java - query.addListenerForSingleValueEvent 不会运行...？

Question

我尝试连接到 firebase 以获取 display_name 和 profile_photo对于我的帖子，例如 Instagram像这个 example image但是当我运行我的应用程序时，它说我的变量上有 nullPointerExceptionmUserAccountSettings 导致我的应用程序无法运行 onDataChanged() 或 onDataCancelled()功能。

我已经尝试从 Profile 类的 mUserAccountSettings 获取数据，实际上它工作得很好，但在 ViewFragment 类中它不起作用

它说:

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.jesslyn.foodylife, PID: 5655 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.jesslyn.foodylife.FoodyLife.models.UserAccountSettings.getProfile_photo()' on a null object reference at com.example.jesslyn.foodylife.FoodyLife.Utils.ViewPostFragment.setupWidgets(ViewPostFragment.java:156) at com.example.jesslyn.foodylife.FoodyLife.Utils.ViewPostFragment.onCreateView(ViewPostFragment.java:117) at android.support.v4.app.Fragment.performCreateView(Fragment.java:2192) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1299) at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1528) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1595) at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:758) at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2363) at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2149) at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2103) at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2013) at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:710) at android.os.Handler.handleCallback(Handler.java:742) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:5544) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629) E/MQSEventManagerDelegate: failed to get MQSService.

这是我在 ViewFragment 类中的代码:

package com.example.jesslyn.foodylife.FoodyLife.Utils;

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.text.LoginFilter;
import android.util.EventLogTags;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ImageView;
import android.widget.RatingBar;
import android.widget.TextView;

import com.example.jesslyn.foodylife.FoodyLife.Utils.BottomNavigationViewHelper;
import com.example.jesslyn.foodylife.FoodyLife.Utils.FirebaseMethods;
import com.example.jesslyn.foodylife.FoodyLife.Utils.GridImageAdapter;
import com.example.jesslyn.foodylife.FoodyLife.Utils.SquareImageView;
import com.example.jesslyn.foodylife.FoodyLife.Utils.UniversalImageLoader;
import com.example.jesslyn.foodylife.FoodyLife.models.Photo;
import com.example.jesslyn.foodylife.FoodyLife.models.UserAccountSettings;
import com.example.jesslyn.foodylife.R;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.auth.GetTokenResult;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.Query;
import com.google.firebase.database.ValueEventListener;
import com.ittianyu.bottomnavigationviewex.BottomNavigationViewEx;

import junit.framework.Test;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;
import java.util.concurrent.Executor;

/**
 * Created by Jesslyn on 12/4/2017.
 */

public class ViewPostFragment extends Fragment {

    private static final String TAG = "ViewPostFragment";
    private SquareImageView mPostImage;
    private BottomNavigationViewEx bottomNavigationViewEx;
    private TextView mBackLabel, mCaption, mUsername, mTimeStamp;
    private ImageView mBackArrow, mEllipses, mProfileImage;
    private RatingBar mStars;
    private String photoUrl;

    private Photo mPhoto;
    private int mActivityNumber = 0;
    private String photoUsername = "";
    private String profilePhotoUrl = "";
    private UserAccountSettings mUserAccountSettings;

    // firebase
    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;
    private FirebaseDatabase mFirebaseDatabase;
    private DatabaseReference myRef;
    private FirebaseMethods mFirebaseMethods;
    private String userID;
    public ViewPostFragment() {
        super();
        setArguments(new Bundle());
    }

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_view_post, container, false);
        mPostImage = (SquareImageView) view.findViewById(R.id.post_image);
        bottomNavigationViewEx = (BottomNavigationViewEx) view.findViewById(R.id.bottomNavViewBar);
        mBackArrow = (ImageView) view.findViewById(R.id.backArrow);
        mBackLabel = (TextView) view.findViewById(R.id.tvBackLabel);
        mCaption = (TextView) view.findViewById(R.id.image_caption);
        mUsername = (TextView) view.findViewById(R.id.username);
        mTimeStamp = (TextView) view.findViewById(R.id.image_time_posted);
        mEllipses = (ImageView) view.findViewById(R.id.iv_ellipses);
        mStars = (RatingBar) view.findViewById(R.id.ratingBar);

        mStars.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                RatingBar bar = (RatingBar) v;
                Log.d(TAG,"VALUE IS : "+bar.getRating()+"         **************************************************");
            }
        });
        mStars.setRating(0);
        mProfileImage = (ImageView) view.findViewById(R.id.profile_photo);

        try {
            mPhoto = getPhotoFromBundle();
            UniversalImageLoader.setImage(mPhoto.getImage_path(), mPostImage, null, "");
        } catch (NullPointerException e) {
            Log.e(TAG, "onCreateView NullPointerException : " + e.getMessage());
        }
        setupBottomNavigationView();
        setupFirebaseAuth();
        getPhotoDetails();
        setupWidgets();
        return view;
    }

    private void getPhotoDetails(){
        Log.d(TAG, "getPhotoDetails: retrieving photo details.");
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference();
        Query query = reference
                .child(getString(R.string.dbname_user_account_settings))
                .orderByChild(getString(R.string.field_user_id))
                .equalTo(mPhoto.getUser_id());
        query.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                Log.d(TAG, "onDataChange: ");
                for ( DataSnapshot singleSnapshot :  dataSnapshot.getChildren()){
                    Log.d(TAG, "onDataChange: " +singleSnapshot.getValue().toString());
                    mUserAccountSettings = singleSnapshot.getValue(UserAccountSettings.class);
                    Log.d(TAG, "onDataChange: "+mUserAccountSettings.getUsername());

                }
                setupWidgets();
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.d(TAG, "onCancelled: query cancelled.");
            }
        });
    }

    private void setupWidgets(){
        String timestampDiff = getTimeStampDifference();
        if(!timestampDiff.equals("0")){
           mTimeStamp.setText(timestampDiff + " DAYS AGO");
        }else{
            mTimeStamp.setText("TODAY");
        }
       UniversalImageLoader.setImage(mUserAccountSettings.getProfile_photo(), mProfileImage, null, "");
       mUsername.setText(mUserAccountSettings.getUsername());
    }


    //get how many days ago of post
    private String getTimeStampDifference() {
        Log.d(TAG, "getTimeStampDifference : getting time difference");
        String difference = "";
        Calendar c = Calendar.getInstance();
        SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy'T'HH:mm:ss'Z'", Locale.ENGLISH);
        sdf.setTimeZone(TimeZone.getTimeZone("Asia/Jakarta"));
        Date timestamp;
        Date today = c.getTime();
        sdf.format(today);
        final String photoTimestamp = mPhoto.getDate_created();

        try {
            timestamp = sdf.parse(photoTimestamp);
            difference = String.valueOf(Math.round(((today.getTime() - timestamp.getTime()) / 1000 / 60 / 60 / 24)));
        } catch (ParseException e) {
            Log.e(TAG, "PARSE EXCEPTION : " + e.getMessage());
            difference = "0";
        }
        return difference;
    }

    private int getActivityFromBundle() {
        Log.d(TAG, "getActivityFromBundle arguments : " + getArguments());

        Bundle bundle = this.getArguments();
        if (bundle != null) {
            return bundle.getInt(getString(R.string.activity_number));
        } else {
            return 0;
        }
    }

    private Photo getPhotoFromBundle() {
        Log.d(TAG, "getPhotoFromBundle arguments : " + getArguments());

        Bundle bundle = this.getArguments();
        if (bundle != null) {
            return bundle.getParcelable(getString(R.string.photo));
        } else {
            return null;
        }
    }

    /**
     * BottomNavigationView setup
     */
    private void setupBottomNavigationView() {
        Log.d(TAG, "setupBottomNavigationView: setting up BottomNavigationView");
        BottomNavigationViewHelper.setupBottomNavigationView(bottomNavigationViewEx);
        BottomNavigationViewHelper.enableNavigation(getActivity(), getActivity(), bottomNavigationViewEx);
        Menu menu = bottomNavigationViewEx.getMenu();
        MenuItem menuItem = menu.getItem(mActivityNumber);
        menuItem.setChecked(true);
    }

    /**
     * ----------------------------------Firebase stuffs-----------------------------------------
     */

    //setup the firebase auth obj
    private void setupFirebaseAuth() {
        Log.d(TAG, "setupFirebaseAuth : setting up firebase auth");
        mAuth = FirebaseAuth.getInstance();
        mFirebaseDatabase = FirebaseDatabase.getInstance();
        myRef = mFirebaseDatabase.getReference();

        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();

                if (user != null) {
                    //user is signed in
                    Log.d(TAG, "user is signed in" + user.getUid());
                } else {
                    //user is signed out
                    Log.d(TAG, "onAuthStateChanged : signed out");
                }
            }
        };
    }

    @Override
    public void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(mAuthListener);

    }

    @Override
    public void onStop() {
        super.onStop();
        if (mAuthListener != null) {
            mAuth.removeAuthStateListener(mAuthListener);
        }
    }
}

Answer 1

这是因为变量mUserAccountSettings是在onDataChange()方法中初始化的，该方法是一个回调，并且是异步执行的。在此之前，它初始化了您的主线程，并尝试在 setupWidgets()

中访问它

一个可能的修复方法是将以下代码从 setupWidgets() 移动到 onDataChange()。

 UniversalImageLoader.setImage(mUserAccountSettings.getProfile_photo(), mProfileImage, null, "");

注意:这并不是为了向您展示任何最佳实践。这个解决方案只是为了表明所有依赖于服务器值的 UI 更新都应该在您从服务器获得响应时发生。