IllegalStateException:NavigationDrawer活动中的Fragment内的RecyclerView为null

Question

我试图制作一个RecyclerView,它将进入我片段的两个页面之一.这些页面放在NavigationDrawer活动中.目标是创建类似Play商店应用主页的内容.

但是我在运行时的代码片段中发现了一个错误.它说:

java.lang.IllegalStateException: mainMenu must not be null
    at com.example.MyApp.app.fragment.MainFragment.onCreate(MainFragment.kt:49)

我一直在看一些SO线程,他们说布局没有正确加载.这导致一些元素没有被链接.另一位在评论中说,问题在于上下文未正确初始化.对我来说情况并非如此(相反,它是RecyclerView).

以下是链接,希望它们可以作为参考.

• TextView不能为null
• 上下文不能为空

在对我的代码进行多次检查之后,我发誓我已经把正确的布局放进去了.编辑:我导入了 kotlinx.android.synthetic.main.^ 我放这个标志的包:^ $ {layout}.这是我的一些文件(如果这个帖子太长,请原谅我):

• MainActivity.kt : AppCompatActivity() ^ activity_main.*

  override fun onCreate(savedInstanceState: Bundle?) {
          super.onCreate(savedInstanceState)
          setContentView(R.layout.activity_main)
  
          // preparing the app bars
          setSupportActionBar(toolBar)
  
          // getting ready for the pages
          val pagerAdapter = MainPagerAdapter(
                  supportFragmentManager,
                  resources.getString(R.string.tab_main),
                  resources.getString(R.string.tab_chat)
          )
          pager.adapter = pagerAdapter
  
          // activating tabs
          tabLayout.setupWithViewPager(pager)
  
          val toggle = ActionBarDrawerToggle(
                  this, mainDrawer, toolbar,
                  R.string.navigation_drawer_open,
                  R.string.navigation_drawer_close)
  
          mainDrawer.addDrawerListener(toggle)
  
  
          navView.setNavigationItemSelectedListener(this)
          toggle.syncState()
  
  }
  

• MainPagerAdapter.kt (fm: FragmentManager, private val page1: String, private val page2: String): FragmentPagerAdapter(fm)

  override fun getItem(position: Int): Fragment? {
      return when (position) {
          0 -> MainFragment()
          1 -> ChatFragment()
          else -> null
      }
  }
  
  override fun getCount() = 2
  
  override fun getPageTitle(position: Int): CharSequence? {
      return when (position) {
          0 -> page1
          1 -> page2
          else -> null
      }
  }
  

• MainFragment.kt : Fragment() ^ content_main.*

  override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?)
      : View? = inflater.inflate(R.layout.content_main, container)
  
  override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
  
      // this is the error
      mainMenu.layoutManager = LinearLayoutManager(this.context)
  
      mainMenu.adapter = MyAdapter(itemList) {
          toast("${it.name} selected")
      }
  }
  

• MyAdapter.kt: RecyclerView.Adapter<MyAdapter.MyHolder>() ^ item_custom.view.*(由Antonio Leiva提供)

  override fun onCreateViewHolder(parent: ViewGroup, viewType: Int)
      = GoodsHolder(parent.inflate(R.layout.item_custom))
  
  override fun getItemCount()
      = itemList.size
  
  override fun onBindViewHolder(holder: MyHolder, position: Int)
      = holder.bind(itemList[position], listener)
  
  
  class MyHolder(v: View): RecyclerView.ViewHolder(v){
      private val item: Item? = null
      private val view = v
  
      fun bind(item: Item, listener: (Item) -> Unit)
              = with (itemView) {
          imgPic.setImageResource(item.pictureId)
          txtName.text = item.name
          txtPrice.text = item.price.toString()
  
      setOnClickListener { listener(item) }
      }
  }
  

• content_main.xml

  <android.support.constraint.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"
      app:layout_behavior="@string/appbar_scrolling_view_behavior"
      tools:context="com.example.MyApp.app.activity.MainActivity"
      >
  
      <!-- the RecyclerView that caused the runtime error -->
      <android.support.v7.widget.RecyclerView
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:id="@+id/mainMenu"/>
  
  </android.support.constraint.ConstraintLayout>
  

• item_custom.xml(这些代码位于CardView内的LinearLayout内)

  <ImageView
      android:id="@+id/imgPic"
      android:layout_width="match_parent"
      android:layout_height="128dp"
      app:srcCompat="@drawable/ic_menu_gallery" />
  
  <TextView
      android:id="@+id/txtName"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Get it while it's hot!"
      android:layout_margin="@dimen/margin_small"
      android:layout_marginTop="@dimen/margin_medium"
      android:maxLines="2"
      android:ellipsize="end"
      android:textStyle="bold"
      />
  
  <TextView
      android:id="@+id/txtPrice"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="$3.000.000"
      android:layout_marginLeft="@dimen/margin_small"
      android:layout_marginStart="@dimen/margin_small"
      android:layout_marginRight="@dimen/margin_small"
      android:layout_marginEnd="@dimen/margin_small"
      android:layout_marginBottom="@dimen/margin_medium"/>
  

• ChatFragment.kt:Fragment()(仅包含onCreateView inflating content_main_chat.xml)

• content_main_chat.xml(仅包含TextView)

Answer 1

您尚未在MainFragment.kt文件中初始化RecyclerView .你必须在下面的行之前初始化它:

mainMenu.layoutManager = LinearLayoutManager(this.context)

您可以通过以下行初始化RecyclerView:

var mainMenu = findViewById(R.id.mainMenu) as RecyclerView

你必须根据需要改变它.