使用Room、ViewModel和LiveData更改数据后重新加载RecyclerView

Question

我试图解决一个问题，但没有成功。每当数据库（数据库室）中特定模型的记录发生变化时，我想更新我的回收器视图。我使用ViewModel来处理模型数据，记录列表存储在LiveData中。

数据库

@Database(entities = arrayOf(Additive::class), version = ElementDatabase.DB_VERSION, exportSchema = false)
    abstract class ElementDatabase() : RoomDatabase() {

        companion object {
            const val DB_NAME : String = "element_db"
            const val DB_VERSION : Int = 1

            fun get(appContext : Context) : ElementDatabase {
                return Room.databaseBuilder(appContext, ElementDatabase::class.java, DB_NAME).build()
            }
        }

        abstract fun additivesModels() : AdditiveDao

    }

模型

@Entity
class Additive {

    @PrimaryKey @ColumnInfo(name = "id")
    var number : String = ""
    var dangerousness : Int = 0
    var description : String = ""
    var names : String = ""
    var notes : String = ""
    var risks : String = ""
    var advice : String = ""
}

刀

@Dao
interface AdditiveDao {

    @Query("SELECT * FROM Additive")
    fun getAllAdditives() : LiveData<List<Additive>>

    @Query("SELECT * FROM Additive WHERE id = :arg0")
    fun getAdditiveById(id : String) : Additive

    @Query("DELETE FROM Additive")
    fun deleteAll()

    @Insert(onConflict = REPLACE)
    fun insert(additive: Additive)

    @Update
    fun update(additive: Additive)

    @Delete
    fun delete(additive: Additive)
}

视图模型

class AdditiveViewModel(application: Application) : AndroidViewModel(application) {

    private var elementDatabase : ElementDatabase
    private val additivesModels : LiveData<List<Additive>>

    init {
        this.elementDatabase = ElementDatabase.get(appContext = getApplication())
        this.additivesModels = this.elementDatabase.additivesModels().getAllAdditives()
    }

    fun getAdditivesList() : LiveData<List<Additive>> {
        return this.additivesModels
    }

    fun deleteItem(additive : Additive) {
        DeleteAsyncTask(this.elementDatabase).execute(additive)
    }

    private class DeleteAsyncTask internal constructor(private val db: ElementDatabase) : AsyncTask<Additive, Void, Void>() {

        override fun doInBackground(vararg params: Additive): Void? {
            db.additivesModels().delete(params[0])
            return null
        }

    }
}

碎片

class AdditivesFragment : LifecycleFragment() {

    private var viewModel : AdditiveViewModel? = null
    private var adapter : AdditivesAdapter? = null

    companion object {
        fun newInstance() : AdditivesFragment {
            val f = AdditivesFragment()
            val args = Bundle()

            f.arguments = args
            return f
        }
    }

    override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater?.inflate(R.layout.fragment_additives, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        this.adapter = AdditivesAdapter(ArrayList<Additive>())
        this.additives_list.layoutManager = GridLayoutManager(this.context, 2, GridLayoutManager.VERTICAL, false)
        this.additives_list.adapter = this.adapter

        this.viewModel = ViewModelProviders.of(this).get(AdditiveViewModel::class.java)

        this.viewModel?.getAdditivesList()?.observe(this, Observer<List<Additive>> { additivesList ->
            if(additivesList != null) {
                this.adapter?.addItems(additivesList)
            }
        })
        super.onActivityCreated(savedInstanceState)
    }
}

现在，我的问题是为什么观察者只被调用一次（在片段的开始），然后没有被再次调用？我如何让观察者不断地倾听数据库中的变化（插入，更新，删除），以便我的回收器视图可以立即更新？非常感谢任何建议。

Answer 1

这就是你犯错误的地方：

 this.viewModel = ViewModelProviders.of(this).get(AdditiveViewModel::class.java)

当您在片段中时，您正在传递这个，这对某些人来说是非常令人不安的，因为它不是语法错误，而是逻辑错误。您必须传递活动！！相反，它将是这样的：

this.viewModel = ViewModelProviders.of(activity!!).get(AdditiveViewModel::class.java)

更新：
在片段内部观察数据时传递viewLifecycleOwner

mainViewModel.data(viewLifecycleOwner, Observer{})

如果你使用片段Ktx，你可以这样初始化viewModel：

private val viewModel by viewModels<MainViewModel>()

如果您已经安装了viewModelFactory：

private val viewModel by viewModels<MainViewModel>{
   viewModelFactory
}

使用这种方法，您无需调用：

// you can omit this statement completely
viewModel = ViewModelProviders.of(this).get(AdditiveViewModel::class.java)

你只需开始观察数据即可。。