尝试获取 Firebase 云数据库数据并将其显示在 RecyclerView 中的片段中答案

【问题标题】：Trying to fetch Firebase Cloud Database data and show it in RecyclerView in a fragment尝试获取 Firebase 云数据库数据并将其显示在 RecyclerView 中的片段中
【发布时间】：2021-11-07 10:03:17
【问题描述】：

我正在尝试从 Firebase Cloud Datastore 获取 RecyclerView 中的数据。但是当我这样做时，我收到以下错误：

java.lang.RuntimeException：无法反序列化对象。类 models.Post 没有定义无参数构造函数。如果你使用 ProGuard，请确保这些构造函数没有被剥离

这是我正在使用的片段：

class forumFragment : Fragment(R.layout.fragment_forum)  {

    private lateinit var myadapter: MyAdapter

    private lateinit var fragmentForumBinding: FragmentForumBinding

    private lateinit var userArrayList: ArrayList<itemViewModel>
    private lateinit var postDao: postDao



    //val mAuth= Firebase.auth.currentUser
    val mAuth : FirebaseUser? = FirebaseAuth.getInstance().currentUser

    //private lateinit var recyclerView:RecyclerView



    private var mDocReference: DocumentReference ?=null
    //= FirebaseFirestore.getInstance().collection(/).document()
    /*override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
       // recyclerView = view?.findViewById(R.id.recyclerViewForum)!!
        val binding = FragmentForumBinding.inflate(layoutInflater)
        fragmentForumBinding = binding

        setUpRecyclerView()
    }*/

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        val binding = FragmentForumBinding.bind(view)
        fragmentForumBinding = binding

        setUpRecyclerView()
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        val view: View = inflater.inflate(R.layout.fragment_forum, container, false)

        val fabButtom = view.findViewById<FloatingActionButton>(R.id.fabSave)

        fabButtom.setOnClickListener{
            if (mAuth !=null){
                fabButtom.visibility = View.INVISIBLE
                fragmentManager?.beginTransaction()?.addToBackStack(null)?.replace(R.id.forumFragment , addPost())?.commit()
                //val navControl = findNavController(R.id.forumFragment)
            }
            else{
             Toast.makeText(activity ,"Please login to make a query!" , Toast.LENGTH_SHORT).show()
            }

        }

        return view

        }

    private fun setUpRecyclerView() {

       // val recyclerView = view?.findViewById<RecyclerView>(R.id.recyclerViewForum)
        postDao = postDao()

        val postsCollection = postDao.postCollection
        val query = postsCollection.orderBy("createdAt" , Query.Direction.DESCENDING)
        val firestoreRecyclerOptions = FirestoreRecyclerOptions.Builder<Post>().setQuery(query , Post::class.java).build()

        myadapter = MyAdapter(firestoreRecyclerOptions)



        fragmentForumBinding.recyclerViewForum.adapter = myadapter
        fragmentForumBinding.recyclerViewForum.layoutManager = LinearLayoutManager(activity)

    }

    override fun onStart() {
        super.onStart()

        myadapter.startListening()
    }

    override fun onStop() {
        super.onStop()
        myadapter.stopListening()
    }
}

models.Post 类看起来像这样：

data class Post (val descrription: String = "",
                 val createdBy: User = User(),

                 val  currentTime:String = "",
                 val currentDate:String = "",
                 val createdAt:Long )

【问题讨论】：

不清楚您使用的是 Cloud Firestore（默认）、Datatore 模式下的 Cloud Firestore 还是实时数据库，因为您可以互换使用这些术语。
以后格式化代码块时，需要使用反引号字符`（通常在波浪号~键上）而不是撇号'。内联代码一个反引号，代码块三个反引号（在自己的行上）。

标签： android firebase kotlin google-cloud-firestore android-recyclerview

【解决方案1】：

为了让 Firebase SDK 将远程数据转换为所需的模型类，它需要有一个无参数的构造函数。

注意：我已将下面的descrription 更正为description。

在您当前的models.Post 类中，它没有createdAt 的默认值：

data class Post (val description: String = "",
                 val createdBy: User = User(),
                 val currentTime: String = "",
                 val currentDate: String = "",
                 val createdAt: Long ) // <-- missing = 0L

应该是这样的：

data class Post (val description: String = "",
                 val createdBy: User = User(),
                 val currentTime: String = "",
                 val currentDate: String = "",
                 val createdAt: Long = 0L )

或者，您可以对models.Post 使用以下声明：

data class Post (val description: String,
                 val createdBy: User,
                 val currentTime: String,
                 val currentDate: String,
                 val createdAt: Long) {
  // empty constructor for Firebase with dummy data
  constructor(): this("", User(), "", "", 0L)
}

【讨论】：