为什么 viewPager 的中间选项卡从不为自己调用 onCreateView？

Question

在最后 2 天里，我把所有的时间都花在了尝试在带有片段的浏览器中找到一个奇怪问题的解决方案。所有片段都包含一个列表视图（它有自己的适配器）。

情况如下：

• 我有一个活动，它为它实例化一个视图分页器和一个适配器，并包含对象适配器类。

公共类 SellerActivity 扩展 AppCompatActivity {

/**
 * The {@link android.support.v4.view.PagerAdapter} that will provide
 * fragments for each of the sections. We use a
 * {@link FragmentPagerAdapter} derivative, which will keep every
 * loaded fragment in memory. If this becomes too memory intensive, it
 * may be best to switch to a
 * {@link android.support.v4.app.FragmentStatePagerAdapter}.
 */
private SectionsPagerAdapter mSectionsPagerAdapter;
private FirebaseAuth mAuth = FirebaseAuth.getInstance();

/**
 * The {@link ViewPager} that will host the section contents.
 */
private ViewPager mViewPager;

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

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    // Create the adapter that will return a fragment for each of the three
    // primary sections of the activity.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.vpContainer2);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs2);
    tabLayout.setupWithViewPager(mViewPager);

    mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        // This method will be invoked when a new page becomes selected.
        @Override
        public void onPageSelected(int position) {
            mViewPager.setCurrentItem(position);
            mSectionsPagerAdapter.getItem(position);
            Toast.makeText(SellerActivity.this,
                    "Selected page position: " + position, Toast.LENGTH_SHORT).show();
        }

        // This method will be invoked when the current page is scrolled
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            // Code goes here
        }

        // Called when the scroll state changes:
        // SCROLL_STATE_IDLE, SCROLL_STATE_DRAGGING, SCROLL_STATE_SETTLING
        @Override
        public void onPageScrollStateChanged(int state) {
            // Code goes here
        }
    });
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_shop, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    Intent i = new Intent(SellerActivity.this, MainActivity.class);
    if (id == R.id.action_home) {
        startActivity(i);
        finish();
        return true;
    }
    if (id == R.id.action_logout) {
        mAuth.signOut();
        startActivity(i);
        finish();
        return false;
    }

    return super.onOptionsItemSelected(item);
}
/**
 * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a PlaceholderFragment (defined as a static inner class below).
        return SellerFragment.newInstance(position + 1, "1");
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
            case 0:
                return "En Attente";
            case 1:
                return "En cours";
            case 2:
                return "Terminées";
        }
        return null;
    }
}

• 然后我有一个如下所示的片段：

公共类 SellerFragment 扩展 Fragment {

private String title;
private int page;

private AllOrders allOrders;

final List<CommandItemForSeller> orders = new ArrayList<>();
final LinkedHashMap<String, Order> saveOrders = new LinkedHashMap<>();

public SellerFragment(){

}

// newInstance constructor for creating fragment with arguments
public static SellerFragment newInstance(int page, String title) {
    SellerFragment fragmentFirst = new SellerFragment();
    Bundle args = new Bundle();
    args.putInt("someInt", page);
    args.putString("someTitle", title);
    fragmentFirst.setArguments(args);
    return fragmentFirst;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    page = getArguments().getInt("someInt", 0);
    title = getArguments().getString("someTitle");
    allOrders = new AllOrders();
    setRetainInstance(true);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_seller, container, false);
    TextView tvLabel = (TextView) view.findViewById(R.id.tvLabel);
    tvLabel.setText(page + " -- " + title);

    DatabaseReference myRef = FirebaseDatabase.getInstance().getReference().child("orders");
    ChildEventListener childEventListener = new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {


            for (DataSnapshot orders : dataSnapshot.getChildren()) {

                String orderString = orders.getValue(String.class);

                //allOrders.addOrder(new Order(orderString, orders.getKey().substring(orders.getKey().length() - 4, orders.getKey().length())));
                saveOrders.put(orders.getKey(), new Order(orderString, orders.getKey().substring(orders.getKey().length() - 4, orders.getKey().length())));

            }

        }

        @Override
        public void onChildChanged (DataSnapshot dataSnapshot, String s){

        }

        @Override
        public void onChildRemoved (DataSnapshot dataSnapshot){

        }

        @Override
        public void onChildMoved (DataSnapshot dataSnapshot, String s){

        }

        @Override
        public void onCancelled (DatabaseError databaseError){

        }
    };

    myRef.addChildEventListener(childEventListener);
    orders.clear();

    allOrders.clear();
    allOrders.addAllOrders(saveOrders.values());
    List<Order> selectedOrders = allOrders.getItemsOrderedByStatus(page);
    for (int i = 0; i < selectedOrders.size(); i++) {
        orders.add(new CommandItemForSeller(selectedOrders.get(i)));
    }

    CommandItemForSellerAdapter aa = new CommandItemForSellerAdapter(getActivity(), R.layout.pending_listview_item, orders);


    ListView lv = (ListView) view.findViewById(R.id.commandListView);
    lv.setAdapter(aa);


    return view;
}

如您所见，每个片段都从 Firebase 数据库调用内容，因此它保持异步。

我的问题是：

当我尝试加载 “同步” 数据（我预先在我的代码中编写的）时，列表内容在第一次创建时由 onCreate 方法设置，因此每个选项卡的每个片段都会得到一些内容。但是，第二个选项卡永远不会刷新新数据。 因此，在异步的情况下，onCreate 方法不会返回任何数据（由于加载时间），并且应该由onCreateView 添加新数据（如刷新），但对于第二个选项卡它永远不会发生！

经过几个小时的调试，我刚刚发现了以下内容：

当我将从标签 1 切换到标签 2 时，会为标签 3 呈现片段。当我将 从选项卡 3 切换到选项卡 2 时，会为选项卡 1 呈现片段。即使 getItem(position) 返回选项卡的正确位置，选项卡 2 现在也不会调用 onCreateView 方法并保持为空。

请..告诉我为什么这个项目会这样，我不想放弃但我真的不明白这里有什么问题。

如果您需要更具体的信息，请告诉我，我可以给您任何您想要的。

Answer 1

使用 FragmentPagerAdapter 时，ViewPager 保持相邻页面“活动”（一个在左侧，一个在右侧）——这意味着当您只有三个片段时，第二个将永远不会被破坏。如果您想在片段进入视图时刷新数据，您必须找到一种不同的方法 - 一个想法是使用 ViewPager.addOnPageChangeListener 方法为页面更改添加侦听器 - https://developer.android.com/reference/android/support/v4/view/ViewPager.html#addOnPageChangeListener(android.support.v4.view.ViewPager.OnPageChangeListener) 然后，您可以在 onPageSelected 回调中触发刷新。

在您的情况下，我认为最简单的解决方案是让 SectionsPagerAdapter 实现 ViewPager.OnPageChangeListener，存储对每个创建的 Fragment 的引用（最好是 WeakReference），然后在适当的 SellerFragment 实例上调用一些刷新方法。