分类器没有伴随对象

Question

我想在我的应用程序中使用 BottomNavigationView，我在使用 kotlin 时遇到了这个问题（以前从未遇到过 java），我看到了这条消息：分类器 'listFragment' 没有伴随对象，因此必须在此处初始化

这是我的代码：

private val mOnNavigationItemSelectedListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
    when (item.itemId) {
        R.id.listNav -> {
//the problem is here in listFragment word below
            setFragment(listFragment)
            return@OnNavigationItemSelectedListener true
        }
        R.id.accountNav -> {
//the problem is here also in accountFragment word below
            setFragment(accountFragment)
            return@OnNavigationItemSelectedListener true
        }
false 
}
private fun setFragment(fragment: Fragment) {
    supportFragmentManager.beginTransaction().replace(R.id.mainFrame , fragment).commit()
}

感谢任何帮助:)

Answer 1

我以这种方式编辑它并且它起作用了：

private val mOnNavigationItemSelectedListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
    when (item.itemId) {
        R.id.listNav -> {
            val mFragment = cartFragment.newInstance()
            openFragment(mFragment)
            return@OnNavigationItemSelectedListener true
        }
        R.id.cartNav -> {
            val mFragment = cartFragment.newInstance()
            openFragment(mFragment)
            return@OnNavigationItemSelectedListener true
        }
        R.id.supportNav -> {
            val mFragment = supportFragment.newInstance()
            openFragment(mFragment)
            return@OnNavigationItemSelectedListener true
        }
        R.id.accountNav -> {
            val mFragment = accountFragment.newInstance()
            openFragment(mFragment)
            return@OnNavigationItemSelectedListener true
        }
    }
    false
}
private fun openFragment(fragment: Fragment) {
    val transaction = supportFragmentManager.beginTransaction()
    transaction.replace(R.id.mainFrame, fragment)
    transaction.addToBackStack(null)
    transaction.commit()
}

片段是这样的：

class listFragment : Fragment() {

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

companion object {
    fun newInstance(): listFragment = listFragment()
}

}

Answer 2

如果有人正在为 when 块中产生的标题错误而苦苦挣扎（就像我一样），当比较 sealed object 时，不要忘记 is 关键字，像这样：

when (someSealedClass) {
        is SomeSealedClass.Foo -> ...
//      ^^ don't forget this
      }

分类器没有伴随对象

classifier does not have a companion object

fragment

kotlin

companion-object

android-studio