java - Android fragment标签式布局，重新选择标签3，listview消失

Question

当我选择选项卡 3 时，我的 ListView 出现，但是当我选择选项卡 1 并重新选择选项卡 3 时， ListView 没有出现。为什么是这样？ https://github.com/jdavey1996/News-Android-App

import android.support.design.widget.TabLayout;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;


public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tab_layout);
        tabLayout.addTab(tabLayout.newTab().setText("Local"));
        tabLayout.addTab(tabLayout.newTab().setText("Top rated"));
        tabLayout.addTab(tabLayout.newTab().setText("All"));
        tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);

        final FragmentAdapter adapter = new FragmentAdapter(getSupportFragmentManager(),tabLayout.getTabCount());
        final ViewPager viewPager = (ViewPager) findViewById(R.id.pager);
        viewPager.setAdapter(adapter);
        viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));

        tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                viewPager.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {

            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {

            }
        });
    }
}





package com.josh_davey.news_app;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;

public class FragmentAdapter extends FragmentPagerAdapter {
    int count;
    public FragmentAdapter(FragmentManager fm, int count) {
        super(fm);
        this.count = count;
    }

    @Override
    public Fragment getItem(int position) {
        switch (position){
            case 0:
                Fragment1 temp = new Fragment1();
                return temp;
            case 1:
                Fragment2 temp2 = new Fragment2();
                return temp2;
            case 2:
                Fragment3 temp3 = new Fragment3();
                return temp3;
        }
        return null;
    }

    @Override
    public int getCount() {
        return count;
    }
}



 package com.josh_davey.news_app;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class Fragment3 extends Fragment {

    public Fragment3() {
        // Required empty public constructor
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        GetData getData = new GetData(getContext(),getActivity());
        getData.execute("lincoln");

    }

    @Override
    public void onResume() {
        super.onResume();

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment3, container, false);
    }
}

如果您需要完整代码的链接，它在我的 github 上。在此先感谢您的帮助 https://github.com/jdavey1996/News-Android-App

Answer 1

默认 ViewPager 的 offscreenPageLimit 为 1。因此，当重新选择 tab1 时，tab3 将被分离。如果 fragment 被分离，Fragment 的 View 将被破坏。
参见:ViewPager.setOffscreenPageLimit

所以我建议你将 offscreenPageLimit 设置为 2。

viewPager.setOffscreenPageLimit(2);

或者你可以像 blew 一样在 Fragment3 中保存 ArticleConstructor 的列表。

获取数据.java:

public class GetData extends AsyncTask<String, String,ArrayList<ArticleConstructor>>{
    Callback callback;

    interface Callback {
        void onArticleConstructorLoaded(ArrayList<ArticleConstructor> articleConstructors);
    }

    public GetData(Callback callback) {
        this.callback = callback;
    }

    ...

    @Override
    protected void onPostExecute(ArrayList<ArticleConstructor> result) {
        callback.onArticleConstructorLoaded(result);
    }
    ....
}

fragment 3.java:

public class Fragment3 extends Fragment implements GetData.Callback {

    private ListView listView;
    private ArrayList<ArticleConstructor> articleConstructors = new ArrayList<>();
    private ArticleArrayAdapter adapter;

    public Fragment3() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment3, container, false);
        listView = (ListView) view.findViewById(R.id.listView);
        adapter = new ArticleArrayAdapter(getActivity(), getContext(), articleConstructors);
        listView.setAdapter(adapter);
        return view;
    }

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

        if (articleConstructors.size() == 0) {
            GetData getData = new GetData(this);
            getData.execute("lincoln");
        }
    }

    @Override
    public void onArticleConstructorLoaded(ArrayList<ArticleConstructor> articleConstructors) {
        this.articleConstructors = articleConstructors;
        adapter.setArticleConstructors(articleConstructors);
    }
}