如何在android中使用kotlin获取片段中的资源ID？

Question

我尝试了下面提到的这段代码，但在运行时崩溃了。发生的错误是 Android Run time:

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

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'

在应用级别的 gradle 文件中和

导入

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

在您的片段的onCreateView 中（例如，如果您的 textview 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：

在你的类中声明 viewOfLayout 而不是 view。

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Kotlin Android Extensions。

您不必再使用findViewById。使用此插件，您可以直接将 UI 组件用作全局字段。在Activities、fragments 和views 中受支持。

例如， 要从下面的布局中引用文本视图，

<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!")
    }
}

在片段中，

// 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!")
    }
}

对于视图，

// 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，您不应该在任何地方使用!!，除非您明确需要NullPointerException。

请改用以下任何人：

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

阅读更多关于Null Safety in Kotlin的信息。

Answer 3

片段中视图的初始化：

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

问题是您访问它太快了。 requireView() 和 view 在 onCreateView 中返回 null。我在 onViewCreated() 中找到了所有视图。 尝试在onViewCreated 方法中执行此操作：

 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");}
    }