如何使用android中的kotlin从片段中获取资源id？

Question

我尝试使用下面提到的代码，但是在运行时会崩溃。发生的错误是Android运行时：

致命异常:主进程: com.root.specialbridge，PID: com.root.specialbridge.fragments.profile_fragment.WallFragments.initializeView(WallFragments.kt:49)：17706 kotlin.KotlinNullPointerException

class WallFragments : Fragment(){

private var wallAdapter: WallAdapter? = null
private var wall_recycler: RecyclerView? = null
private val wallArrayList: ArrayList<Wall>? = null
private var mainlayout: LinearLayout? = null
private var no_result_found_layout: RelativeLayout? = null
private var userProfileWallInterface: UserProfileWallInterface? = null
internal var wallActivityBeanse: MutableList<WallActivityBeans> = ArrayList()

override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    val view = inflater!!.inflate(R.layout.wall_fragments, container, false)
    userProfileWallInterface = UserProfileWallPresentation(activity, this)
    initializeView()
    wallAdapter = WallAdapter(activity, wallActivityBeanse)
    wall_recycler!!.adapter = wallAdapter

    return view
}
fun initializeView() {
    wall_recycler = view!!.findViewById(R.id.wall_recycler_id) as RecyclerView
    mainlayout = view!!.findViewById(R.id.mainlayout) as LinearLayout
    no_result_found_layout = view!!.findViewById(R.id.no_result_found_layout) as RelativeLayout
    wall_recycler!!.layoutManager = LinearLayoutManager(activity)
    wall_recycler!!.setHasFixedSize(true)
    if (AuthPreference(activity).isGetMemberProfile) {
        userProfileWallInterface!!.getMemberProfileWall(view!!)

    } else {
        userProfileWallInterface!!.getUserProfileWall(AuthPreference(activity).token, AuthPreference(activity).user.id, view!!)

    }
}   
companion object {
    val instance: WallFragments
        get() = WallFragments()  }}

Answer 1

添加

apply plugin: 'kotlin-android-extensions'

在应用程序级别的gradle文件和

导入

import kotlinx.android.synthetic.main.fragment_your_fragment_name.view.*

在片段的onCreateView中(例如，如果文本视图的id是textView)

override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
    // Inflate the layout for this fragment
    val view = inflater!!.inflate(R.layout.fragment_splashfragment, container, false)
    view.textView.text = "hello"   //add your view before id else getting nullpointer exception
    return view
}

更新：

在类中而不是视图中声明viewOfLayout。

class yourfragment:Fragment(){

    private lateinit var viewOfLayout: View
    override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
        // Inflate the layout for this fragment
        viewOfLayout = inflater!!.inflate(R.layout.fragment_splashfragment, container, false)
        viewOfLayout.textView.text = "hello"   //add your view before id else will get nullpointer exception
        return viewOfLayout
    }

}

Answer 2

介绍了Kotlin Android扩展。

您不必再使用findViewById了。使用这个插件，您可以直接将UI组件作为全局字段使用。支持Activities、fragments和views。

例如，要从下面的布局引用文本视图，

<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/hello"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="Hello World, MyActivity"/>

</android.support.constraint.ConstraintLayout>

在活动中你可以简单地写，

// Using R.layout.activity_main from the main source set
import kotlinx.android.synthetic.main.activity_main.*

class MyActivity : Activity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        // Instead of findViewById(R.id.hello) as TextView
        hello?.setText("Hello, world!")
    }
}

碎片，

// Using R.layout.fragment_content from the main source set
import kotlinx.android.synthetic.main.fragment_content.*

class ContentFragment : Fragment() {

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

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        // Instead of view.findViewById(R.id.hello) as TextView
        hello?.setText("Hello, world!")
    }
}

为了表达意见，

// Using R.layout.item_view_layout from the main source set
import kotlinx.android.synthetic.main.item_view_layout.*

class ItemViewHolder(view: View) : RecyclerView.ViewHolder(view) {

    fun bindData(data: String) {
        // Instead of itemView.findViewById(R.id.hello) as TextView
        itemView.hello?.setText(data)
    }

}

而且，除非您想要显式地使用!!，否则不应该在任何地方都使用NullPointerException。

相反，请使用以下内容中的任何人：

1. 使用安全调用- ?.进行空检查，例如。nullableVariable?.method()
2. 使用?.let{ }使用非空对象，例如。nullableVariable?.let { it.method() }
3. 使用elvis操作符- ?:为可空变量提供备份值，例如。nullableVariable ?: <backupValue>。

阅读更多关于Kotlin中的空安全的信息。

Answer 3

以片段形式初始化视图：

wall_recycler=view.findViewById<RecyclerView>(R.id.wall_recycler_id)
mainlayout = view.findViewById<LinearLayout>(R.id.mainlayout)

问题是你访问它太快了。requireView()和view在onCreateView中返回null，.I查找onViewCreated()中的所有视图。尝试在onViewCreated方法中这样做：

 override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {

 wall_recycler=requireView().findViewById<RecyclerView>(R.id.wall_recycler_id)
 mainlayout = requireView().findViewById<LinearLayout>(R.id.mainlayout)
 mainlayout.setOnClickListener { Log.d(TAG, "onViewCreated(): hello world");}
    }