ViewPager 第二個片段為空白

Question

我知道以前有人問過這個問題，但沒有一個解決方案對我有用。 我在 TabLayout 中有一個 ViewPager，每個選項卡都有片段。 現在，這些片段中的每一個都有一個從 Internet 獲取數據的 RecyclerView，並且只有第一個 RecyclerView 被填充。 我不知道我是怎么搞砸的。 我知道這只是因為我沒有正確使用它，但我真的不知道怎么做。 我真的很感激一些幫助。

看起來像這樣，第二個選項卡只是空的：

這是我的代碼：

代碼

MainActivity.java

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.viewpager2.widget.ViewPager2;
import android.os.Bundle;
import com.google.android.material.tabs.TabLayout;
import com.mapbox.mapboxsdk.Mapbox;
import com.mapbox.mapboxsdk.maps.MapView;
import com.mapbox.mapboxsdk.maps.Style;

public class MainActivity extends AppCompatActivity {

    private MapView mapView;

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

        // Mapbox Access token
        Mapbox.getInstance(getApplicationContext(), getString(R.string.mapbox_api_key));

        setContentView(R.layout.activity_main);

        mapView = findViewById(R.id.mapView);
        mapView.onCreate(savedInstanceState);
        mapView.getMapAsync(mapboxMap -> mapboxMap.setStyle(Style.DARK, style -> {


        }));

        ViewPager2 pager = findViewById(R.id.view_pager);

        ViewPageAdapter pageAdapter = new ViewPageAdapter(getSupportFragmentManager(), getLifecycle());
        pager.setAdapter(pageAdapter);

        pager.setOrientation(ViewPager2.ORIENTATION_HORIZONTAL);

        TabLayout tabLayout = findViewById(R.id.tabLayout2);
        tabLayout.setTabMode(TabLayout.MODE_SCROLLABLE);

        new StatsLoader("https://api.covid19api.com/summary", pageAdapter, pager, tabLayout).execute();
        new NewsLoader("https://newsapi.org/v2/top-headlines?q=coronavirus&sortBy=popularity", pageAdapter, pager, tabLayout).execute();

    }
}

排序JSONArray.java

import android.util.Log;
import org.json.JSONObject;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

class SortJsonArray {

    void sortArray(List list, final String keyName, final boolean ascending) {

        Collections.sort(list, new Comparator<JSONObject>() {

            private final String KEY_NAME = keyName;

            @Override
            public int compare(JSONObject o1, JSONObject o2) {

                String val1 = "";
                String val2 = "";

                try {
                    val1 = String.valueOf(o1.get(KEY_NAME));
                    val2 = String.valueOf(o2.get(KEY_NAME));
                } catch (Exception e) {
                    Log.e("Sort Exception", "Issue when sorting JSONArray", e);
                    e.printStackTrace();
                }

                if (IntCheckHelper.isInteger(val1)) {
                    if (ascending) {
                        return Integer.valueOf(val1).compareTo(Integer.valueOf(val2));
                    }
                    else {
                        return Integer.valueOf(val2).compareTo(Integer.valueOf(val1));
                    }
                }
                else {
                    if (ascending) {
                        return val1.compareToIgnoreCase(val2);
                    }
                    else {
                        return val2.compareToIgnoreCase(val1);
                    }
                }

            }

        });

    }

}

StatsFragment.java

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

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import org.json.JSONArray;

public class StatsFragment extends Fragment {

    View view;
    RecyclerView recyclerView;
    JSONArray covidData;
    String dataUrl;

    public StatsFragment(JSONArray data, String url) {
        covidData = data;
        dataUrl = url;
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        view = inflater.inflate(R.layout.view_page, container, false);
        recyclerView = view.findViewById(R.id.recyclerView2);
        LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity());
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setHasFixedSize(false);
        RecyclerView.Adapter<CustomListAdapter.ListViewHolder> mAdapter = new CustomListAdapter(covidData, dataUrl);
        recyclerView.setAdapter(mAdapter);
        return view;
    }

}

CustomListAdapter.java

import android.view.LayoutInflater;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import org.json.JSONArray;
import org.json.JSONException;

import java.text.NumberFormat;

public class CustomListAdapter extends RecyclerView.Adapter<CustomListAdapter.ListViewHolder> {

    private JSONArray mDataset;
    private String murl;

    static class ListViewHolder extends RecyclerView.ViewHolder {

        TextView textView;
        TextView textView2;
        ImageView imageView;
        ListViewHolder(LinearLayout v) {
            super(v);
            textView = v.findViewById(R.id.name);
            textView2 = v.findViewById(R.id.cases);
            imageView = v.findViewById(R.id.image);
        }

    }

    CustomListAdapter(JSONArray myDataset, String url) {
        mDataset = myDataset;
        murl = url;
    }

    @NonNull
    @Override
    public CustomListAdapter.ListViewHolder onCreateViewHolder(ViewGroup parent,
                                                               int viewType) {
        // create a new view
        LinearLayout linearLayout = (LinearLayout) LayoutInflater.from(parent.getContext())
                .inflate(R.layout.list_view, parent, false);

        // TextView v = (TextView) linearLayout.findViewById(R.id.info_text);

        return new ListViewHolder(linearLayout);
    }

    @Override
    public void onBindViewHolder(ListViewHolder holder, int position) {

        String textViewString = "";
        String textView2String = "";

        try {
            if (murl.contains("api.covid19api.com")) {
                textViewString = mDataset.getJSONObject(position).getString("Country");
                textView2String = NumberFormat.getInstance().format(mDataset.getJSONObject(position).getInt("TotalConfirmed"));
            }
            else if (murl.contains("newsapi.org")) {
                textViewString = mDataset.getJSONObject(position).getString("title");
                textView2String = mDataset.getJSONObject(position).getString("description");
            }
            holder.textView.setText(textViewString);
            holder.textView2.setText(textView2String);
        } catch (JSONException e) {
            e.printStackTrace();
        }

    }

    @Override
    public int getItemCount() {
        return mDataset.length();
    }

}

StatsLoader.java

import android.os.AsyncTask;
import android.util.Log;
import androidx.annotation.Nullable;
import androidx.viewpager2.widget.ViewPager2;

import com.google.android.material.tabs.TabLayout;
import com.google.android.material.tabs.TabLayoutMediator;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import javax.net.ssl.HttpsURLConnection;

public class StatsLoader extends AsyncTask<String, Void, JSONArray> {

    Exception exception;
    String urlString = "";
    static JSONArray covidData = new JSONArray();
    ViewPageAdapter pageAdapter;
    ViewPager2 viewPager2;
    TabLayout tabLayout;

    public StatsLoader(String url, ViewPageAdapter adapter, ViewPager2 pager, TabLayout tabs) {
        super();
        urlString = url;
        pageAdapter = adapter;
        viewPager2 = pager;
        tabLayout = tabs;
    }

    @Nullable
    @Override
    public JSONArray doInBackground(String ... urls) {
        HttpsURLConnection connection = null;
        BufferedReader reader = null;

        try {

            URL url = new URL(urlString);
            connection = (HttpsURLConnection) url.openConnection();
            connection.connect();
            InputStream inputStream = connection.getInputStream();
            reader = new BufferedReader(new InputStreamReader(inputStream));

            StringBuilder buffer = new StringBuilder();
            String line = "";

            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }

            JSONObject json = new JSONObject(buffer.toString());
            covidData = json.getJSONArray("Countries");
            ArrayList<Object> list = new ArrayList<>();
            for (int i = 0; i < covidData.length(); i++) {
                list.add(covidData.get(i));
            }
            SortJsonArray sortJsonArray = new SortJsonArray();
            sortJsonArray.sortArray(list, "TotalConfirmed", false);
            covidData = new JSONArray();
            for (Object object : list) {
                covidData.put(object);
            }
            return covidData;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                connection.disconnect();
            }
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return null;
    }

    protected void onPostExecute(JSONArray coviddata) {

        if (this.exception == null) {
            Log.d("Check", "Works!");
            pageAdapter.addFragment(new StatsFragment(coviddata, urlString), "Stats");
            viewPager2.setAdapter(pageAdapter);
            new TabLayoutMediator(tabLayout, viewPager2,
                    (tab, position) -> {
                        switch (position) {
                            case 0:
                                tab.setText("Stats");
                                break;
                            case 1:
                                tab.setText("News");
                                break;
                            case 2:
                                tab.setText("Symptoms");
                                break;
                            case 3:
                                tab.setText("Safety");
                                break;
                            default:
                                break;
                        }
                    }).attach();
        }

    }


}

ViewPageAdapter.java

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.lifecycle.Lifecycle;
import androidx.viewpager2.adapter.FragmentStateAdapter;

import java.util.ArrayList;

public class ViewPageAdapter extends FragmentStateAdapter {

    private final ArrayList<Fragment> fragments = new ArrayList<>();
    private final ArrayList<String> titles = new ArrayList<>();

    public ViewPageAdapter(@NonNull FragmentManager fragmentManager, @NonNull Lifecycle lifecycle) {
        super(fragmentManager, lifecycle);
    }

    public void addFragment(Fragment fragment, String title) {
        fragments.add(fragment);
        titles.add(title);
    }

    @NonNull
    @Override
    public Fragment createFragment(int position) {
        return fragments.get(position);
    }

    @Override
    public int getItemCount() {
        return fragments.size();
    }

}

新聞Fragment.java

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import org.json.JSONArray;

public class NewsFragment extends Fragment {

    View view;
    RecyclerView recyclerView;
    JSONArray news;
    String newsUrl;

    public NewsFragment(JSONArray data, String url) {
        news = data;
        newsUrl = url;
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        view = inflater.inflate(R.layout.view_page, container, false);
        recyclerView = view.findViewById(R.id.recyclerView2);
        LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity());
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setHasFixedSize(false);
        RecyclerView.Adapter<CustomListAdapter.ListViewHolder> mAdapter = new CustomListAdapter(news, newsUrl);
        recyclerView.setAdapter(mAdapter);
        return view;
    }

}

NewsLoader.java

import android.os.AsyncTask;
import android.util.Log;
import androidx.annotation.Nullable;
import androidx.viewpager2.widget.ViewPager2;
import com.google.android.material.tabs.TabLayout;
import com.google.android.material.tabs.TabLayoutMediator;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

import javax.net.ssl.HttpsURLConnection;

public class NewsLoader extends AsyncTask<String, Void, JSONArray> {

    Exception exception;
    String urlString = "";
    static JSONArray news = new JSONArray();
    ViewPageAdapter pageAdapter;
    ViewPager2 viewPager2;
    TabLayout tabLayout;

    public NewsLoader(String url, ViewPageAdapter adapter, ViewPager2 pager, TabLayout tabs) {
        super();
        urlString = url;
        pageAdapter = adapter;
        viewPager2 = pager;
        tabLayout = tabs;
    }

    @Nullable
    @Override
    public JSONArray doInBackground(String ... urls) {
        HttpURLConnection connection = null;
        BufferedReader reader = null;

        try {

            URL url = new URL(urlString);
            connection = (HttpsURLConnection) url.openConnection();
            connection.addRequestProperty("Authorization", "Bearer baef544b7dbe4ff8b15bb502d1fd5e1a");
            connection.connect();
            InputStream inputStream = connection.getInputStream();
            reader = new BufferedReader(new InputStreamReader(inputStream));

            StringBuilder buffer = new StringBuilder();
            String line = "";

            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }

            JSONObject json = new JSONObject(buffer.toString());
            news = json.getJSONArray("articles");
            return news;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                connection.disconnect();
            }
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return null;
    }

    protected void onPostExecute(JSONArray newsData) {

        if (this.exception == null) {
            Log.d("Check", "Works!");
            pageAdapter.addFragment(new NewsFragment(newsData, urlString), "News");
            viewPager2.setAdapter(pageAdapter);
            new TabLayoutMediator(tabLayout, viewPager2,
                    (tab, position) -> {
                        switch (position) {
                            case 0:
                                tab.setText("Stats");
                                break;
                            case 1:
                                tab.setText("News");
                                break;
                            case 2:
                                tab.setText("Symptoms");
                                break;
                            case 3:
                                tab.setText("Safety");
                                break;
                            default:
                                break;
                        }
                    }).attach();
        }

    }
}

布局

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout 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">

    <com.google.android.material.appbar.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <com.google.android.material.appbar.CollapsingToolbarLayout
            android:id="@+id/collapsing_toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_gravity="top"
            android:fitsSystemWindows="true"
            app:contentScrim="?attr/colorPrimary"
            app:layout_scrollFlags="scroll|exitUntilCollapsed"
            app:statusBarScrim="@null"
            app:titleEnabled="false"
            android:minHeight="?attr/actionBarSize" >

            <fragment
                android:id="@+id/mapView"
                android:name="com.mapbox.mapboxsdk.maps.MapFragment"
                android:layout_width="match_parent"
                android:layout_height="350sp"
                android:apiKey="@string/mapbox_api_key"
                android:clickable="true"
                android:enabled="true"
                android:focusable="true"
                app:layout_collapseMode="parallax" />

            <androidx.appcompat.widget.Toolbar
                android:id="@+id/appbar"
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_marginBottom="48dp"
                android:gravity="top"
                app:layout_collapseMode="pin"
                app:layout_scrollFlags="scroll|exitUntilCollapsed"
                app:popupTheme="@style/ThemeOverlay.AppCompat.Dark"
                app:title="" />

            <com.google.android.material.tabs.TabLayout
                android:id="@+id/tabLayout2"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_gravity="bottom"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toBottomOf="parent">

            </com.google.android.material.tabs.TabLayout>

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

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom"
        app:layout_behavior="@string/appbar_scrolling_view_behavior">

        <androidx.viewpager2.widget.ViewPager2
            android:id="@+id/view_pager"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="parent"
            tools:listitem="@layout/view_page" >

        </androidx.viewpager2.widget.ViewPager2>


    </androidx.constraintlayout.widget.ConstraintLayout>


</androidx.coordinatorlayout.widget.CoordinatorLayout>

list_view.xml

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

    <androidx.cardview.widget.CardView
        android:id="@+id/card_view"
        android:layout_gravity="center"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        card_view:cardCornerRadius="14sp"
        android:layout_margin="15sp">

        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:padding="40sp">

            <ImageView
                android:id="@+id/image"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_toLeftOf="@+id/name"
                android:layout_toStartOf="@+id/name"
                android:contentDescription="Image of country flag" />

            <TextView
                android:id="@+id/name"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:paddingBottom="10sp" />

            <TextView
                android:id="@+id/cases"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_below="@+id/name" />

        </RelativeLayout>

    </androidx.cardview.widget.CardView>

</LinearLayout>

view_page.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView2"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="parent"
        tools:listitem="@layout/list_view">

    </androidx.recyclerview.widget.RecyclerView>

</androidx.constraintlayout.widget.ConstraintLayout>

API 密鑰

pk.eyJ1IjoiZGF5ZW1zYWVlZCIsImEiOiJja2E2d3FyZTYwZHh5MnptdzFjcXMycTRxIn0.hpzQHB9-_OTMC1iGkkvYpw

問題在於代碼的 NewsFragment 和 NewsLoader 部分，因為那是沒有出現的部分。 我添加了 rest 作為上下文，以防萬一有人需要它。 謝謝！

編輯：根據評論建議添加了 SortJSONArray 代碼和 API 鍵。

Answer 1

只需修復您的布局...數據僅存在

    <androidx.recyclerview.widget.RecyclerView
    android:id="@+id/recyclerView2"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    tools:listitem="@layout/list_view">

</androidx.recyclerview.widget.RecyclerView>

消除：

app:layout_constraintTop_toBottomOf="parent"