2 files changed, 41 insertions, 0 deletions
diff --git a/android/src/main/kotlin/net/mullvad/mullvadvpn/ui/AdvancedFragment.kt b/android/src/main/kotlin/net/mullvad/mullvadvpn/ui/AdvancedFragment.kt
index 6cc28fd865..fbace21ff7 100644
--- a/android/src/main/kotlin/net/mullvad/mullvadvpn/ui/AdvancedFragment.kt
+++ b/android/src/main/kotlin/net/mullvad/mullvadvpn/ui/AdvancedFragment.kt
@@ -6,7 +6,12 @@ import android.view.View
 import android.view.ViewGroup
 import net.mullvad.mullvadvpn.R
 
+private const val MIN_MTU_VALUE = 1280
+private const val MAX_MTU_VALUE = 1420
+
 class AdvancedFragment : ServiceDependentFragment(OnNoService.GoBack) {
+    private lateinit var wireguardMtuInput: CellInput
+
     override fun onSafelyCreateView(
         inflater: LayoutInflater,
         container: ViewGroup?,
@@ -18,6 +23,9 @@ class AdvancedFragment : ServiceDependentFragment(OnNoService.GoBack) {
             parentActivity.onBackPressed()
         }
 
+        wireguardMtuInput =
+            CellInput(view.findViewById(R.id.wireguard_mtu_input), MIN_MTU_VALUE, MAX_MTU_VALUE)
+
         return view
     }
 }
diff --git a/android/src/main/kotlin/net/mullvad/mullvadvpn/ui/CellInput.kt b/android/src/main/kotlin/net/mullvad/mullvadvpn/ui/CellInput.kt
new file mode 100644
index 0000000000..5a582216da
--- /dev/null
+++ b/android/src/main/kotlin/net/mullvad/mullvadvpn/ui/CellInput.kt
@@ -0,0 +1,33 @@
+package net.mullvad.mullvadvpn.ui
+
+import android.text.Editable
+import android.text.TextWatcher
+import android.widget.EditText
+import net.mullvad.mullvadvpn.R
+
+class CellInput(val input: EditText, val minValue: Int, val maxValue: Int) {
+    private val resources = input.context.resources
+
+    private val validInputColor = resources.getColor(R.color.white)
+    private val invalidInputColor = resources.getColor(R.color.red)
+
+    init {
+        input.addTextChangedListener(InputWatcher())
+    }
+
+    inner class InputWatcher : TextWatcher {
+        override fun beforeTextChanged(text: CharSequence, start: Int, count: Int, after: Int) {}
+
+        override fun onTextChanged(text: CharSequence, start: Int, count: Int, after: Int) {}
+
+        override fun afterTextChanged(text: Editable) {
+            val value = text.toString().trim().toIntOrNull()
+
+            if (value != null && value >= minValue && value <= maxValue) {
+                input.setTextColor(validInputColor)
+            } else {
+                input.setTextColor(invalidInputColor)
+            }
+        }
+    }
+}