神秘的“渐进减速”运行循环中的问题/ drawRect [英] Mysterious "progressive slowing" problem in run loop / drawRect
问题描述
这是一个真正的谜。
想象你正在做一个复杂的图像绘制一个复杂的图像（CGLayer），你正在更新的图像到屏幕
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.
你知道这样做的方法是：在后台运行大型绘图过程，并根据需要调用setNeedsDisplays的前台，要更新的图像。这是很简单的，用两行代码实现。
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.
但是，当你这样做，有一个神秘的问题： 绘图周期，每次增加，变得不可用 。此外，时间可能会不定期地变化。
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.
这是一个已知的iOS错误，还是？有没有人看过这个？
Is this a known iOS bug, or? Has anyone seen this before?
这是一个非常简单，非常容易理解的应用程序，演示的问题：
Here is an extremely trivial, extremely easy to understand app, that demos the problem:
http://www.fileswap.com/dl/p8lU3gAi/stepwiseDrawingV2.zip.html
输出示例如下。
有没有人知道为什么会发生这种情况？这是操作系统非常奇怪的行为。
Does anyone know why this happens? It is very strange behaviour from the OS.
稍后 ...一个令人惊讶的解决方案由FELZ出。 Felz 复制 。这完全阻止了奇怪的行为。
Later ... an astounding work-around has been figured out by FELZ. Felz copies the CGLayer once each round. This completely stops the strange behaviour.
然而，仍然没有真正清楚的了解什么是正在发生的：因此字面上，当时使用的时候，奇怪问题展示？
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?
这里有一个很长的输出示例...
And here's a long example of the output ...
请注意，双重奇怪的结果如下：它展现了正常的奇怪行为：时间增加每一个。然而，偶尔，时间会下降到令人难以置信的快几轮，然后返回。奇怪啊？此外，如果您在模拟器上运行，请在Mac上快速点击无关的应用程序，以获得甚至更糟糕的结果。
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.
虽然Felz给出了完美的工作答案，实际机制仍然是一个谜。
Although Felz has given a perfect working answer, the actual mechanism is still a mystery.
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推荐答案
首先让我说这是一个有趣的谜题。我有很多乐趣在这一个工作。好问题和好的示例代码。
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.
以下是我可能的答案：
我看到的问题如下。 CGLayer是伟大的重复绘画，因为他们缓存他们的渲染到平面图像。当附加操作附加到上下文时，高速缓存无效。当这种情况发生时，CGLayer被再次渲染。减慢是因为代码维护了一个单独的CGLayer（ offscreenPrefabCGL ），它在 paintActualGutsOfHugeImage 中循环的每次迭代中修改其上下文。因为 offscreenPrefabCGL的上下文中的操作序列随着每个花而增长，每个渲染需要更多的时间。这解释了每个显示器的时间增加。
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.
我看到两个解决方案：
- PaintView。而不是使完整的视图无效，只使用真正需要重绘的CGRect。根据我的理解，这是@ v01d提出的解决方案。
- 展开图层。而不是持有单个CGLayer，在每次迭代复制CGLayer。这在图层中维护一个平面图像，渲染速度非常快。
我已实施（2），效果良好。前100个花更快，然后它将稳定在120毫秒（在设备）。
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.
我修改的版本位于：
http://dl.dropbox.com/u/9866261/felz_mod_stepwiseDrawingV2.zip
这篇关于神秘的“渐进减速”运行循环中的问题/ drawRect的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
