Kotlin: lateinit var 需要初始化

Question

我在这里为困境和问题而苦苦挣扎。困境是我想让 ManagerUsers 屏幕保持简单，但谷歌正在推动碎片化。稍后我可以看到此屏幕的片段的用途，但就目前而言，这似乎有点矫枉过正。无论如何我应该使用它们吗？

其次，如果我现在确实避免片段，我将努力完成 ClickListener 的设置以简单地更改 ListView 中的用户名。 Kotlin 似乎至少有 6 种不同的方法来做大多数事情，这让人很难理解。 Google 的教程强制您使用片段，而其他人的教程并没有真正涵盖我正在尝试做的事情。当我导航到“管理用户”活动时，应用程序崩溃了。 LogCat 显示 userSelect 需要初始化。我不知道该怎么做。

用户列表适配器.kt

package com.neillbarrett.debitsandcredits

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import androidx.recyclerview.widget.DiffUtil
import androidx.recyclerview.widget.ListAdapter
import androidx.recyclerview.widget.RecyclerView
import com.neillbarrett.debitsandcredits.database.UsersTable
import com.neillbarrett.debitsandcredits.databinding.ActivityManageUsersBinding

class UserListAdapter(private val userSelect: (UsersTable?) -> Unit) :
    ListAdapter<UsersTable, UserListAdapter.UserViewHolder>(UsersComparator()) {

    override fun onBindViewHolder(holder: UserViewHolder, position: Int) {
        val current = getItem(position)
        holder.bind(current, userSelect)
    }

    class UserViewHolder(private val binding: ActivityManageUsersBinding) : RecyclerView.ViewHolder(binding.root) {

        fun bind(usersTable: UsersTable?, userSelect: (UsersTable?) -> Unit) {

            binding.root.setOnClickListener( View.OnClickListener {
                userSelect(usersTable)
                binding.etEditName.setText(R.layout.activity_list_of_users.toString())
            })
        }

        companion object {
            fun create(parent: ViewGroup) : UserViewHolder {
                val view: View = LayoutInflater.from(parent.context)
                    .inflate(R.layout.activity_manage_users, parent, false)
                return UserViewHolder(ActivityManageUsersBinding.bind(view))
            }
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.activity_manage_users, parent, false)
        return UserViewHolder.create(parent)
    }

    class UsersComparator : DiffUtil.ItemCallback<UsersTable>() {
        override fun areItemsTheSame(oldItem: UsersTable, newItem: UsersTable): Boolean {
            return oldItem == newItem
        }

        override fun areContentsTheSame(oldItem: UsersTable, newItem: UsersTable): Boolean {
            return oldItem.userName == newItem.userName
        }
    }
}

管理用户.kt

package com.neillbarrett.debitsandcredits

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.text.TextUtils
import android.widget.Button
import android.widget.EditText
import android.widget.Toast
import androidx.activity.viewModels
import androidx.lifecycle.Observer
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import com.neillbarrett.debitsandcredits.database.CreditsAndDebitsApp
import com.neillbarrett.debitsandcredits.database.UsersTable
import com.neillbarrett.debitsandcredits.databinding.ActivityManageUsersBinding

class ManageUsers : AppCompatActivity() {

    lateinit var binding: ActivityManageUsersBinding
    lateinit var recyclerView: RecyclerView
    lateinit var editTextAddUser: EditText
    lateinit var editTextChangeUser: EditText
    lateinit var newUser: String
    lateinit var userSelect: ((UsersTable?) -> Unit)
    var position: Long = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityManageUsersBinding.inflate(layoutInflater)
        val view = binding.root
        setContentView(view)
        //setContentView(R.layout.activity_manage_users)

        val userViewModel: UserViewModel by viewModels {
            UserViewModelFactory((application as CreditsAndDebitsApp).repository)
        }

        recyclerView = findViewById(R.id.rec_view_userList)
        editTextAddUser = findViewById(R.id.et_UserName)
        editTextChangeUser = findViewById(R.id.et_Edit_Name)

        val adapter = UserListAdapter(userSelect)
        binding.recViewUserList.adapter = adapter
        recyclerView.adapter = adapter
        recyclerView.layoutManager = LinearLayoutManager(this)

        userViewModel.allUsers.observe(this, Observer() {user ->
            user?.let { adapter.submitList(it) }
        })

        val btnAddUser = findViewById<Button>(R.id.btn_AddUser)
        btnAddUser.setOnClickListener {
            if (TextUtils.isEmpty(editTextAddUser.text)) {
                Toast.makeText(this, "User name cannot be empty", Toast.LENGTH_SHORT).show()
            } else {
                newUser = editTextAddUser.text.toString()
                userViewModel.insertUser(UsersTable(0, newUser))
            }
        }

        val btnChangeUser = findViewById<Button>(R.id.btn_ChangeUserName)
        btnChangeUser.setOnClickListener {
            if (recyclerView.getChildAdapterPosition(it) == -1) {
                Toast.makeText(this, "Select a name.", Toast.LENGTH_SHORT).show()
            } else {
                if (editTextChangeUser.text.toString() == recyclerView.adapter.toString()) {
                    Toast.makeText(this, "Name has not been changed.", Toast.LENGTH_SHORT).show()
                } else {
                    Toast.makeText(this, "Name would have been changed.", Toast.LENGTH_SHORT).show()
                    /*val rvItemRecId: Long
                    rvItemRecId = adapter.getItemId(position.toInt())
                    userViewModel.updateUser(UsersTable(rvItemRecId.toInt(), adapter.toString()))*/
                }
            }
        }
    }
}

UserViewModel.kt

package com.neillbarrett.debitsandcredits

import androidx.lifecycle.*
import com.neillbarrett.debitsandcredits.database.UsersTable
import kotlinx.coroutines.launch
import java.lang.IllegalArgumentException

class UserViewModel(private val repository: UserRepository) : ViewModel() {

    val allUsers: LiveData<List<UsersTable>> = repository.allUsers.asLiveData()

    fun insertUser(user: UsersTable) = viewModelScope.launch {
        repository.insertUser(user)
    }

    fun updateUser(user: UsersTable) = viewModelScope.launch {
        repository.updateUser(user)
    }

    fun deleteUser(user: UsersTable) = viewModelScope.launch {
        repository.deleteUser(user)
    }
}

class UserViewModelFactory(private val repository: UserRepository) : ViewModelProvider.Factory{
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(UserViewModel::class.java)) {
            @Suppress("UNCHECKED_CAST")
            return UserViewModel(repository) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

Answer 1

如果您检查您的ManageUsers活动实施

lateinit var userSelect: ((UsersTable?) -> Unit)

userSelect 是 lateinit 并且必须在将引用传递给 UserListAdapter 适配器之前对其进行初始化。像这样的东西

userSelect = {
        // do something
    }

在传递给适配器之前，或者

val userSelect: ((UsersTable?) -> Unit) = {
     
}

申报时。