在Kotlin中將值從Activity傳遞給Fragment

Question

我在項目中創建了一個底部導航活動，其中包含一個活動和兩個片段。 在Main Activity中，我將值存儲在變量中，但是如果將值傳遞給片段，則會收到NullPointer Exception錯誤。 我在項目中使用kotlin，對您的幫助表示贊賞。 期望

Get Value into Fragment from MainActivity. MainActivity--->TestOneFragment

使用語言

Kotlin

主要活動

class Test : AppCompatActivity(), BottomNavigationView.OnNavigationItemSelectedListener
{
    private val KEY_POSITION = "keyPosition"
    private var navPosition: BottomNavigationPosition = BottomNavigationPosition.ONE
    private lateinit var toolbar: Toolbar
    private lateinit var bottomNavigation: BottomNavigationView

    override fun onCreate(savedInstanceState: Bundle?)
    {
        super.onCreate(savedInstanceState)
        restoreSaveInstanceState(savedInstanceState)
        setContentView(R.layout.activity_test)
        toolbar = findViewById(R.id.toolbar)
        bottomNavigation = findViewById(R.id.bottom_navigation)
        setSupportActionBar(toolbar)
        initBottomNavigation()
        initFragment(savedInstanceState)
        var Name:String=intent.getStringExtra("name")
        println("Test CLLicked: $Name")


        //This code is to pass the value to Fragment
        var bundle=Bundle()
        bundle.putString("name",Name)
        var frag=TestFragment()
        frag.arguments=bundle
    }

    override fun onSaveInstanceState(outState: Bundle?)
    {
        outState?.putInt(KEY_POSITION, navPosition.id)
        super.onSaveInstanceState(outState)
    }

    override fun onNavigationItemSelected(item: MenuItem): Boolean
    {
        navPosition = findNavigationPositionById(item.itemId)
        return switchFragment(navPosition)
    }

    private fun restoreSaveInstanceState(savedInstanceState: Bundle?)
    {
        savedInstanceState?.also {
            val id = it.getInt(KEY_POSITION, BottomNavigationPosition.ONE.id)
            navPosition = findNavigationPositionById(id)
        }
    }

    private fun initBottomNavigation()
    {
        bottomNavigation.active(navPosition.position)
        bottomNavigation.setOnNavigationItemSelectedListener(this)
    }

    private fun initFragment(savedInstanceState: Bundle?)
    {
        savedInstanceState ?: switchFragment(BottomNavigationPosition.ONE)
    private fun switchFragment(navPosition: BottomNavigationPosition): Boolean {
        return supportFragmentManager.findFragment(navPosition).let {
            if (it.isAdded) return false
            supportFragmentManager.detach() // Extension function
            supportFragmentManager.attach(it, navPosition.getTag()) // Extension function
            supportFragmentManager.executePendingTransactions()
        }
    }
    private fun FragmentManager.findFragment(position: BottomNavigationPosition): Fragment
    {
        return findFragmentByTag(position.getTag()) ?: position.createFragment()
    }
}

TestOneFragment

 override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,savedInstanceState: Bundle?): View? {
    val testName= arguments!!.getString("name")
....
}

錯誤

kotlin.KotlinNullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)

Answer 1

這是用於創建Fragments的newInstance模式的示例。

這是在companion object ，這幾乎只是說“這些東西是靜態的”的一種方式。

首先，您應該為Bundle名稱定義常量，這將有助於使所有內容保持一致。 接下來，定義一個采用您的參數（例如name的newInstance方法。

在其中，您將創建Fragment並將其返回。 這樣，您的Activity就不必擔心Bundle或其他任何東西。 您所有的邏輯都在一個Fragment內，用於存儲/檢索，所有這些都在您的Fragment 。

class TestOneFragment {

    companion object {
        const val ARG_NAME = "name"


        fun newInstance(name: String): TestOneFragment {
            val fragment = TestOneFragment()

            val bundle = Bundle().apply {
                putString(ARG_NAME, name)
            }

            fragment.arguments = bundle

            return fragment
        }
    }

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val name = arguments?.getString(ARG_NAME)
        // ...
    }
}

現在，您可以通過執行以下操作輕松獲得Fragment 。

class Test : AppCompatActivity(), BottomNavigationView.OnNavigationItemSelectedListener {

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

        val name = intent.getStringExtra("name")

        // Creating the new Fragment with the name passed in.
        val fragment = TestFragment.newInstance(name)
    }

}

希望有幫助！