Activity & Fragment Lifecycles: Edge Cases

Copy codeonPause → onStop → onSaveInstanceState → onDestroy
→ onCreate (savedInstanceState != null) → onStart → onResume

Copy codeoverride fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    outState.putInt("scrollY", binding.scrollView.scrollY)
    // ViewModel data is NOT saved here — ViewModel survives rotation
}

override fun onRestoreInstanceState(savedInstanceState: Bundle) {
    super.onRestoreInstanceState(savedInstanceState)
    binding.scrollView.scrollY = savedInstanceState.getInt("scrollY")
}

Copy codeonBackPressedDispatcher.addCallback(this) {
    if (viewModel.hasUnsavedChanges()) {
        showDiscardDialog()
    } else {
        isEnabled = false
        onBackPressedDispatcher.onBackPressed()
    }
}

Copy code// ❌ Wrong assumption
override fun onPause() {
    videoPlayer.pause()  // But user might still be watching in multi-window!
}

// ✅ Check visibility
override fun onPause() {
    if (!isInMultiWindowMode) {
        videoPlayer.pause()
    }
}

Copy codeoverride fun onUserLeaveHint() {
    enterPictureInPictureMode(PictureInPictureParams.Builder().build())
}

Copy codeFragment lifecycle:  onCreate ────────────────────────── onDestroy
View lifecycle:               onCreateView ─ onDestroyView

Copy code// ❌ Observing with 'this' (fragment lifecycle)
// viewModel data delivered after onDestroyView → crash accessing binding
viewModel.data.observe(this) {
    binding.textView.text = it.name  // binding may be null!
}

// ✅ Observing with viewLifecycleOwner — stops when view is destroyed
viewModel.data.observe(viewLifecycleOwner) {
    binding.textView.text = it.name  // safe
}

Copy code// ❌ Only called when fragment is the current page
override fun onResume() {
    loadData()  // skipped for offscreen pages!
}

// ✅ Use repeatOnLifecycle with STARTED
viewLifecycleOwner.lifecycleScope.launch {
    repeatOnLifecycle(Lifecycle.State.STARTED) {
        viewModel.uiState.collect { render(it) }
    }
}

Copy codeclass ConfirmDialog : DialogFragment() {
    override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
        return AlertDialog.Builder(requireContext())
            .setTitle("Confirm")
            .setPositiveButton("Yes") { _, _ ->
                // communicate result via shared ViewModel or Fragment Result API
                setFragmentResult("confirm", bundleOf("ok" to true))
            }
            .create()
    }
}