iphone - UITableView 流畅的滚动性能

Question

是什么导致加载图像时 UITableView 上的滚动如此不稳定？是因为它们需要添加为 subview 吗？还是因为图片需要缓存？两个？

Answer 1

因为它们是作为 subview 添加的，并且绘制 subview 需要进行大量工作，因为它们必须被合成。当 Core Graphics 或 Core Animation 使用这些 subview 绘制多个单元格时更是如此。

Loren Brichter (atebits)更好地解释它:

Much like on Mac OS X, there are two drawing systems on the iPhone. One is CoreGraphics, the other is LayerKit CoreAnimation. CoreGraphics does drawing on the CPU, CoreAnimation does drawing on whatever it thinks is fastest - most likely the GPU.

The GPU on the iPhone hates blending, that’s why Apple recommends that you keep as many of your views opaque as possible. Sometimes you have no choice - if you have a label over an image you are forced to make the label transparent otherwise you get a big ugly block around your text.

What’s a developer to do? Pre-blend of course… with CoreGraphics into your own view. If you blend your stuff together into a single static view on demand (e.g. when a table view moves a cell onscreen), it’s a little bit more expensive for the first frame, but every frame after that CoreAnimation is just dealing with one big, opaque texture… which it loves. It’s more than just the blending too. If you think about what is happening in terms of overdraw, having one big view per table cell is a big win because CoreAnimation will only touch a single given pixel on the screen once rather than multiple times (potentially, depending on how much overlap your old view hierarchy had).