如何从Thread返回值？

Question

I want to pass arrayList to adapter constructor. Arraylist is returned by query. Query must be in Thread. How can i do that?

CostDao.kt

@Dao
interface CostDAO {

    @Query("select * from Cost")
    fun getAll() : List<Cost>

MainActivity.kt

class ComparatorActivity : AppCompatActivity(), OpenDatabase.OpenDatabaseListener {
    private var db: AppDatabase? = null


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

        val open = OpenDatabase(this)
        open.setOpenDatabaseListener(this)
        open.load()
        val recyclerView = findViewById<RecyclerView>(R.id.recyclerView)
        Threads()


//i need to pass the arraylist
        val adapter = MyAdapter(this, )

        recyclerView.layoutManager = LinearLayoutManager(this, LinearLayout.VERTICAL, false)
        recyclerView.adapter = adapter

}
    fun Threads() {
        Thread {
            val List = db!!.costDAO().getAll()
        }.start()
}

How can i return List from Thread? and then pass it to adapter constructor.

Answer 1

you can pass an empty list to adapter constructor and when the data retrieved from the database call adapter.notifydatasetchanged(), but the list must be the same object.

class ComparatorActivity : AppCompatActivity(), OpenDatabase.OpenDatabaseListener {
    private var db: AppDatabase? = null
    var list = listOf<Cost>()
    private lateinit var adapter: MyAdapter

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

        val open = OpenDatabase(this)
        open.setOpenDatabaseListener(this)
        open.load()
        val recyclerView = findViewById<RecyclerView>(R.id.recyclerView)

        adapter = MyAdapter(this,list)
        Threads()

        recyclerView.layoutManager = LinearLayoutManager(this, LinearLayout.VERTICAL, false)
        recyclerView.adapter = adapter
    }
    fun Threads() {
        Thread {
            list = db!!.costDAO().getAll()
            adapter.notifyDataSetChanged()
        }.start()
}

Answer 2

Create a new instance of the Adapter when data is ready and set it to your recyclerView :

Thread {
    val itemsList...// from db
    runOnUiThread {
        recyclerView.adapter = MyAdapter(this, itemsList)
    }
}.start()

you could alternatively create a setItems method (in your adapter ) and assign it that way:

class MyAdapter... {

    private val items = mutableListOf<Cost>()

    fun setItems(items: List<Cost>) {
        this.items.clear()
        this.items.addAll(items)
        notifyDataSetChanged()
    }
}

and

Thread {
    val itemsList...// from db
    runOnUiThread {
        (recyclerView.adapter as MyAdapter).setItems(itemsList)
    }
}.start()

I also suggest using AsyncTask instead of a regular Thread