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iphone - run loop/drawRect 中神秘的 "progressive slowing"问题

coder 2023-09-21 原文

这是一个真正的谜。

假设您正在执行在屏幕外绘制复杂图像 (CGLayer) 的传统过程，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，并根据需要调用前台的 setNeedsDisplays，从而更新图像。这很简单，只需两行代码即可实现。

但是，当您这样做时，会出现一个神秘的问题:每次绘制周期所花费的时间，每次都在增加，变得无法使用。此外，时间可能会不规律地变化。

这是一个已知的 iOS 错误，还是？有人以前见过这个吗？

这是一个非常简单、非常容易理解的应用程序，它演示了这个问题:

http://www.fileswap.com/dl/p8lU3gAi/stepwiseDrawingV2.zip.html

输出示例如下。

有人知道为什么会这样吗？这是操作系统的非常奇怪的行为。

稍后 ... FELZ 想出了一个惊人的解决方法。 Felz 每轮复制 CGLayer 一次。这完全阻止了奇怪的行为。

然而，对于到底发生了什么，仍然没有真正清楚的理解:因此，从字面上看，当这个奇怪的问题出现时，时间用在了哪里？

这是一个很长的输出示例......

请注意，您有时会得到如下所示的“双重怪异”结果:它表现出“正常”的怪异行为:每转一圈时间都会增加。然而，偶尔时间会下降到“非常快”几轮，然后返回。很奇怪吧？此外，如果您在模拟器上运行，请快速前后点击 Mac 上不相关的应用程序以获得“更奇怪”的结果。

虽然 Felz 给出了完美的工作答案，但实际机制仍然是个谜。

:26:56.697 stepwiseDrawing[5334:1a03] time difference was     0
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最佳答案

首先让我说这是一个有趣的谜题。我在这方面工作很开心。很好的问题和很好的示例代码。

这是我对可能的答案的看法:

我看到的问题如下。 CGLayer 非常适合重复绘制，因为它们将渲染缓存到平面图像中。当附加操作附加到上下文时，缓存将失效。发生这种情况时，CGLayer 会再次渲染。速度变慢的原因是代码维护了一个 CGLayer (offscreenPrefabCGL)，它在 paintActualGutsOfHugeImage 循环的每次迭代中都修改了上下文。由于 offscreenPrefabCGL 的 上下文中的操作序列随着每朵花的增长而增长，因此每次渲染都需要更多时间。这解释了每次显示的时间增加。

我看到两个解决方案:

  1. 将新渲染剪辑到 PaintView 中的特定位置。不是使整个 View 失效，而是仅使真正需要重新绘制的 CGRect 失效。据我了解，这是@v01d 提出的解决方案。
  2. 展平图层。不是持有单个 CGLayer，而是在每次迭代时复制 CGLayer。这会在层中保持平面图像，渲染速度非常快。

我实现了 (2) 并取得了良好的效果。前 100 朵花更快，然后它会在 120 毫秒内稳定下来(在设备中)。我用多达 2000 朵花测试了我的修改版本。

我的修改版本在:

http://dl.dropbox.com/u/9866261/felz_mod_stepwiseDrawingV2.zip

关于iphone - run loop/drawRect 中神秘的 "progressive slowing"问题，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/4786754/

progressiveampstepwiseDrawingdifference1a03iphonecocoaiosdrawrect

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