Challenge 2: Pass Data to the Second Activity — Solution

// MainActivity.kt — inside onCreate, after binding is set up
binding.goToDetailButton.setOnClickListener {
    val message = binding.messageInput.text.toString()

    val intent = Intent(this, DetailActivity::class.java)
    intent.putExtra("MESSAGE", message)
    startActivity(intent)
}

// DetailActivity.kt
class DetailActivity : AppCompatActivity() {
    private lateinit var binding: ActivityDetailBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityDetailBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val message = intent.getStringExtra("MESSAGE") ?: "No message received"
        binding.detailText.text = message
    }
}

Notes:
- binding.messageInput.text.toString() reads the EditText's current
  content at the moment the button is clicked, not continuously — a
  fresh read each click.
- intent.getStringExtra("MESSAGE") returns String? (nullable) since the
  key might not exist at all — the ?: "No message received" Elvis
  fallback (Kotlin Fundamentals Chapter 3) handles that case cleanly,
  covering both "DetailActivity launched with no extra" and "the key
  genuinely wasn't set" scenarios.
- Using a shared constant (e.g. const val EXTRA_MESSAGE = "MESSAGE",
  referenced from both Activities) instead of the raw string literal
  "MESSAGE" in both places would remove the risk of a typo mismatch
  between sender and receiver — worth doing in real code, simplified
  here for clarity.