如何修复这个bug使用静态编程语言，找不到片段构造函数？

Question

我正在使用Kotlin开发Android应用程序。在我的应用程序中包含带有ViewPager的Tab，因此我实现了这两个tabs.when我移动到另一个活动并编译到制表符查看活动，应用程序获得fsource停止，logcat显示在错误下方。

java.lang.RuntimeException：无法启动活动ComponentInfo{com.crypto.wallet/com.crypto.wallet.activities.MainActivity}：android.support.v4.app.片段$实例化异常：无法实例化片段com.crypto.wallet.activities.接收片段：找不到片段构造函数

MyFragment.kt

@SuppressLint("ValidFragment")
class ReceiveFragment(private val transactionList: List<TransactionEntity>, val appDatabase: AppDatabase, private val direction: TransactionAdapterDirection,
                      val networkDefinitionProvider: NetworkDefinitionProvider) : Fragment(){

    private var linearLayoutManager: LinearLayoutManager? = null

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

    }

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {

        val rootView = inflater.inflate(R.layout.receive_fragment, container, false)
        val recyclerView = rootView.findViewById<RecyclerView>(R.id.transaction_recycler_in) as RecyclerView
        linearLayoutManager = LinearLayoutManager(getActivity(), LinearLayout.VERTICAL, false)
        recyclerView.layoutManager = linearLayoutManager
        recyclerView.adapter = TransactionRecyclerAdapter(transactionList,appDatabase,direction,networkDefinitionProvider)
        recyclerView.setHasFixedSize(true);
        return rootView
    }
}

查看ager.kt

override fun getItem(position: Int): Fragment? {
        var fragment: Fragment? = null
        when (position) {
            //0 -> fragment = ReceiveFragment(MytransactionList,MyappDatabase,Myincoming,MynetworkDefinitionProvider)
            0 -> fragment = ReceiveFragment(MyIT,MyAppDatabase,MyIncoming,MyNetwork)
            1 -> fragment = SendFragment()
        }

        return fragment
    }

主要的kt OnCreate（）

val viewPager = findViewById<ViewPager>(R.id.viewpager)
                if (viewPager != null) {
                    val adapter = ViewPagerAdapter(supportFragmentManager,it,appDatabase,INCOMING,networkDefinitionProvider)
                    viewPager.adapter = adapter
                }

 val pref = applicationContext.getSharedPreferences("MyPref", 0) // 0 - for private mode
                val editor = pref.edit()
                val gson = Gson()
                val json = gson.toJson(it)
                editor.putString("MyObject", json)
                editor.apply()

OnResume（）

val prefs = applicationContext.getSharedPreferences("MyPref", 0)
        val gson = Gson()
        val json = prefs.getString("MyObject", "")
        val person1: List<TransactionEntity> = gson.fromJson(json, ArrayList<TransactionEntity>()::class.java)

更新：在我尝试了这个之后，我得到了同样的错误（我尝试了Bundle和Shared pref）：

companion object {
        @JvmStatic
        fun newInstance(myIT: List<TransactionEntity>, myAppDatabase: AppDatabase, myIncoming: TransactionAdapterDirection,
                        myNetwork: NetworkDefinitionProvider,
                        bundle: Bundle) =
                ReceiveFragment(myIT, myAppDatabase, myIncoming, myNetwork,bundle).apply {
                    arguments = Bundle().apply {
                       /* val prefs = getActivity()!!.getApplicationContext().getSharedPreferences("myPrefs", 0)
                        val gson = Gson()
                        val json = prefs.getString("MyObject", "")
                        val person1: List<TransactionEntity> = gson.fromJson(json,
                                ArrayList<TransactionEntity>()::class.java)
                        Log.d("Karthi", "Frag-GSON " + person1)*/

                        val dd = bundle.getSerializable("MySerializable")
                        Log.d("Karthi", "Frag-GSON " + dd)
                    }
                }
    }

主要的kt:

val bundle = Bundle()
                val gson_budle = Gson()
                val json_bundle = gson_budle.toJson(it)
                bundle.putSerializable("MySerializable", json_bundle)

                val sharedPreferences = getSharedPreferences(myPreferences, Context.MODE_PRIVATE)
                val editor = sharedPreferences.edit()
                val gson = Gson()
                val json = gson.toJson(it)
                editor.putString("MyObject", json)
                editor.putString("MyObject_json_bundle", json_bundle)
                Log.d("Karthi","After - IT" + json)
                Log.d("Karthi","After - IT" + json_bundle)
                editor.apply()

致命异常：主进程：com。加密。钱包，PID:29313 java。lang.RuntimeException：无法启动活动组件信息{com.crypto.wallet/com.crypto.wallet.activities.MainActivity}：android。支持v4.app。片段$InstantiationException：无法实例化片段com。加密。钱包活动。ReceiveFragment：在android上找不到片段构造函数。支持v4.app。碎片控制器。android上的restoreAllState（FragmentController.java:152）。支持v4.app。碎片活动。android上的onCreate（FragmentActivity.java:330）。支持v7.app。AppCompatActivity。在com上创建（AppCompatActivity.java:84）。加密。钱包活动。主要活动。onCreate（主活动。kt:194）

Answer 1

如前所述，

如果无参数构造函数不可用，则在某些情况下，在状态还原期间会发生运行时异常。

所以请像这样更改代码

class ReceiveFragment : Fragment{

   private val transactionList: List<TransactionEntity>? = null
   val appDatabase: AppDatabase? = null
   private val direction: TransactionAdapterDirection? = null
   val networkDefinitionProvider: NetworkDefinitionProvider? = null

   //Add empty constructor to avoid the exception
   constructor() : super()

   constructor(transactionList: List<TransactionEntity>, appDatabase: AppDatabase, direction: TransactionAdapterDirection, networkDefinitionProvider: NetworkDefinitionProvider){
      this.transactionList = transactionList
      this.appDatabase = appDatabase
      this.direction = direction
      this.networkDefinitionProvider = networkDefinitionProvider
   }
}

Answer 2

您可以做的是创建一个具有零参数的默认构造函数

companion object {
    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment LoginFragment.
     */
    // TODO: Rename and change types and number of parameters
    @JvmStatic
    fun newInstance(param1: String, param2: String) =
            ReceiveFragment().apply {
                arguments = Bundle().apply {
                    putString(ARG_PARAM1, param1)
                    putString(ARG_PARAM2, param2)
                }
// Or from your code, you can use Gson
   val prefs = applicationContext.getSharedPreferences("MyPref", 0)
   val gson = Gson()
   val json = prefs.getString("MyObject", "")
   val person1: List<TransactionEntity> = gson.fromJson(json, 
   ArrayList<TransactionEntity>()::class.java)

            }
}

你可以像这样使用它

   0 -> receiveFragment= ReceiveFragment.newInstance(firstParam, secondParam)

您可以在onCreate中接收此参数

 override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    arguments?.let {
        email = it.getString(ARG_PARAM1)
        password = it.getString(ARG_PARAM2)

    }
  // Or from your code, you can use Gson
   val prefs = applicationContext.getSharedPreferences("MyPref", 0)
   val gson = Gson()
   val json = prefs.getString("MyObject", "")
   val person1: List<TransactionEntity> = gson.fromJson(json, 
   ArrayList<TransactionEntity>()::class.java)
}

你的片段构造函数像这样

class ReceiveFragment: Fragment()

希望这有帮助

Answer 3

当您获得错误的简短描述时，您将看到：

避免在片段中使用非默认构造函数：使用默认构造函数加上片段setArguments（Bundle）而不是更少。从片段留档：

每个片段都必须有一个空构造函数，以便在恢复其活动状态时可以实例化它。强烈建议子类不要有其他带参数的构造函数，因为当片段重新实例化时，不会调用这些构造函数；相反，参数可以由调用方提供，并由片段检索。

例如：

 fun newInstance(bundle : Bundle) : ReceiveFragment{
        val fragment = ReceiveFragment()
         fragment.arguments=bundle           
        return fragment
    }

现在调用片段：

 0 -> fragment = ReceiveFragment.newInstance(new Bundle)

更多信息：http://developer.android.com/reference/android/app/Fragment.html#Fragment（）