通过 range 来获取对应内容的 Frame，获取到的Frame 基于 当前的UITextView

import Foundation

extension UITextView {
    
    // MARK: -- 通过Range 获取对应的frame（相对于 textView）
    public func getRangeRect(_ textView: UITextView, _ range: NSRange) -> CGRect {
        
        let beginning = textView.beginningOfDocument
        
        guard let start = textView.position(from: beginning, offset: range.location) else {
            return .zero
        }
        guard let end = textView.position(from: start, offset: range.length) else {
            return .zero
        }
        
        guard let textRange = textView.textRange(from: start, to: end) else {
            return .zero
        }
    
        return textView.firstRect(for: textRange)
    }
}