How to detect TextField cursor lies on which line in Android Jetpack compose

Question

How to detect TextField cursor lies on which line in Android Jetpack compose.

Answer 1

You can get indexes of new line characters '\n' or others and then loop through each of them to get line count.

Result

fun getLine(textFieldValue: TextFieldValue): Int {
    val text = textFieldValue.text
    val selection = textFieldValue.selection.start
    val lineList = mutableListOf< Int>()
    text.forEachIndexed { index: Int, c: Char ->
        if (c == '\n') {
            lineList.add(index)
        }
    }

    if (lineList.isEmpty()) return 1

    lineList.forEachIndexed { index, lineEndIndex ->
        if (selection <= lineEndIndex){
            return index + 1
        }
    }

    return  lineList.size + 1
}

If you wish to get not only line but row too you can do it by using a map

fun getLineAndRowOfSelection(textFieldValue: TextFieldValue): Pair<Int, Int> {
    val text = textFieldValue.text

    val selection = textFieldValue.selection.start
    val lineMap = linkedMapOf<Int, Int>()
    var lineCount = 1

    text.forEachIndexed { index: Int, c: Char ->
        if (c == '\n') {
            lineMap[index] = lineCount
            lineCount++
        }
    }

    if (lineMap.isEmpty()) {
        return Pair(1, selection)
    } else if (lineMap.keys.last() < selection) {
        // Selection is in a row after last new line char
        val key = lineMap.keys.last()
        val lastLine = lineMap[key] ?: 0
        return Pair(lastLine + 1, selection - key - 1)
    } else {
        // Selection is before last new line char
        var previousLineIndex = -1
        lineMap.forEach { (lineEndIndex, line) ->
            if (selection <= lineEndIndex) {
                // First line
                return if (previousLineIndex == -1) {
                    Pair(line, selection)
                } else {
                    Pair(line, selection - previousLineIndex - 1)
                }
            }

            previousLineIndex = lineEndIndex
        }
    }

    return Pair(-1, -1)
}

Demo

@Preview
@Composable
private fun TextSelectionTest() {
    Column(
        modifier = Modifier.padding(top = 30.dp)
    ) {
        var textFieldValue by remember {
            mutableStateOf(TextFieldValue())
        }

        val lineAndRow = getLineAndRowOfSelection(textFieldValue)
        val line = getLine(textFieldValue)

        Text(
            "Text ${textFieldValue.text}, " +
                    "selection: ${textFieldValue.selection}\n" +
                    "line and row: $lineAndRow\n" +
                    "line: $line"
        )

        TextField(value = textFieldValue, onValueChange = { textFieldValue = it })
    }
}