android - RecyclerView上 Unresolved reference

Question

我能够从Firebase检索数据，但是当我尝试将其加载到groupie适配器中时，recyclerview Activity 未加载，并显示以下错误“ Unresolved reference :由于接收器类型不匹配，以下候选者均不适用....“，我是否缺少依赖关系？？

这是我的 Activity 代码，在fetchUser方法下是出现错误的地方:

import android.os.Bundle
import android.util.Log
import androidx.appcompat.app.AppCompatActivity
import com.google.firebase.database.DataSnapshot
import com.google.firebase.database.DatabaseError
import com.google.firebase.database.FirebaseDatabase
import com.google.firebase.database.ValueEventListener
import com.xwray.groupie.GroupAdapter
import com.xwray.groupie.GroupieViewHolder
import com.xwray.groupie.Item
import kotlinx.android.synthetic.main.activity_new_message.*

class NewMessageActivity : AppCompatActivity() {

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


        // Activity screen title
        supportActionBar?.title = "Select User"

        val adapter = GroupAdapter<GroupieViewHolder>()


        adapter.add(UserItem())
        adapter.add(UserItem())
        adapter.add(UserItem())

        recycler_view_new_message.adapter = adapter

        fetchUsers()

    }
}

private fun fetchUsers() {
    val ref = FirebaseDatabase.getInstance().getReference("/users")
    ref.addListenerForSingleValueEvent(object: ValueEventListener {

        override fun onDataChange(p0: DataSnapshot) {
            val adapter = GroupAdapter<GroupieViewHolder>()


            p0.children.forEach {
                Log.d("NewMessage", it.toString())
                val user = it.getValue(User::class.java)
                adapter.add(UserItem())
            }
            recycler_view_new_message.adapter = adapter /// THIS IS WHERE I GET THE UNRESOLVED REFERENCE ERROR
        }

        override fun onCancelled(p0: DatabaseError) {

        }
    })


}

class UserItem: Item<GroupieViewHolder>() {
    override fun bind(viewHolder: GroupieViewHolder, position: Int) {
    }

    override fun getLayout(): Int {
        return R.layout.user_row_new_message
    }

}

这是gradle依赖项:

    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'androidx.appcompat:appcompat:1.0.2'
    implementation 'androidx.core:core-ktx:1.0.2'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1'

    implementation 'com.google.firebase:firebase-analytics:17.2.0'
    implementation 'com.google.firebase:firebase-auth:19.2.0'

    implementation 'com.google.firebase:firebase-storage:19.1.0'
    implementation 'com.google.firebase:firebase-database:19.1.0'
    implementation 'de.hdodenhof:circleimageview:3.0.1'
    implementation 'androidx.recyclerview:recyclerview:1.1.0'
    implementation "com.xwray:groupie:2.7.0"

    implementation 'com.squareup.picasso:picasso:2.71828'


}

apply plugin: 'com.google.gms.google-services'

这是我的recyclerView XML配置:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    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"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".NewMessageActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler_view_new_message"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


</androidx.constraintlayout.widget.ConstraintLayout>

解决方案:
最终这是一个非常愚蠢的错误，我意识到我已经将fetchUsers()方法留在NewMessageActivity类之外，所以我将该方法移回了类中，并且能够引用RecyclerView组件。

Answer 1

在设置适配器之前，使用以下代码定义对RecyclerView的引用。即

在类中位于它上方的onCreate函数外部。如下定义您的RecyclerView。
lateinit var recycler_view_new_message: RecyclerView

在 recycler_view_new_message.adapter =适配器之前的onCreate函数内部，添加以下代码。
recycler_view_new_message = findViewById<RecyclerView>(R.id. recycler_view_new_message)