在 Fragment 中调用 ViewPager

Question

我正在 ViewPager 的帮助下设计一个引导屏幕。 ViewPager 位于我的 MainActivity class 下的片段内。当我将 ViewPager 适配器 class 传递到片段中的 ViewPager 时，会出现错误！它抛出 NullPointerException 并且在断点调试后我发现我的 PagerAdapter class 内的 inflater 是空的。我已经尝试了一些替代方法来让充气机工作，但没有成功！我将 post 我的代码如下：

ViewPager 片段 class:

public class ViewPagerFragment extends Fragment {

ViewPager viewpager;
LinearLayout dotsLayout;
SliderAdapter sliderAdapter;

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

}

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

    viewpager = (ViewPager) view.findViewById(R.id.viewPagerFragment);
    dotsLayout = (LinearLayout) view.findViewById(R.id.dotsLayout);

    sliderAdapter = new SliderAdapter(view.getContext());
   
    viewpager.setAdapter(sliderAdapter); //The error is thrown here
    

    // Inflate the layout for this fragment
    return view;

}

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

    //Toast.makeText(view.getContext(),"Here",Toast.LENGTH_LONG).show();


}}

ViewPager 适配器Class:

public class SliderAdapter extends PagerAdapter {

Context context1;
LayoutInflater inflater;

public int[] slide_images = {
        R.drawable.band,
        R.drawable.studio,
        R.drawable.videoshoot
};
public String[] slide_headings = {
        "REHEARSAL PAD", "RECORDING STUDIO", "SESSION VIDEOSHOOT"
};
public String[] slide_desc ={
        String.valueOf(R.string.rehearsal_desc),
        String.valueOf(R.string.studio_desc),
        String.valueOf(R.string.videoshoot_desc)
};

public  SliderAdapter(Context context){

    context1 = context;
}

@Override
public int getCount() {
    return slide_headings.length;
}

@Override
public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
    return view== (ConstraintLayout)object;
}

@NonNull
@Override
public Object instantiateItem(@NonNull ViewGroup container, int position) {

    //ViewPager Sliding Images and Desc
      inflater = (LayoutInflater)context1.getSystemService(context1.LAYOUT_INFLATER_SERVICE);
    //inflater = LayoutInflater.from(context1);

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

    ImageView slideImageView = (ImageView) view.findViewById(R.id.SliderImageView);
    TextView slideHeading = (TextView) view.findViewById(R.id.SliderTextView1);
    TextView slideDesc = (TextView) view.findViewById(R.id.SliderTextView2);

    slideImageView.setImageResource(slide_images[position]);
    slideHeading.setText(slide_headings[position]);
    slideDesc.setText(slide_desc[position]);

    container.addView(view);

    return view;
}

@Override
public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {

    container.removeView((ConstraintLayout)object);
}}

异常日志如下：

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.soundflixstudios, PID: 13363 java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference at com.example.soundflixstudios.onboarding.ViewPagerFragment.onCreateView(ViewPagerFragment.java:43) at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2698) at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:320) at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1187) at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224) at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997) at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953) at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1849) at androidx.fragment.app.FragmentManager.run(FragmentManager.java:413) at android.os.Handler.handleCallback(Handler.java:873) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:224) at android.app.ActivityThread.main(ActivityThread.java:7099) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:536) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:876)

感谢任何帮助，是的，我在 Whosebug 中经历了多次 post，但没有找到任何解决方案，因此提出了一个新问题！提前致谢！

编辑：我为“view_pager_fragment”布局添加了 xml

enter code here<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
android:background="@color/white"
android:orientation="vertical"
tools:context=".onboarding.ViewPagerFragment">

<androidx.viewpager2.widget.ViewPager2
    android:id="@+id/sliderViewPager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_gravity="center"
    android:layout_weight="1">

</androidx.viewpager2.widget.ViewPager2>

<LinearLayout
    android:id="@+id/dotsLayout"
    android:layout_width="wrap_content"
    android:layout_height="66dp"
    android:layout_gravity="center"
    android:layout_weight="1"
    android:orientation="horizontal">

</LinearLayout>

Answer 1

能否添加 xml 文件 (viewPagerFragment)。在正常情况下，当您缺少对 xml 的引用或缺少创建 xml 标记（不同屏幕- landscape/portrait）

时，将显示 nullPointerException

Answer 2

根本原因

在view_pager_fragment.xml文件中，你声明了一个ID为sliderViewPager的ViewPager2，但在ViewPagerFragmentclass中，你使用了一个ViewPager变量并为其分配一个 ID 为 viewPagerFragment 的视图。

解决方案

步骤 1. 将 fragment_view_pager.xml 更改为：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/white"
    android:orientation="vertical">

    <androidx.viewpager.widget.ViewPager
        android:id="@+id/sliderViewPager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_gravity="center"
        android:layout_weight="1">

    </androidx.viewpager.widget.ViewPager>

    <LinearLayout
        android:id="@+id/dotsLayout"
        android:layout_width="wrap_content"
        android:layout_height="66dp"
        android:layout_gravity="center"
        android:layout_weight="1"
        android:orientation="horizontal">
    </LinearLayout>
</LinearLayout>

步骤 2. 在 ViewPagerFragment class 中，将您的代码从

viewpager = (ViewPager) view.findViewById(R.id.viewPagerFragment);

至

viewpager = (ViewPager) view.findViewById(R.id.sliderViewPager);

在 Fragment 中调用 ViewPager

Calling ViewPager inside a Fragment

android

android-studio

nullpointerexception

android-viewpager2