display system, and it shows in terms of the amount and complexity of code you have to write. For complex 3-D work, this is what you will need to use, but for 2-D and even rudimentary 3-D I recommend looking first to Core Animation because of the code it can save you. If performance is unacceptable, then should you go to OpenGL ES.
Now, just because you need to use one of these technologies to work with part of your interface does not mean that it can't coexist with the others. UIViews are backed by Core Animation layers, and even OpenGL ES renders into a CALayer which can be placed in a view. Again, use the highest level of abstraction that is appropriate for that part of your interface.