//- Add separator to text each 3 number //- Handle selection when selection position is after separator

NumberTextWatcher.kt

package com.nasserkhosravi.uikit

import android.text.Editable
import android.text.SpannableStringBuilder
import android.text.TextWatcher
import android.widget.EditText
import android.widget.TextView

/**
 * Your [EditText] should have ltr layout direction
 */
class NumberTextWatcher(
    private val edView: EditText,
    private val listener: ((EditText, CharSequence?) -> Unit)?
) : TextWatcher {
    private var beforeText: String = ""

    override fun afterTextChanged(s: Editable?) {
        listener?.invoke(edView, s.toString())
    }

    /*
     * start: is the start index of the red highlighted text (that is about to be deleted)
     * count: is the length of the red highlighted text (that is about to be deleted)
     * after: is the length of the green highlighted text (that is about to be added)
     * */
    override fun beforeTextChanged(p0: CharSequence?, start: Int, count: Int, after: Int) {
        beforeText = p0.toString()
    }

    /*
    * start: is the start index of the green highlighted text (that just got added).
    *        This is the same as the start of beforeTextChanged.
    * before: is the length of the red highlighted text (that just got deleted).
    *         This is the same as the count of beforeTextChanged.
    * count: is the length of the green highlighted text (that just got added).
    *        This is the same as the after of beforeTextChanged.
    * */
    override fun onTextChanged(p0: CharSequence?, start: Int, before: Int, count: Int) {
        if (p0.isNullOrEmpty()) {
            return
        }
        // 1. get cursor position : p0 = start + before
        val initialCursorPosition = start + before

        val targetString = if (isSeparatorRemoved(p0)) {
            StringBuilder(p0).deleteCharAt(initialCursorPosition - 2).toString()
        } else {
            p0
        }
        //2. get digit count after cursor position : c0
        val numOfDigitsToRightOfCursor = getNumberOfDigits(
            beforeText.substring(
                initialCursorPosition,
                beforeText.length
            )
        )
        val newAmount = formatAmount(targetString.toString())

        edView.removeTextChangedListener(this)
        if (newAmount.toString() == "0")
            edView.setText("")
        else {
            edView.setText(newAmount, TextView.BufferType.SPANNABLE)
            //set new cursor position
            edView.setSelection(getNewCursorPosition(numOfDigitsToRightOfCursor, newAmount.toString()))
        }
        edView.addTextChangedListener(this)

    }

    private fun isSeparatorRemoved(p0: CharSequence): Boolean {
        val beforeDigit = getNumberOfDigits(beforeText)
        val newDigit = getNumberOfDigits(p0.toString())
        return newDigit == beforeDigit
    }

    private fun formatAmount(amount: String): SpannableStringBuilder {
        val sNonNumeric = removeNonNumeric(amount)
        val result = sNonNumeric.formatEachBySeparator(3, ",")
        return SpannableStringBuilder(result)
    }

    private fun removeNonNumeric(numberString: String): String {
        val strings = StringBuilder()
        for (i in numberString) {
            if (i.isDigit()) {
                strings.append(i)
            }
        }
        return strings.toString()
    }

    private fun getNewCursorPosition(digitCountToRightOfCursor: Int, numberString: String): Int {
        var position = 0
        var c = digitCountToRightOfCursor
        for (i in numberString.reversed()) {
            if (c == 0)
                break

            if (i.isDigit())
                c--
            position++


        }
        return numberString.length - position
    }

    private fun getNumberOfDigits(text: String): Int {
        var count = 0
        for (i in text)
            if (i.isDigit())
                count++
        return count
    }

    private fun String.formatEachBySeparator(each: Int, separator: String): String {
        val target = this
        val result = StringBuilder()
        for (index in target.indices) {
            if (index % each == 0) {
                if (index > 0) {
                    result.append(separator)
                }
            }
            result.append(target[index])
        }
        return result.toString()
    }

}