代码运行效果如图：

import Foundation

class TestVC: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let testV = MyCustomTextView(frame: CGRect(x: 0, y: 130, width: SCREEN_WIDTH - 50, height: 170))
        self.view.addSubview(testV)
        testV.backgroundColor = .darkGray
                
        let ps = NSMutableParagraphStyle()
        ps.lineSpacing = 7
        ps.alignment = .left
        
       let attr = NSMutableAttributedString(string: "字", attributes: [
            .font: UIFont.systemFont(ofSize: 20),
            .foregroundColor: UIColor.blue,
            .paragraphStyle: ps.copy()
        ])
        
        testV.attributedText = attr
    }
}

extension TestVC: UITextViewDelegate {
    func textViewDidChange(_ textView: UITextView) {
        let text = textView.text ?? ""
        
        let ps = NSMutableParagraphStyle()
        ps.lineSpacing = 7
        ps.alignment = .left
        
        let attr = NSMutableAttributedString(string: text, attributes: [
            .font: UIFont.systemFont(ofSize: 20),
            .foregroundColor: UIColor.blue,
            .paragraphStyle: ps.copy()
        ])
        
        textView.attributedText = attr
    }
}

class MyCustomTextView: UITextView {
    override func caretRect(for position: UITextPosition) -> CGRect {
        var rect = super.caretRect(for: position)
        var text = self.text
        // 计算字符串为空时的textView rect, 避免删完字符串时出现的光标变高和上移问题
        // 如果自己还要做换行效果, 那么只在self.text.isEmpty时用sizeToFit
        if self.text.isEmpty {
            //  用"AAgg"可以算出字符串最大高度, 因为一个往上占据位置, 一个往下占据位置.
            text = "AAgg"
        }
        
        let label = UILabel()
        label.font = self.font
        label.text = text
        label.sizeToFit()
        rect.size.height = label.height
        
        return rect
    }
}