Kotlin collaboration - flow+room database

Import dependence ：

implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.2.0"
def room_version = "2.3.0"
implementation "androidx.room:room-runtime:$room_version"
implementation "androidx.room:room-ktx:$room_version"
kapt "androidx.room:room-compiler:$room_version"
implementation 'androidx.recyclerview:recyclerview:1.2.1'

/**
 * entities     A table stored in a database , But many 
 * version       Version number of the database 
 * exportSchema     Whether to generate json file , Used to view the structure of the database 
 */
@Database(entities = [User::class],version = 1,exportSchema = false)
abstract class AppDataBase : RoomDatabase() {

    abstract fun UserDao():UserDao  //Dao  object 

    companion object{
        private var instance:AppDataBase?=null

        fun getInstance(context:Context):AppDataBase{
                                // Object lock 
            return instance?: synchronized(this) {
                                    // Context                            Database name 
                Room.databaseBuilder(context,AppDataBase::class.java,"user_db")
                    .build().also { instance=it }
            }
        }
    }
}

database dataBase file

@Dao //Dao  Class declaration 
interface UserDao  {
    // Here is a conflict ,  If two records are the same, they will be replaced 
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insert(user: User)

    // There is no need to suspend （ return flow or livedata No need ）
    @Query("SELECT * FROM user")
    fun getAll(): Flow<List<User>>
}

database Dao class

//Room Entity class declaration 
@Entity
data class User(
        // Primary key 
        @PrimaryKey val uid:Int,
        // Field names stored in the database 
        @ColumnInfo(name = "first_name")val  firstName:String,
        // Field names stored in the database 
        @ColumnInfo(name = "last_name") val lastName :String
)

Database entity class

class UserViewModel(app: Application) :AndroidViewModel(app) {

    fun insert(uid:String,firstName:String,lastName:String){
            // coroutines 
            viewModelScope.launch { 
                AppDataBase.getInstance(getApplication()).UserDao()
                        // insert data 
                    .insert(User(uid.toInt(),firstName,lastName))
                Log.d("ning","insert user:$uid")
            }
    }

    fun getAll(): Flow<List<User>>{
        return AppDataBase.getInstance(getApplication())
            .UserDao().getAll().catch {
                e-> // There is an abnormality in the upstream , Under the guarantee of not breaking flow Where principles are involved, it is recommended to use catch function 
                e.printStackTrace()
            }.flowOn(Dispatchers.IO)// Switching thread 
    }
}

class BindingViewHolder (val binding:ViewBinding):RecyclerView.ViewHolder(binding.root){


}

Packaged ViewHolder

class UserAdapter(private val context: Context) : RecyclerView.Adapter<BindingViewHolder>() {
    private val data=ArrayList<User>()

    fun setData(data:List<User>){
        this.data.clear();
        this.data.addAll(data)
        notifyDataSetChanged()
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BindingViewHolder {
        val inflate = ItemUserBinding.inflate(LayoutInflater.from(context), parent, false)
        return BindingViewHolder(inflate)
    }

    override fun onBindViewHolder(holder: BindingViewHolder, position: Int) {
       val item=data[position]
        val itemUserBinding = holder.binding as ItemUserBinding
        itemUserBinding.text.text="${item.uid},${item.firstName},${item.lastName}"
    }

    override fun getItemCount(): Int {
       return data?.size
    }
}

class UserFragment :Fragment() {
        // Here you create an object directly 
        private val viewModel by viewModels<UserViewModel> ()

    private val mBinding: FragmentUserBinding by lazy {
        FragmentUserBinding.inflate(layoutInflater)
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return mBinding.root
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        mBinding.apply {
            btnAddUser.setOnClickListener {
                viewModel.insert(
                    etUserId.text.toString(),
                    etFirstName.text.toString(),
                    etLastName.text.toString()

                )
            }
        }
        context?.also {
            val adapter=UserAdapter(it)
            mBinding.recyclerView.adapter=adapter
            // coroutines 
            lifecycleScope.launchWhenCreated {
                //flow Take out 
                viewModel.getAll().collect {
                    value ->
                    adapter.setData(value)

                }
            }
        }

    }

}

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <data>

    </data>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        tools:context=".fragment.UserFragment"
        >

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:padding="5dip">

            <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:orientation="horizontal">

                <EditText
                    android:id="@+id/et_user_id"
                    android:layout_width="0dip"
                    android:layout_height="wrap_content"
                    android:layout_weight="1"
                    android:hint="USER ID" />

                <EditText
                    android:id="@+id/et_first_name"
                    android:layout_width="0dip"
                    android:layout_height="wrap_content"
                    android:layout_weight="1"
                    android:hint="FIRST NAME" />

                <EditText
                    android:id="@+id/et_last_name"
                    android:layout_width="0dip"
                    android:layout_height="wrap_content"
                    android:layout_weight="1"
                    android:hint="LAST NAME" />

            </LinearLayout>

            <Button
                android:id="@+id/btn_add_user"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="ADD USER" />
        </LinearLayout>

        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/recyclerView"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical"
            app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager" />
    </LinearLayout>
</layout>

This is the Lord XML file

Effect display ：