UIView在布局中最常用的三个属性是frame、bounds和center，CALayer也有类似的属性，分别为frame、bounds、position、anchorPoint.
Position
/* The position in the superlayer that the anchor point of the layer's
* bounds rect is aligned to. Defaults to the zero point. Animatable. */
通过position定义可以知道position是layer中的anchorPoint点在superLayer中的位置坐标.position点是相对superLayer的.
AnchorPoint
/* Defines the anchor point of the layer's bounds rect, as a point in
* normalized layer coordinates - '(0, 0)' is the bottom left corner of
* the bounds rect, '(1, 1)' is the top right corner. Defaults to
* '(0.5, 0.5)', i.e. the center of the bounds rect. Animatable. */
anchorPoint是相对于自身layer，anchorPoint点(锚点)的值是用相对bounds的比例值来确定的，iOS坐标系中(0,0), (1,1)分别表示左上角、右下角.
FlyElephant.png
anchorPoint相当于支点，可以用作旋转变化、平移、缩放.
如果修改anchorPoint则layer的frame会发生改变，position不会发生改变.修改position与anchorPoint中任何一个属性都不影响另一个属性.
原始视图:
let bgView:UIView = UIView(frame: CGRect(x: 100, y: 100, width: 200, height: 200))
bgView.backgroundColor = UIColor.red
self.view.addSubview(bgView)
let midView:UIView = UIView(frame: CGRect(x: 50, y: 50, width: 100, height: 100))
midView.backgroundColor = UIColor.blue
bgView.addSubview(midView)
print("FlyElephant---中间视图的frame:\(midView.layer.frame)")
print("FlyElephant---中间视图的position:\(midView.layer.position)")
print("FlyElephant---中间视图的anchorPoint:\(midView.layer.anchorPoint)")
修改锚点:
let bgView:UIView = UIView(frame: CGRect(x: 100, y: 400, width: 200, height: 200))
bgView.backgroundColor = UIColor.red
self.view.addSubview(bgView)
let midView:UIView = UIView(frame: CGRect(x: 50, y: 50, width: 100, height: 100))
midView.backgroundColor = UIColor.blue
midView.layer.anchorPoint = CGPoint(x: 1, y: 1)
bgView.addSubview(midView)
print("anchorPoint---中间视图的frame:\(midView.layer.frame)")
print("anchorPoint---中间视图的position:\(midView.layer.position)")
print("anchorPoint---中间视图的anchorPoint:\(midView.layer.anchorPoint)")
视图位置计算公式:
frame.origin.x = position.x - anchorPoint.x * bounds.size.width；
frame.origin.y = position.y - anchorPoint.y * bounds.size.height；
FlyElephant.png
如果修改anchorPoint之后不想让视图位置发生改变，那么可以在修改锚点之后重新对frame赋值.
参考资料:
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CoreAnimation_guide/CoreAnimationBasics/CoreAnimationBasics.html#//apple_ref/doc/uid/TP40004514-CH2-SW15
