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.
I need to make a design decision of how to approach an app which needs to render few 3D objects on top of an image texture.
Result needs to be rendered and saved as an UIImage.
Graphical design (client) expects standard UIKit controls to manipulate 3D world.
OpenGL view (CAEAGLLayer layer) needs to be inside UIScrollView for cheap and natural scroll and zooming implementation.
There are just few extra controls to manipulate rotation scale and transition of the few 3D objects.
There is not many triangles expected (100-200 at most) and 2-3 textures (1 mask). It does not even have to be refreshed constantly, just when some transformations and zoom changes.
CAEAGLLayer does not need to be opaque.
I would go with Core Animation solution but rendering 3D transformed CALayers to CGContextRef is not supported (neither is masking).
What is real performance cost when putting CAEAGLLayer inside UIScrollView and mixing it with few UIKit views?
How much triangles per second can I expect to be rendered with smooth frame rate (30fps will do), so I can make the best decision possible?
I know there are similar questions out there already, but none of the answers provides specific numbers, which could help with estimating expected rendering results.
Per Allan Schaffer, who gives the WWDC and WWDC on tour OpenGL speeches, OpenGL itself is not so much a special case — it's that anything that changes will cause everything on top of it to be recomposited. I spoke to him specifically about an app with a live updating video view underneath a live updating OpenGL view and he said that sort of thing is the pathological worst case. It's generally not that expensive to throw a few quite static views on top, such as using a UILabel to display the current score over an OpenGL game.
In your case, I think you can probably largely avoid the problem. Don't put the GL view inside the scroll view, but rather make the scroll view non-opaque and put the GL view behind it. Catch scrollViewDidScroll: (and the corresponding zoom messages) and make related OpenGL adjustments. I can speak from experience and say I've done exactly that on an iPad 1 with no performance issues. Particularly for the sort of model you're talking about I don't imagine a problem.
I'm creating an interactive e-book for the iPad. This book will contain multiple pages that will consist of a lot of animations (frame and motion animations), transitions,... I was wondering what my development options are, should I use OpenGL, Quartz,...?
I've use UIImageView.animationImages before and found that it had really bad performance. What's the best way to draw frame based animations?
Does anybody have some good pointers to resources on this?
thanks in advance,
Thomas
I guess that depends a bit on what you'll be drawing. If you have a need for 3D, then OpenGL is the way to go, but it doesn't sound like it. I have a feeling Quartz2D is going to be just fine for your 2D drawing needs. I've done drawing with both and they have a very similar API. I think the downside of using all the raw power of OpenGL is that you have then signed up for doing most of the work yourself. I don't recommend attempting to using Core Animation high level APIs to manipulate OpenGL views.
If you do use Quartz2D and "normal" UIViews instead of OpenGL/EAGLView, then you can take advantage the many pre-canned animations Apple already build with Core Animation. This include the card flip left/right, resizing, moving (x/y translation), rotation and the ever popular e-book page curl.
The best example of iBook like custom page curl functionality I could find is this example code from High Caffeine Content. However, you don't have to bring that much math to the table if you just want to use the out of the box Core Animation stuff. The bad performance you may have encountered could have been due to anything, including older/slower hardware. They have revved the graphics chips on the new devices.
I'm writing a game that displays 56 hexagon pieces filling the screen in the shape of a board. I'm currently drawing each piece using a singleton rendering class that when called to draw a piece, creates a path from 6 points based of the coordinate passed in. This path is filled with a solid color and then a 59x59 png with an alpha to white gradient is overlayed over the drawing to give the piece a shiny look. Note I'm currently doing this in Core Graphics.
My first thought is that creating a path everytime I draw is costly and seems like I can somehow do this once and then reuse it, but I'm not sure of the best approach for this. When I look at the bottlenecks with Shark, it looks like the drawing of the png is the most taxing part of the process. I've tried just rendering the png overlay or just rendering the path without the overlay and both give me some frame gains, although removing the png overlay yields the most frames.
My current thought is that at startup, I should render 6 paths (1 for each color piece I have) and overlay them with the png and then store an image of these pieces and then just redraw the pieces each time I need them. Is there an effecient machanism for storing something you've drawn once and redrawing it? It kinda just sounds like I'd be running into the whole drawing pngs too often thing again, but maybe there's a less taxing method that does a similar thing...
Any suggestions are much appreciated.
Thanks!
You might try CGLayer or CALayer.
General thoughts:
Game programming on iPhone usually necessitates OpenGL. Core Graphics is a bit easier to work with, but OpenGL is optimized for speed.
Prerender this "shiny look" into the textures as much as is possible (as in: do it in Photoshop before you even insert them into your project). Alpha blending is hell on performance.
Maybe try PVRTC (also this tutorial) as it's a format used by iPhone's GPU's manufacturer. Then again, this could make things worse depending on where your bottleneck is.
If you really need speed you have to go the OpenGL route. Be careful if you want to mix OpenGL and Core Animation, they can conflict.
OpenGL is a pain if you haven't done much with it. It sounds like you could use Core Animation and make each tile a layer. CA doesn't call the redraw again unless you change something, so you should be able to just move that layer around without taking a big hit. Also note that CA stores the layer in the texture memory so it should be much faster.
Some others have mentioned that you should use OpenGL. Here's a nice introduction specifically for the iPhone: OpenGL ES from the Ground Up: Table of Contents
You might also want to look at cocos2d. It seems to be significantly faster than using CoreAnimation in my tests, and provides lots of useful stuff for games.
I'm considering re-writing the menu system of my iPhone app to use Open GL just to improve the smoothness of scrolling a big image (480x1900px) across the screen.
I'm looking at doing this as a way to improve on using the method/solution as described here (Smoother UIView). This solution was a big improvement over the previous implementation but it's still not perfect and as this is the first thing the user will see I'd like it to be as flawless as possible.
Will switching to OpenGL give me the sort of smooth scrolling I'm looking for? I've stayed clear of OpenGL until now as this is my first app and core animation has handled everything else I've thrown at it well enough, would be good to know if this alternative implementation is likely to work! thanks
You should be using CATiledLayer for images that big. From the UIView docs:
Regardless of which version of iPhone
OS is running, you should consider
using a CATiledLayer object if you
need to create views larger than 1024
x 1024 in size.
Bill Dudney posted the CATiledLayer example from his Core Animation book on his blog. Unless you're building a graphics intensive game, you shouldn't need to drop down into OpenGL. Core Animation should give you all of the performance you need.