访问 Fragment 中的父活动按钮会导致应用程序崩溃

Question

大家好，我是 Android 开发的初学者。我目前正在学习导航组件，但停留在这个应用程序中。我正在将侦听器设置为 FirstFragment.java 和 SecondFragment.java 内的 Activity_main.xml 中存在的按钮，但我的应用程序崩溃了。我不知道我做错了什么。

Main Activity.java

package com.ahstore.inventory;

import android.os.Bundle;

import com.google.android.material.snackbar.Snackbar;

import androidx.appcompat.app.AppCompatActivity;

import android.view.View;

import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.fragment.NavHostFragment;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;

import com.ahstore.inventory.databinding.ActivityMainBinding;

import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private AppBarConfiguration appBarConfiguration;
    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

        setSupportActionBar(binding.toolbar);

        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_content_main);
        appBarConfiguration = new AppBarConfiguration.Builder(navController.getGraph()).build();
        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);




        binding.fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public boolean onSupportNavigateUp() {
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_content_main);
        return NavigationUI.navigateUp(navController, appBarConfiguration)
                || super.onSupportNavigateUp();
    }
}

activity_main.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:id="@+id/coordinatorLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <com.google.android.material.appbar.AppBarLayout
        android:id="@+id/appBarLayout"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:theme="@style/Theme.Inventory.AppBarOverlay"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/Theme.Inventory.PopupOverlay" />

    </com.google.android.material.appbar.AppBarLayout>


    <HorizontalScrollView
        android:id="@+id/horizontalScrollView"
        android:layout_width="409dp"
        android:layout_height="50dp"
        android:layout_marginStart="10dp"
        android:layout_marginEnd="10dp"
        android:scrollIndicators="none"
        android:scrollbarAlwaysDrawHorizontalTrack="false"
        android:scrollbarAlwaysDrawVerticalTrack="false"
        android:scrollbars="none"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/appBarLayout">

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:orientation="horizontal">

            <Button
                android:id="@+id/button1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginRight="5dp"
                android:layout_weight="1"
                android:text="Button" />

            <Button
                android:id="@+id/button2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginRight="5dp"
                android:layout_weight="1"
                android:text="Button" />

        </LinearLayout>
    </HorizontalScrollView>

    <fragment
        android:id="@+id/nav_host_fragment_content_main"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/horizontalScrollView"
        app:navGraph="@navigation/nav_graph" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="16dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:srcCompat="@android:drawable/ic_input_add"
        tools:ignore="SpeakableTextPresentCheck,SpeakableTextPresentCheck" />



</androidx.constraintlayout.widget.ConstraintLayout>

FirstFragment.java

package com.ahstore.inventory;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.navigation.fragment.NavHostFragment;

import com.ahstore.inventory.databinding.FragmentFirstBinding;

public class FirstFragment extends Fragment {

    private FragmentFirstBinding binding;

    @Override
    public View onCreateView(
            LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {
View view=inflater.inflate(R.layout.fragment_first,container,false);


        return view;

    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        Button button1=view.findViewById(R.id.button1);
        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                NavHostFragment.findNavController(FirstFragment.this)
                        .navigate(R.id.action_FirstFragment_to_SecondFragment);
            }
        });



    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        binding = null;
    }

}

fragment_first.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"
    tools:context=".FirstFragment">

    <ScrollView
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

            <TextView
                android:id="@+id/textView"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="First" />

        </LinearLayout>
    </ScrollView>
</androidx.constraintlayout.widget.ConstraintLayout>

SecondFragment.java >

package com.ahstore.inventory;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.navigation.fragment.NavHostFragment;

import com.ahstore.inventory.databinding.FragmentSecondBinding;

public class SecondFragment extends Fragment {

    private FragmentSecondBinding binding;

    @Override
    public View onCreateView(
            LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {

        View view=inflater.inflate(R.layout.fragment_second,container,false);

        return view;

    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        Button button=view.findViewById(R.id.button2);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                NavHostFragment.findNavController(SecondFragment.this)
                        .navigate(R.id.action_SecondFragment_to_FirstFragment);
            }
        });
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        binding = null;
    }

}

fragment_second.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:id="@+id/cl"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".SecondFragment">

    <TextView
        android:id="@+id/textview_second"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="am second"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

错误

2022-03-04 16:54:11.473 2392-2392/com.ahstore.inventory E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.ahstore.inventory, PID: 2392
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ahstore.inventory/com.ahstore.inventory.MainActivity}: android.view.InflateException: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Error inflating class fragment
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3800)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3976)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2315)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8550)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
     Caused by: android.view.InflateException: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Error inflating class fragment
     Caused by: android.view.InflateException: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Error inflating class fragment
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.ahstore.inventory.FirstFragment.onViewCreated(FirstFragment.java:35)
        at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:2987)
        at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:546)
        at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:282)
        at androidx.fragment.app.FragmentStore.moveToExpectedState(FragmentStore.java:112)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1647)
        at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:3128)
        at androidx.fragment.app.FragmentManager.dispatchViewCreated(FragmentManager.java:3065)
        at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:2988)
        at androidx.fragment.app.FragmentStateManager.ensureInflatedView(FragmentStateManager.java:392)
        at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:281)
        at androidx.fragment.app.FragmentLayoutInflaterFactory.onCreateView(FragmentLayoutInflaterFactory.java:140)
        at androidx.fragment.app.FragmentController.onCreateView(FragmentController.java:135)
        at androidx.fragment.app.FragmentActivity.dispatchFragmentsOnCreateView(FragmentActivity.java:319)
        at androidx.fragment.app.FragmentActivity.onCreateView(FragmentActivity.java:298)
        at android.view.LayoutInflater.tryCreateView(LayoutInflater.java:1067)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:995)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:959)
        at android.view.LayoutInflater.rInflate(LayoutInflater.java:1121)
        at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:1082)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:680)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:532)
        at com.ahstore.inventory.databinding.ActivityMainBinding.inflate(ActivityMainBinding.java:75)
        at com.ahstore.inventory.databinding.ActivityMainBinding.inflate(ActivityMainBinding.java:69)
        at com.ahstore.inventory.MainActivity.onCreate(MainActivity.java:32)
        at android.app.Activity.performCreate(Activity.java:8198)
2022-03-04 16:54:11.474 2392-2392/com.ahstore.inventory E/AndroidRuntime:     at android.app.Activity.performCreate(Activity.java:8182)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3773)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3976)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        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:2315)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8550)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
 

Answer 1

以下几点将帮助您更好地理解您的问题

1. 您的活动和片段有各自的布局文件（xml）
2. 视图绑定在它们的布局文件中。简而言之，如果 activity_main.xml 中存在 id 为 A 的元素，则只有当正在膨胀的布局为 activity_main.xml 时才能访问它。如果您的 Activity 的 contentView 是 activity_main.xml，它将能够访问布局文件中的所有 View。
3. 在片段中，布局在 onCreateView 方法中膨胀，该方法返回片段的结果 View。针对该 View 对象调用 findViewById(id) 方法。该方法将在Viewonly内查找具有给定id的View。如果它能够找到View，它将返回它，否则它将返回null。在 Activity 中查找 View 也是如此（您不需要为 Activity 指定 View，如下所示它会自动引用在setContentView中指定的布局）。

现在我们已经清楚了上述几点，我们可以看到您面临的问题了。首先，ID 为 button1 和 button2 的按钮位于您的 activity_main.xml 布局文件中。但是，您正尝试从 Fragment1 和 Fragment2 类访问这些内容。但是您的 Fragment1 和 Fragment2 类有自己的布局文件，但没有这两个按钮。因此，当您尝试检索按钮（使用 findViewById）时，它会返回 null，因为布局中不存在按钮。然后您尝试设置一个点击侦听器。由于按钮对象为 null，因此调用其上的任何方法都会导致抛出 Null Pointer Exception。

我们现在明白您做错了什么。现在，我们如何解决这个问题。首先，您需要了解您想要实现的目标是什么。在这种情况下，我建议您将两个按钮移动到各自的 Fragment 布局，即 ID 为 button1 的 View 将转到 Fragment1，而 View 将转到 Fragment1。 id button2 将转到 Fragment2。这应该有助于解决您面临的 NPE。

在某些情况下，Fragment 可能想要访问其父 Activity 中存在的 View。实现此目的的方法之一是

1. 在 Activity 类中创建一个公共方法，该方法返回具有给定 ID 的 View。

   public View getViewById(int id) {
       返回 findViewById(id);
   }
   

2. 通过这样做在您的Fragment中使用此方法

   ((MainActivity) getActivity()).getViewById(R.id.button1)
   

其他方法涉及使用接口，我相信随着您继续学习更多内容，您将会了解这些方法。

总而言之，您有两种方法可以解决您的问题。将按钮移动到片段布局文件或创建一个公共方法来检索这些视图。