Android Pin/Code Entry Hack

fragment_pin_entry.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp">

    <EditText
        android:id="@+id/textInput"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="@string/enter_pin"
        android:importantForAutofill="no"
        android:inputType="number"
        android:maxLength="4"
        android:visibility="invisible"
        android:textIsSelectable="false"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/digit1"
        android:layout_width="72dp"
        android:layout_height="80dp"
        android:layout_marginTop="64dp"
        android:gravity="center"
        app:layout_constraintEnd_toStartOf="@+id/digit2"
        app:layout_constraintHorizontal_chainStyle="spread"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/digit2"
        android:layout_width="72dp"
        android:layout_height="80dp"
        android:gravity="center"
        app:layout_constraintBaseline_toBaselineOf="@+id/digit1"
        app:layout_constraintEnd_toStartOf="@+id/digit3"
        app:layout_constraintHorizontal_chainStyle="spread"
        app:layout_constraintStart_toEndOf="@+id/digit1" />

    <TextView
        android:id="@+id/digit3"
        android:layout_width="72dp"
        android:layout_height="80dp"
        android:gravity="center"
        app:layout_constraintBaseline_toBaselineOf="@+id/digit1"
        app:layout_constraintEnd_toStartOf="@+id/digit4"
        app:layout_constraintHorizontal_chainStyle="spread"
        app:layout_constraintStart_toEndOf="@+id/digit2" />

    <TextView
        android:id="@+id/digit4"
        android:layout_width="72dp"
        android:layout_height="80dp"
        android:gravity="center"
        app:layout_constraintBaseline_toBaselineOf="@+id/digit1"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_chainStyle="spread"
        app:layout_constraintStart_toEndOf="@+id/digit3" />

    <View
        android:id="@+id/digit1Underline"
        android:layout_width="56dp"
        android:layout_height="8dp"
        app:layout_constraintEnd_toEndOf="@+id/digit1"
        app:layout_constraintStart_toStartOf="@+id/digit1"
        app:layout_constraintTop_toBottomOf="@+id/digit1" />

    <View
        android:id="@+id/digit2Underline"
        android:layout_width="56dp"
        android:layout_height="8dp"
        app:layout_constraintEnd_toEndOf="@+id/digit2"
        app:layout_constraintStart_toStartOf="@+id/digit2"
        app:layout_constraintTop_toTopOf="@+id/digit1Underline" />

    <View
        android:id="@+id/digit3Underline"
        android:layout_width="56dp"
        android:layout_height="8dp"
        app:layout_constraintEnd_toEndOf="@+id/digit3"
        app:layout_constraintStart_toStartOf="@+id/digit3"
        app:layout_constraintTop_toTopOf="@+id/digit1Underline" />

    <View
        android:id="@+id/digit4Underline"
        android:layout_width="56dp"
        android:layout_height="8dp"
        app:layout_constraintEnd_toEndOf="@+id/digit4"
        app:layout_constraintStart_toStartOf="@+id/digit4"
        app:layout_constraintTop_toTopOf="@+id/digit1Underline" />

</androidx.constraintlayout.widget.ConstraintLayout>

View.kt

        // Within onViewCreated 
        fun updateViewBasedOnPin(pin: String) {
            binding.digit1.text = pin.getOrNull(0)?.toString()
            binding.digit2.text = pin.getOrNull(1)?.toString()
            binding.digit3.text = pin.getOrNull(2)?.toString()
            binding.digit4.text = pin.getOrNull(3)?.toString()
            binding.digit1Underline.setBackgroundColor(if (pin.isNotEmpty()) Color.YELLOW else Color.DKGRAY)
            binding.digit2Underline.setBackgroundColor(if (pin.length > 1) Color.YELLOW else Color.DKGRAY)
            binding.digit3Underline.setBackgroundColor(if (pin.length > 2) Color.YELLOW else Color.DKGRAY)
            binding.digit4Underline.setBackgroundColor(if (pin.length > 3) Color.YELLOW else Color.DKGRAY)
        }

        binding.textInput.afterTextChanged { pin ->
            updateViewBasedOnPin(pin)
        }

        updateViewBasedOnPin("")

        listOf(
            binding.digit1,
            binding.digit2,
            binding.digit3,
            binding.digit4
        ).forEach {
            it.setOnClickListener {
                if (binding.textInput.requestFocus()) {
                    val imm = getSystemService(requireContext(), InputMethodManager::class.java)
                    imm?.showSoftInput(view, InputMethodManager.SHOW_FORCED)
                }
            }
        }