活动到片段通信:当我试图从活动更新片段中的文本视图时，得到了null指针异常

Question

我有一个活动和碎片。其中，我想将值从活动发送到片段。从活动am调用片段中的方法，该方法更新Text视图。但是我无法更新UI，我得到了null指针异常

首先，我在案例2中检查了这个链接链接：我遵循了这段代码，能够将值从活动发送到片段，效果很好。但当我在fragment.sendData("Some Text")内部剪下这条线时按钮点击并给予粘贴在按钮外的点击其获取空指针异常

fragment.sendData("Some Text")是如何在按钮点击内部工作而不是在按钮点击外部工作的？

我还尝试了相同的概念，添加了代码和错误行。所以有人能帮我为什么不更新fragment>中的文本视图吗；

错误：

com.example.fragments E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.fragments, PID: 3104
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fragments/com.example.fragments.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
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$H.handleMessage(ActivityThread.java:2016)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.example.fragments.FragmentTwo.displayUI(FragmentTwo.kt:30)
at com.example.fragments.MainActivity.loadDetailsView(MainActivity.kt:38)
at com.example.fragments.MainActivity.onCreate(MainActivity.kt:32)
at android.app.Activity.performCreate(Activity.java:7825)
at android.app.Activity.performCreate(Activity.java:7814)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1306)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)

主要活动

import android.os.Bundle
import android.util.LogAv
import androidx.appcompat.app.AppCompatActivity
class MainActivity : AppCompatActivity() {
lateinit var detailsView:FragmentTwo
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
loadDetailsView()
}
private fun loadDetailsView() {
detailsView= FragmentTwo()
supportFragmentManager.beginTransaction().replace(R.id.frameLayout,detailsView).commit()
detailsView.displayUI("HELLO")
}  
}

activity_main

<?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=".MainActivity">
<FrameLayout
android:layout_width="match_parent" android:id="@+id/frameLayout"
android:layout_height="match_parent"></FrameLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

碎片二

package com.example.fragments
import android.content.Context
import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import kotlinx.android.synthetic.main.row_names.*
class FragmentTwo:Fragment() {

fun displayUI(string: String)
{
txtName.setText(string)
}

override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
val view:View
view=inflater.inflate(R.layout.fragment_two,container,false)
return view
}
}

fragment_two.xml

<?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">
<TextView
android:layout_width="415dp"
android:layout_height="200dp"
android:text="HAI"
android:id="@+id/txtName"
android:textSize="16sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.201"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.09" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

Var txtName为null。你在fragment_two.xml中设置了它的ID吗？

Answer 2

您不能在事务替换方法之后立即访问片段方法。您可以使用AndroidX片段1.3.0-beta01 的新api在它们之间传递数据

关于文本更改

Activity.supportFragmentManager.setFragmentResult("some_key", bundle_of_data)

并从Create 上的片段开始监听

Fragment.setFragmentResultListener()