在最后的两天里，我花了所有的时间试图在带有片段的viewpager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)。
情况是这样的：
- 我有一个活动，它实例化了一个查看页程序和一个适配器，并包含对象适配器类。
public class SellerActivity extensions 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;
}
}
- 然后我有一个片段，看起来像这样：
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，每个片段都会调用来自 Firebase 数据库的内容，因此它保持异步。
我的问题是：
当我尝试加载"同步">数据(我之前在我的代码中编写)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获得一些内容。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onCreateView应该添加新数据(例如刷新)，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，我刚刚发现以下内容：
当我从选项卡1 切换到选项卡 2时，将为选项卡 3 渲染片段。当我从选项卡3 切换到选项卡 2 时，将为选项卡 1 渲染片段。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，请告诉我我可以为您提供任何您想要的信息。
当使用FragmentPagerAdapter时，ViewPager会保持相邻页面"活动"(一个在左边，一个在右边) - 这意味着当你只有三个片段时，第二个永远不会被销毁。如果你想在片段进入视图时刷新数据，你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，您可以从 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 SectionPagerAdapter 实现 ViewPager.OnPageChangeListener，存储对每个创建的片段(最好是 WeakReference)的引用，然后在适当的 SellerFragment 实例上调用一些刷新方法。