java - Android Studio 中的 LogCat 错误导致应用程序运行时崩溃

Question

在这个 fragment 中，我添加了 ZXingscanner View ，代码根本没有显示任何错误，但是当我运行应用程序时，它每次都会崩溃并停止。这个应用程序有 3 个选项卡，每个选项卡中都有 fragment ，其中一个选项卡的 fragment 就是这个扫描仪，但这个 View 由于某种原因破坏了应用程序。这是我的 fragment 类的代码:

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.os.Vibrator;
import android.support.annotation.NonNull;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;

import com.google.zxing.BarcodeFormat;
import com.google.zxing.Result;
import com.hist_area.imeda.histarea.R;

import java.util.ArrayList;
import java.util.List;

import me.dm7.barcodescanner.zxing.ZXingScannerView;


public class EmptyFragment extends Fragment implements ZXingScannerView.ResultHandler {
    public static EmptyFragment create() {
        return new EmptyFragment();
    }
    Vibrator vibrator;


    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        }

        private ZXingScannerView mScannerView;
        private LinearLayout qrCameraLayout;

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

            qrCameraLayout = (LinearLayout) v.findViewById(R.id.camera_preview);
            mScannerView = new ZXingScannerView(getActivity().getApplicationContext());
            mScannerView.setLayoutParams(new LinearLayout.LayoutParams(
                    LinearLayout.LayoutParams.MATCH_PARENT,
                    LinearLayout.LayoutParams.MATCH_PARENT));
            qrCameraLayout.addView(mScannerView);

            List<BarcodeFormat> formats = new ArrayList<>();
            formats.add(BarcodeFormat.QR_CODE);
            mScannerView.setFormats(formats);

            return v;
        }

        @Override
        public void onResume ()
        {
            super.onResume();
            mScannerView.setResultHandler(this); // Register ourselves as a handler for scan results.
            mScannerView.startCamera();          // Start camera on resume
        }

        @Override
        public void onPause ()
        {
            super.onPause();
            mScannerView.stopCamera();
        }

        @Override
        public void handleResult ( final Result result)
        {
            vibrator.vibrate(369);
            Log.e("handleresult", result.getText());
            //Hold result
            AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
            builder.setTitle("             Scan Result");
            builder.setPositiveButton("Scan Again", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    mScannerView.resumeCameraPreview(EmptyFragment.this);
                }
            });
            builder.setNeutralButton("Main Page", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    mScannerView.stopCamera();
                    MainPageFragment.create();
                }


            });

            builder.setMessage(result.getText());
            AlertDialog alertDialog = builder.create();
            alertDialog.show();
        }


}

下面是 XML LinearLayout 的代码，其中包含此 View :

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


    <View
        android:id="@+id/camera_scanner"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout> 

这是 LogCat 错误:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.addView(android.view.View)' on a null object reference
                                                                                at com.hist_area.imeda.histarea.fragment.EmptyFragment.onCreateView(EmptyFragment.java:52)
                                                                                at android.support.v4.app.Fragment.performCreateView(Fragment.java:2239)
                                                                                at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1332)
                                                                                at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1574)
                                                                                at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1641)
                                                                                at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:794)
                                                                                at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2415)
                                                                                at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2200)
                                                                                at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2153)
                                                                                at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:2034)
                                                                                at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:651)
                                                                                at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:143)
                                                                                at android.support.v4.view.ViewPager.populate(ViewPager.java:1239)
                                                                                at android.support.v4.view.ViewPager.populate(ViewPager.java:1087)
                                                                                at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1613)
                                                                                at android.view.View.measure(View.java:18788)
                                                                                at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
                                                                                at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
                                                                                at android.view.View.measure(View.java:18788)
                                                                                at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                                                                                at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
                                                                                at android.view.View.measure(View.java:18788)
                                                                                at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
                                                                                at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
                                                                                at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
                                                                                at android.view.View.measure(View.java:18788)
                                                                                at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                                                                                at android.view.View.measure(View.java:18788)
                                                                                at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
                                                                                at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
                                                                                at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
                                                                                at android.view.View.measure(View.java:18788)
                                                                                at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
                                                                                at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                                                                                at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643)
                                                                                at android.view.View.measure(View.java:18788)
                                                                                at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100)
                                                                                at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216)
                                                                                at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1452)
                                                                                at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
                                                                                at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013)
                                                                                at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
                                                                                at android.view.Choreographer.doCallbacks(Choreographer.java:670)
                                                                                at android.view.Choreographer.doFrame(Choreographer.java:606)
                                                                                at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
                                                                                at android.os.Handler.handleCallback(Handler.java:739)
                                                                                at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                at android.os.Looper.loop(Looper.java:148)
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                                at java.lang.reflect.Method.invoke(Native Method)
                                                                                at co
08-04 17:29:54.075 4819-4819/com.hist_area.imeda.histarea I/Process: Sending signal. PID: 4819 SIG: 9

Answer 1

您尚未在布局文件中为此元素提供 ID。这样做:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/camera_preview" <!-- This was missing -->
>
 <View
    android:id="@+id/camera_scanner"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />
 </LinearLayout>