sizeToFit和sizeThatFits

//文档
//显示是UIView的extension。UILabel, UITextField, UITextView都可以调用
extension UIView {
    ...
    open func sizeThatFits(_ size: CGSize) -> CGSize // return ‘best‘ size to fit given size. does not actually resize view. Default is return existing view size 获取最优size
    open func sizeToFit() // calls sizeThatFits: with current view bounds and changes bounds size. 调用sizeThatFits获取最优size，并修改为最优size
}

//使用
//sizeToFit
textView.sizeToFit()

//使用较多的是sizeThatFits，可用来在固定width时，根据text获取textView的高度
let size = textView.sizeThatFits(CGSize(width: textView.frame.size.width, height: CGFloat.greatestFiniteMagnitude))