让我想起了用 Quartz 想顺时针画一个 2pi 的圆，会什么也画不出来。

苹果文档：

Note that using values very near 2π can be problematic. For example,
setting `startAngle' to 0, `endAngle' to 2π, and `clockwise' to true will
draw nothing. (It's easy to see this by considering, instead of 0 and 2π,
the values ε and 2π - ε, where ε is very small.) Due to round-off error,
however, it's possible that passing the value `2 * M_PI' to approximate
2π will numerically equal to 2π + δ, for some small δ; this will cause a
full circle to be drawn.

If you want a full circle to be drawn clockwise, you should set
`startAngle' to 2π, `endAngle' to 0, and `clockwise' to true. This avoids
the instability problems discussed above.