class MainActivity : AppCompatActivity() {

    private lateinit var listAdapter : ItemsAdapter

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

        //create stuff for the ViewModel
        val database = ItemDatabase.getInstance(this)
        val viewModelFactory = ViewModelFactory(ItemRepository(database.itemDao(), Executors.newSingleThreadExecutor()))
        val viewModel = ViewModelProviders.of(this, viewModelFactory).get(MainViewModel::class.java)

        //observe the result and adap UI accordingly
        viewModel.getItems().observe(this, Observer {
            it?.let { res : Resource<List<Item>> ->
                //if there is data, always display it
                it.data?.let {
                    listAdapter.submitList(it)
                }

                when (res.status) {
                    STATUS_LOADING ->  {
                        progressBar.visibility = VISIBLE
                        error.visibility = GONE
                    }
                    STATUS_ERROR -> {
                        progressBar.visibility = GONE
                        error.visibility = VISIBLE
                    }
                    STATUS_SUCCESS -> {
                        progressBar.visibility = GONE
                        error.visibility = GONE
                    }
                }
            }
        })

        listAdapter = ItemsAdapter()
        val recyclerView : RecyclerView = findViewById(R.id.recyclerView)
        recyclerView.apply {
            setHasFixedSize(true)
            adapter = listAdapter
            layoutManager = LinearLayoutManager(context)
        }
    }
}