iphone - 运行循环/drawRect 中的神秘 "progressive slowing"问题

这是一个真正的谜。

想象一下，您正在执行在屏幕外绘制复杂图像 (CGLayer) 的常规过程，并且您在绘制时间歇性地将图像更新到屏幕上。

如您所知，这样做的方法是:在后台运行大型绘图过程，并根据需要调用前台的 setNeedsDisplays，从而更新图像。这是微不足道的，只需两行代码即可实现。

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，每次都增加，变得不可用 。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？有没有人见过这个？

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

下面的输出示例。

有谁知道为什么会这样？从操作系统来看，这是非常奇怪的行为。

后来 ... 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，仍然没有真正清楚地了解到底发生了什么:因此，从字面上看，当这个奇怪的问题出现时，时间在哪里使用？

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”怪异行为:每次运行时间都会增加。但是，有时时间会下降到“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，请快速来回单击 Mac 上不相关的应用程序，以获得“更奇怪”的结果。

尽管 Felz 给出了一个完美的工作答案，但实际的机制仍然是个谜。

: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。我在这方面工作很有趣。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，缓存失效。发生这种情况时，CGLayer 将再次呈现。速度变慢是因为代码维护了一个单独的 CGLayer ( offscreenPrefabCGL )，它在 paintActualGutsOfHugeImage 的每次循环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着每朵花的增长而增长，因此每次渲染需要更多时间。这解释了每次显示的时间增加。

我看到两种解决方案:

将新渲染剪辑到 PaintView 中的特定区域。不是使整个 View 无效，而是仅使真正需要重新绘制的 CGRect 无效。据我了解，这是@ v01d提出的解决方案。

展平图层。每次迭代时复制 CGLayer，而不是持有单个 CGLayer。这会在图层中保持一个渲染速度非常快的平面图像。

我实现了(2)，结果很好。前 100 朵花更快，然后会在 120 毫秒内稳定(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