Instantly format a UITextView using AttributedString

I'm developing a macOS rich-text editor that applies pre-defined style for each line of the text view.

To format the lines, I'm using NSAttributedString, then, I'm inserting that string into my UITextView. To make things easier, I'm using a tool called SwiftRichString.

My code looks like below. It's straight-forward and works fine.

import Cocoa
import SwiftRichString
import AppKit

class ViewController: NSViewController {
    @IBOutlet var textView: NSTextView!
    
    override func viewDidLoad() {
        super.viewDidLoad()

        // Format the string
        let style = Style {
            $0.font = NSFont(name: "Arial", size: 20)
            $0.color = NSColor.black
            $0.alignment = .center
        }
        let attributedText = "Hello World!".set(style: style)

        // Add formatted string to the text view
        textView.textStorage?.append(attributedText)
    }

    override var representedObject: Any? {
        didSet {
        // Update the view, if already loaded.
        }
    }

}

Current situation:

User is typing a formatted line. Then when user hits Return and types something, format of the new line returns back to the default style of the UITextView.

What I want:

User is typing a certain formatted line, then he hits Return. The next line should be formatted to another pre-defined style on-the-go.

Example:

1. User is typing the Title line. Current style is (Arial, bold, 20pt).
2. He hits Return.
3. Next line should be styled as Normal Text using a pre-defined style (Arial, 12pt).

Important Note:

In my above code, I was able to format the line easily because it's hard-coded. My real issue is, how can I instantly format the next line, because the next line will be entered by the user. The style should be applied to the next line before user begins writing it.

from Instantly format a UITextView using AttributedString