UIViewAnimationOptions

有时候想用UIview写出不是匀速的动画，

       UIView.animate(withDuration: duration, delay: 0.0, options: .curveEaseOut, animations: { [weak self] in

        }) { (_) in

        }

参数options是UIViewAnimationOptions类型，包含很多选项，其中

UIViewAnimationOptionCurveEaseInOut：开始和结束慢，中间快

Specify an ease-in ease-out curve, which causes the animation to begin slowly, accelerate through the middle of its duration, and then slow again before completing.

UIViewAnimationOptionCurveEaseIn：开始慢，结束时候加速

An ease-in curve causes the animation to begin slowly, and then speed up as it progresses.

UIViewAnimationOptionCurveEaseOut：开始快，结束时候变慢

An ease-out curve causes the animation to begin quickly, and then slow as it completes.

UIViewAnimationOptionCurveLinear：匀速

A linear animation curve causes an animation to occur evenly over its duration.