android - 如何在 fragment 中初始化 "lateinit binding"？

Question

我试图从 Fragment 向我的按钮添加一个监听器，但它总是给我这个错误:

 Process: com.example.projectdrivemark, PID: 27776
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.projectdrivemark/com.example.MainActivity}: kotlin.UninitializedPropertyAccessException: lateinit property binding has not been initialized
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3341)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3485)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2045)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7478)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:549)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:941)
     Caused by: kotlin.UninitializedPropertyAccessException: lateinit property binding has not been initialized
        at com.example.tempConverter.TempConverterFragment.<init>(TempConverterFragment.kt:16)
        at com.example.MainActivity.onCreate(MainActivity.kt:28)
        at android.app.Activity.performCreate(Activity.java:7989)
        at android.app.Activity.performCreate(Activity.java:7978)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3316

这是 MainActivity 代码:

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import androidx.fragment.app.Fragment
import androidx.recyclerview.widget.LinearLayoutManager
import com.example.projectdrivemark.R
import com.example.projectdrivemark.databinding.ActivityMainBinding
import com.example.recyclerView.MockDatabase.Companion.createMockData
import com.example.recyclerView.PostAdapter
import com.example.recyclerView.RecyclerViewFragment
import com.example.tempConverter.TempConverterFragment
import com.example.uploaderView.UploaderFragment

class MainActivity : AppCompatActivity(), PostAdapter.OnPostClickListener {
    private lateinit var binding: ActivityMainBinding
    val dummyList = createMockData()
    val adapter = PostAdapter(dummyList, this)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        title = "First Kotlin App"
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val recyclerView = RecyclerViewFragment()
        val tempConverterView = TempConverterFragment()
        val uploaderView = UploaderFragment(this)
        setFragmentView(recyclerView)

        binding.bottomNavBar.setOnNavigationItemSelectedListener {
            when(it.itemId){
                R.id.listView ->  setFragmentView(recyclerView)
                R.id.tempConverterView -> setFragmentView(tempConverterView)
                R.id.videoUploaderView -> setFragmentView(uploaderView)
            }
            true
        }
    }

    private fun setFragmentView(fragment: Fragment){
        supportFragmentManager.beginTransaction().apply {
            replace(R.id.main_fragment_view, fragment)
            //Will return to previous page when tap "Back Button" on the phone
            addToBackStack(null)
            commit()
        }
    }

    override fun onEditPost(position: Int){
        val clickedPost = dummyList[position]
        clickedPost.title = "Updated title"
        clickedPost.body = "Updated body"
        adapter.notifyItemChanged(position)
    }

    override fun onDeletePost(position: Int) {
        dummyList.removeAt(position)
  

  adapter.notifyItemRemoved(position)
}

这是我的 TempConverterFragment 的样子:

import android.content.Intent
import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.Toast
import androidx.fragment.app.Fragment
import com.example.MainActivity
import com.example.projectdrivemark.R
import com.example.projectdrivemark.databinding.FragmentTempConverterBinding

class TempConverterFragment: Fragment(){
    private lateinit var binding: FragmentTempConverterBinding
    val celsiusButton = binding.celsiusButton


    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val binding = inflater.inflate(R.layout.fragment_temp_converter, container, false)
        celsiusButton.setOnClickListener {
            celsiusFunction(celsiusButton)
        }
        return binding.rootView

    }

    fun celsiusFunction(view: View){
        val fahrenheitView = binding!!.userTemp
        val fahrenheitValue = fahrenheitView.text.toString()

        if(!fahrenheitValue.isBlank()){
            val celsiusCovertedValue = (fahrenheitValue.toDouble() - 32) * 5/9
            val celsiusValue = String.format("%.2f", celsiusCovertedValue)
            Toast.makeText(activity,
                    "$fahrenheitValue fahrenheit is $celsiusValue degrees celsius",
                    Toast.LENGTH_LONG).show()
        }else {
            Toast.makeText(activity, "You must enter a value to convert", Toast.LENGTH_LONG).show()
        }
    }

    fun farenheitFunction(view: View){
        val celsiusView = binding.userTemp
        val celsiusValue = celsiusView.text.toString()

        if(!celsiusValue.isBlank()){
            val farenheitConvertedValue = celsiusValue.toDouble() * 9/5 + 32
            val farenheitValue = String.format("%.2f", farenheitConvertedValue)
            Toast.makeText(activity,
                    "$celsiusValue degrees celsius is $farenheitValue farenheit",
                    Toast.LENGTH_LONG).show()
        }else{
            Toast.makeText(activity, "You must enter a value to convert", Toast.LENGTH_LONG).show()
        }
    }
}

下面是我的 Xml Fragment 的样子:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.MainActivity">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="487dp"
        android:layout_height="320dp"
        android:layout_marginBottom="16dp"
        android:adjustViewBounds="true"
        android:cropToPadding="false"
        android:scaleType="fitXY"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:srcCompat="@drawable/termometer" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="62dp"
        android:layout_marginLeft="62dp"
        android:layout_marginTop="24dp"
        android:layout_marginEnd="76dp"
        android:layout_marginRight="76dp"
        android:text="Enter desired temperature to convert"
        android:textAppearance="@style/TextAppearance.AppCompat.Large"
        android:textSize="18sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/imageView" />

    <EditText
        android:id="@+id/userTemp"
        style="@android:style/Widget.DeviceDefault.EditText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="140dp"
        android:layout_marginLeft="140dp"
        android:layout_marginTop="24dp"
        android:layout_marginEnd="140dp"
        android:layout_marginRight="140dp"
        android:ems="10"
        android:hint="Enter Temperature Here"
        android:inputType="numberDecimal"
        android:selectAllOnFocus="false"
        android:singleLine="true"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView" />

    <Button
        android:id="@+id/celsiusButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="70dp"
        android:layout_marginLeft="70dp"
        android:layout_marginTop="32dp"
        android:onClick="celsiusFunction"
        android:text="to Celsius"
        app:backgroundTint="@color/purple_200"
        app:layout_constraintEnd_toStartOf="@+id/farenheitButton"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/userTemp" />

    <Button
        android:id="@+id/farenheitButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="41dp"
        android:layout_marginLeft="41dp"
        android:layout_marginTop="32dp"
        android:layout_marginEnd="72dp"
        android:layout_marginRight="72dp"
        android:onClick="farenheitFunction"
        android:text="to Farenheit"
        app:backgroundTint="@color/teal_200"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/celsiusButton"
        app:layout_constraintTop_toBottomOf="@+id/userTemp" />
</androidx.constraintlayout.widget.ConstraintLayout>

如何初始化一个 lateinit 绑定(bind)？还是只使用 view.findViewById 更好？

Answer 1

在我们 fragment 中的 onCreateView 中绑定(bind)之前删除“val”。您已经声明它并像下面这样初始化它

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? 
{
    binding = FragmentTempConverterBinding.inflate(inflater, container, false)
    return binding.root
    }