MainActivity.kt

package com.example.myapplication

import android.content.Context
import android.content.Intent
import android.os.Bundle
import android.text.Editable
import android.text.TextUtils
import android.text.TextWatcher
import android.util.Patterns
import android.view.KeyEvent
import android.view.inputmethod.InputMethodManager
import android.widget.Button
import android.widget.EditText
import android.widget.LinearLayout
import androidx.appcompat.app.AlertDialog
import androidx.appcompat.app.AppCompatActivity


class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val isKeyPressed = KeyEvent.KEYCODE_BACK


        //hide keyboard when touch outside
        val focus = findViewById<LinearLayout>(R.id.linearLayout2)
        focus.setOnClickListener { v ->
            val imm = getSystemService(Context.INPUT_METHOD_SERVICE) as? InputMethodManager
            imm?.hideSoftInputFromWindow(v.windowToken, 0)
        }

        fun isValidEmail(email: String): Boolean {
            return !TextUtils.isEmpty(email) && Patterns.EMAIL_ADDRESS.matcher(email).matches()
        }

        val button = findViewById<Button>(R.id.btnSend)
        val em = findViewById<EditText>(R.id.email)
        val sub = findViewById<EditText>(R.id.subj)
        val msg = findViewById<EditText>(R.id.message)


        //check form fields are empty
        button.setEnabled(false)
        val editTexts = listOf(em, sub, msg)
        for (editText in editTexts) {
            editText.addTextChangedListener(object : TextWatcher {
                override fun onTextChanged(s: CharSequence, start: Int, before: Int, count: Int) {
                    var et1 = em.text.toString().trim()
                    var et2 = sub.text.toString().trim()
                    var et3 = msg.text.toString().trim()

                    button.isEnabled = et1.isNotEmpty()
                            && isValidEmail(em.text.toString())
                            && et2.isNotEmpty()
                            && et3.isNotEmpty()
                    if (!isValidEmail(em.text.toString()) && et1.isNotEmpty())
                        em.error = "Email not valid!"
                }

                override fun beforeTextChanged(
                        s: CharSequence, start: Int, count: Int, after: Int) {
                }

                override fun afterTextChanged(
                        s: Editable) {
                }
            })
        }


        //add event
        button.setOnClickListener {
            val text = findViewById<EditText>(R.id.email)
            val email = text.text.toString()

            val builder = AlertDialog.Builder(this)
            //set title for alert dialog
            builder.setTitle("Send Email")
            //set message for alert dialog
            builder.setMessage("Do you really want to send this email to $email")
            builder.setIcon(android.R.drawable.ic_dialog_alert)
            //performing no action
            builder.setNegativeButton("No") { dialog, _ -> dialog.dismiss() }
            //performing yes action
            builder.setPositiveButton("Yes") { dialog, _ ->
                val intent = Intent(this, MainActivity2::class.java)
                startActivity(intent)
            }
            // Create the AlertDialog
            val alertDialog: AlertDialog = builder.create()
            // Set other dialog properties
            alertDialog.setCancelable(false)
            alertDialog.show()
        }
    }
    //exit the app!
    override fun onBackPressed() {
        finishAffinity()
    }
}