MainActivity.kt

fun signInUser(email:String, password:String){
        auth.signInWithEmailAndPassword(email, password)
            .addOnCompleteListener(this) { task ->
                if (task.isSuccessful) {
                    // Sign in success, update UI with the signed-in user's information
                    val user = auth.currentUser
                } else {
                    // If sign in fails, display a message to the user.
                    Toast.makeText(baseContext, "Authentication failed.",
                        Toast.LENGTH_SHORT).show()
                }
            }
    }