这是一个真正的谜。
想象一下，您正在进行在屏幕外绘制复杂图像的传统过程（CGLayer），并且在绘制时间歇性地将图像更新到屏幕上。
如您所知，执行此操作的方法是：在后台运行大型绘图过程，并根据需要调用setNeedsDisplays的前景，从而更新图像。这是微不足道的，可以通过两行代码实现。
然而，当你这样做时，会出现一个神秘的问题： 每个绘图周期所花费的时间，每次增加，变得无法使用 。此外，时间可能不规律地变化。
这是一个已知的iOS错误，还是？有没有人见过这个？
这是一个非常简单，易于理解的应用程序，可以解决问题：
http://www.fileswap.com/dl/p8lU3gAi/stepwiseDrawingV2.zip.html
下面的输出示例。
有谁知道为什么会这样？操作系统的行为非常奇怪。
稍后 ...... FELZ已经找到了令人震惊的解决方案。 Felz 复制 CGLayer每轮一次。这完全阻止了这种奇怪的行为。
然而，对于究竟发生了什么仍然没有真正清楚的理解：因此从字面上看，当奇怪的问题出现时，时间在哪里？
这是输出的一个很长的例子......
请注意，您有时会得到一个“双重奇怪”的结果，如下所示：它表现出“正常”奇怪的行为：每次都会增加时间。然而，偶尔会有几轮时间“非常快”，然后返回。怪啊？此外，如果您在模拟器上运行，请快速点击Mac上不相关的应用程序，以获得“更奇怪”的结果。
尽管费尔兹给出了一个完美的工作答案，但实际的机制仍然是一个谜。
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答案 0 :(得分：10)
首先让我说这是一个有趣的谜题。我在这个上工作很开心。不错的问题和很好的示例代码。
以下是我对可能答案的看法：
我看到的问题如下。 CGLayer非常适合重复绘画，因为它们将渲染缓存为平面图像。将附加操作附加到上下文时，缓存无效。发生这种情况时，再次渲染CGLayer。减速是因为代码维护单个CGLayer（ offscreenPrefabCGL ），其在 paintActualGutsOfHugeImage 中的循环的每次迭代中都修改了其上下文。由于 offscreenPrefabCGL的上下文中的操作序列随每个花朵而增长，因此每个渲染需要更多时间。这解释了每个显示器的时间增加。
我看到两个解决方案：
我实施了（2）并取得了良好的效果。前100朵花更快，然后它将稳定在120毫秒（在设备中）。我用最多2000朵花测试了我的修改版本。
我的修改版本位于：
http://dl.dropbox.com/u/9866261/felz_mod_stepwiseDrawingV2.zip
答案 1 :(得分：3)
<强> EDIT2：
这是一个有效（无减速）的调试项目：http://bit.ly/eukSgR
这个项目绝不是最佳选择。
这里有趣的是需要位图上下文。 CGLayer缓存您的图像绘制并导致您的渐进式减速。使用位图上下文，它被强制渲染;然后你只需将这些像素推送到drawRect中的UIView的CALayer：。
我想要获得更高的速度，你可以使用setNeedsDisplayInRect：来最小化要重绘的区域。
但是，除了可能是优化或两个优化之外，通过UIView推送像素总是比通过说GL完全在GPU上执行此操作慢。
答案 2 :(得分：0)
作为一项实验，我建议您更新动画进度视图，而不尝试使用或读取未完成的大图像位图或正在进行的图层绘制。从未完成的合成中读取或绘制到另一个绘图上下文可能对GPU管道或绘图状态做了有趣的事情。尝试在胡椒方法中绘制其他东西。一个简单的柜台或其他东西。