问题
Here's a real mystery.
Imagine you are doing the conventional process of drawing a complex image offscreen (CGLayer), and you are updating the image on to the screen intermittently as you draw.
As you know the way to do this is: run the large drawing process in the background, and call to the foreground for setNeedsDisplays as desired, causing the image to be updated. This is trivial and achieved with two lines of code.
However, when you do this, there is a mysterious problem: the time taken for each drawing cycle, increases each time, becoming unusable. Further, the time can vary erratically.
Is this a known iOS bug, or? Has anyone seen this before?
Here is an extremely trivial, extremely easy to understand app, that demos the problem:
http://www.fileswap.com/dl/p8lU3gAi/stepwiseDrawingV2.zip.html
Output example below.
Does anyone know why this happens? It is very strange behaviour from the OS.
Later ... an astounding work-around has been figured out by FELZ. Felz copies the CGLayer once each round. This completely stops the strange behaviour.
However there is still no really clear understanding of what the heck exactly was happening: hence literally, where is the time being used when the bizarre problem exhibits?
And here's a long example of the output ...
Notice you sometimes get a "doubly bizarre" result like the following: It is exhibiting the "normal" bizarre behavior: the time increases each go around. However, occasionally the time will drop to "incredibly quick" for a couple rounds, and then return. Weird huh? Further, if you are running on your simulator, click back and fore quickly to unrelated apps on your Mac for "even weirder" results.
Although Felz has given a perfect working answer, the actual mechanism is still a mystery.
:26:56.697 stepwiseDrawing[5334:1a03] time difference was 0
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回答1:
First let me say this was an interesting puzzle. I had a lot of fun working on this one. Nice question and nice sample code.
Here is my take on a possible answer:
The issue I see is the following. CGLayer are great for repeatedly painting because they cache their rendering into a flat image. The cache is invalidated when additional operations are appended to the context. When this happens, the CGLayer is rendered again. The slowdown is cause because the code maintains a single CGLayer (offscreenPrefabCGL) that has its context modified in every iteration of the loop in paintActualGutsOfHugeImage. Because the sequence of operations in offscreenPrefabCGL's context grows with each flower, each render takes more time. This explains the increase in time for each display.
I see two solutions:
- Clip the new rendering into a particular in the PaintView. Instead of invalidating the complete view, invalidate only the the CGRect that really needs repainting. This, as I understand it, is the solution proposed by @v01d.
- Flatten the layer. Instead of holding a single CGLayer, copy the CGLayer on each iteration. This maintains a flat image in the Layer that is very fast to render.
I Implemented (2) with good results. The first 100 flowers are faster and then it will stabilize in 120ms (in Device). I tested my modified version with up to 2000 flowers.
My modified version is in:
http://dl.dropbox.com/u/9866261/felz_mod_stepwiseDrawingV2.zip
回答2:
EDIT2:
Here's a working (no slowdown) debug project: http://bit.ly/eukSgR
This project is by no means optimal.
What is interesting here is the need of a bitmap context. A CGLayer caches your image draws and causes your progressive slowdown. With a bitmap context it is forced to render; and then you are simply pushing those pixels to the UIView's CALayer in drawRect:.
I guess to get more speed, you can use setNeedsDisplayInRect: to minimise the area to redraw.
But, other than maybe an optimisation or two here and there, pushing pixels through a UIView will always be slower than doing this on the GPU completely via say GL.
回答3:
As one experiment, I suggest you update your animated progress view without trying to draw with or read from the unfinished large image bitmap or layer drawing in progress. Reading from or drawing with an unfinished compositing to another drawing context may be doing interesting things to the GPU pipeline or drawing state. Try drawing something else in your pepper method. A simple counter or something.
来源：https://stackoverflow.com/questions/4786754/mysterious-progressive-slowing-problem-in-run-loop-drawrect