An InputFilter for Android EditText that limits the number of newlines the user can type into it.

NewlineLimitingInputFilter.kt

import android.text.InputFilter
import android.text.Spanned

class NewlineLimitingInputFilter(val maxLines: Int) : InputFilter {
    var monitor: LimitMonitor? = null

    var numNewlines = 0
    override fun filter(source: CharSequence, start: Int, end: Int, dest: Spanned, dstart: Int, dend: Int): CharSequence? {
        // subtract all newlines in dest range
        for (char in dest.subSequence(dstart, dend)) {
            if (char == '\n') {
                numNewlines--
            }
        }

        var replacement: StringBuilder? = null

        // add newlines in source
        var char: Char
        for (i in start until end) {
            char = source[i]
            if (replacement != null) {
                if (char != '\n')
                    replacement.append(char)
                else if (numNewlines >= maxLines)
                    monitor?.onLimitExceeded(this)
                else
                    numNewlines++
            }
            else if (char == '\n') {
                if (numNewlines >= maxLines) {
                    monitor?.onLimitExceeded(this)
                    replacement = StringBuilder(source.subSequence(start, i))
                }
                else
                    numNewlines++
            }
        }

        return replacement
    }

    interface LimitMonitor {
        fun onLimitExceeded(filter: InputFilter)
    }
}