关于UIView

mark一些属性：

clipsToBounds  = YES 使得超出UIView的部分不进行显示

layer层

masksToBounds = YES

/* When true an implicit mask matching the layer bounds is applied to

 * the layer (including the effects of the `cornerRadius' property). If

 * both `mask' and `masksToBounds' are non-nil the two masks are

 * multiplied to get the actual mask values. Defaults to NO.

 * Animatable. */