运行循环/ drawRect中的神秘“渐进式减速”问题
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，有一个神秘的问题： 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
输出示例如下。
有谁知道为什么会这样？ 这是操作系统非常奇怪的行为。
后来 …… FELZ已经找到了令人震惊的解决方案。 Felz每轮复制一次CGLayer。 这完全阻止了这种奇怪的行为。
然而，仍然没有真正清楚地了解究竟发生了什么：因此从字面上看，当奇怪的问题出现时，使用的时间在哪里？
这是输出的一个很长的例子……
请注意，您有时会得到一个“双重奇异”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。 但是，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，请快速单击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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stepwiseDrawing[5334:1a03] time difference was 282 :27:25.880 stepwiseDrawing[5334:1a03] time difference was 294 :27:26.174 stepwiseDrawing[5334:1a03] time difference was 294 :27:26.470 stepwiseDrawing[5334:1a03] time difference was 296 :27:26.475 stepwiseDrawing[5334:1a03] time difference was 5 :27:26.777 stepwiseDrawing[5334:1a03] time difference was 302 :27:27.077 stepwiseDrawing[5334:1a03] time difference was 299 :27:27.373 stepwiseDrawing[5334:1a03] time difference was 297 首先让我说这是一个有趣的难题。 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了一个CGLayer（ offscreenPrefabCGL ），它在paintActualGutsOfHugeImage的循环的每次迭代中都修改了它的上下文。 因为offscreenPrefabCGL上下文中的操作序列随每个花朵而增长，所以每个渲染都需要更多时间。 这解释了每个显示器的时间增加。
我看到两个解决方案：
- 将新渲染剪辑到PaintView中的特定渲染中。 而不是使整个视图无效，只使实际需要重新绘制的CGRect无效。 据我所知，这是@ v01d提出的解决方案。
- 展平图层。 而不是持有单个CGLayer，在每次迭代时复制CGLayer。 这样可以在图层中保持一个非常快速渲染的平面图像。
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。
我的修改版本是：
http://dl.dropbox.com/u/9866261/felz_mod_stepwiseDrawingV2.zip
EDIT2：
这是一个有效的（没有减速）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; 然后你只是将这些像素推送到drawRect：中的UIView的CALayer。
我想要获得更高的速度，你可以使用setNeedsDisplayInRect：来最小化重绘区域。
但是，除了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上执行此操作要慢。
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会对GPU管道或绘图状态做有趣的事情。 尝试在胡椒方法中绘制其他东西。 一个简单的柜台或什么的。