FragmentTabHost 片段适配器在返回选项卡时为空，片段/视图保留答案

【问题标题】：FragmentTabHost Fragment Adapters Are Empty Upon Returning To Tab, Fragment/Views RemainFragmentTabHost 片段适配器在返回选项卡时为空，片段/视图保留
【发布时间】：2017-01-05 03:54:51
【问题描述】：

我发现了几个关于此的问题，但没有一个对我有帮助。每个问题都与我没有在我的片段中实现的其他功能和视图有关，问题不在于我需要交换我的方法，让 FragmentManager 到我的片段中的 getChildFragmentManager()，因为我不需要得到一个FragmentManager 那里。

我猜我的问题源于片段，而不是主要活动中的 FragmentTabHost，但我不太确定。完全没有。我所知道的是，当您在选项卡之间分页时，适配器内容会消失，但片段本身不会消失。所有视图仍然正常工作，因此每个片段的功能保持不变。

只有在我添加了一个选项卡更改侦听器以了解何时为我的聊天片段初始化适配器后才会出现此问题。

请注意，选项卡的内容在第一次初始化时很好，但是当您返回选项卡时，适配器中的内容为空。这意味着在创建 FragmentTabHost 时尚未初始化的选项卡，隐藏的选项卡尚未初始化，因此在您第一次翻页时它们仍然可以工作。

通过调试，我可以看到在转换发生时会出现此问题，并且在使用会话期间所有适配器都将保持为空。我将这段代码放在我的 tabHost.setOnTabChangedListener 调用中的初始检查之前：

//Before paging back to an initialized tab for the first time, the adapters of the initialized tab is populated.
Log.d("test", "pre");
new Handler().postDelayed(new Runnable() {
    @Override
    public void run() {
        //At this point, the adapter is empty.
        Log.d("test", "post");
    }
}, 50);

两个片段如下：

public class GroupTasksFragment extends Fragment {

public ArrayAdapter<String> adapter;
private Context context;
public ListView taskListView;

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

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

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_group_tasks, container, false);
    taskListView = (ListView) rootView.findViewById(R.id.tasksList);
    adapter = new ArrayAdapter<>(context, android.R.layout.simple_list_item_1, new ArrayList<String>());
    taskListView.setAdapter(adapter);
    return rootView;
}

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    this.context = context;
}

@Override
public void onDetach() {
    super.onDetach();
}
}

public class GroupChatFragment extends Fragment{

public ArrayAdapter<String> adapter;
private Context context;
public ListView chatListView;

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

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

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_group_chat, container, false);
    chatListView = (ListView) rootView.findViewById(R.id.chatList);
    adapter = new ArrayAdapter<>(context, android.R.layout.simple_list_item_1, new ArrayList<String>());
    chatListView.setAdapter(adapter);
    return rootView;
}

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    this.context = context;
}

@Override
public void onDetach() {
    super.onDetach();
}
}

FragmentTabHost 的主要活动（我排除了只接受输入并将内容发送到 PubNub 的方法）：

public class GroupContentActivity extends AppCompatActivity {

private GroupChatFragment chatFrag;
private GroupTasksFragment taskFrag;
private FragmentTabHost tabHost;
private PubNub connection;
private String groupName;
private String nickName;
private boolean chatFragInitialized = false;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_group_content);
    tabHost = (FragmentTabHost) findViewById(android.R.id.tabhost);
    tabHost.setup(this, getSupportFragmentManager(), android.R.id.tabcontent);

    tabHost.addTab(tabHost.newTabSpec("tasks").setIndicator("Tasks"),
            GroupTasksFragment.class, null);

    tabHost.addTab(tabHost.newTabSpec("chat")
            .setIndicator("Chat"), GroupChatFragment.class, null);

    groupName = getIntent().getStringExtra("groupName");
    nickName = getIntent().getStringExtra("nickName");
    PNConfiguration config = new PNConfiguration();
    config.setPublishKey(Constants.publishKey);
    config.setSubscribeKey(Constants.subscribeKey);
    connection = new PubNub(config);



    tabHost.setOnTabChangedListener(new TabHost.OnTabChangeListener() {
        @Override
        public void onTabChanged(String tabId) {
            if (!chatFragInitialized && tabId.equals("chat")) {
                chatFragInitialized = true;
                new Handler().postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        chatFrag = (GroupChatFragment) getSupportFragmentManager().findFragmentByTag("chat");
                        connection.history()
                                .channel(groupName)
                                .count(50)
                                .async(new PNCallback<PNHistoryResult>() {
                                    @Override
                                    public void onResponse(PNHistoryResult result, PNStatus status) {
                                        for (PNHistoryItemResult item : result.getMessages()) {
                                            final String[] sForm = item.getEntry().getAsString().split(">>>>");
                                            String m = "";
                                            if (sForm.length > 2) {
                                                for (int x = 1; x < sForm.length; x++) {
                                                    m += sForm[x];
                                                }
                                            } else {
                                                m = sForm[1];
                                            }

                                            final String mCopy = m;

                                            runOnUiThread(new Runnable() {
                                                @Override
                                                public void run() {
                                                    switch (sForm[0]) {
                                                        case "groupCreated":
                                                            chatFrag.adapter.clear();
                                                            break;
                                                        case "chat":
                                                            chatFrag.adapter.add(mCopy);
                                                    }
                                                }
                                            });

                                        }
                                    }
                                });
                    }
                }, 50);
            }
        }
    });

    new Handler().postDelayed(new Runnable() {
        @Override
        public void run() {
            taskFrag = (GroupTasksFragment) getSupportFragmentManager().findFragmentByTag("tasks");

            connection.history()
                    .channel(groupName)
                    .count(50)
                    .async(new PNCallback<PNHistoryResult>() {
                        @Override
                        public void onResponse(PNHistoryResult result, PNStatus status) {
                            for (PNHistoryItemResult item : result.getMessages()) {
                                final String[] sForm = item.getEntry().getAsString().split(">>>>");
                                String m = "";
                                if (sForm.length > 2) {
                                    for (int x = 1; x < sForm.length; x++) {
                                        m += sForm[x];
                                    }
                                } else {
                                    m = sForm[1];
                                }

                                final String mCopy = m;

                                runOnUiThread(new Runnable() {
                                    @Override
                                    public void run() {
                                        switch (sForm[0]) {
                                            case "addTask":
                                                if (taskFrag.adapter.getPosition(mCopy) < 0) {
                                                    taskFrag.adapter.add(mCopy);
                                                }
                                                break;
                                            case "deleteTask":
                                                if (taskFrag.adapter.getPosition(mCopy) >= 0) {
                                                    taskFrag.adapter.remove(mCopy);
                                                }
                                                break;
                                            case "groupCreated":
                                                taskFrag.adapter.clear();
                                                break;
                                        }
                                    }
                                });

                            }
                        }
                    });

            connection.addListener(new SubscribeCallback() {
                @Override
                public void status(PubNub pubnub, PNStatus status) {
                    if (status.getCategory() == PNStatusCategory.PNUnexpectedDisconnectCategory) {
                        Toast.makeText(getApplicationContext(), "You were disconnected!", Toast.LENGTH_SHORT).show();
                    } else if (status.getCategory() == PNStatusCategory.PNConnectedCategory) {
                        if (status.getCategory() == PNStatusCategory.PNConnectedCategory) {
                            pubnub.publish().channel(groupName).message("chat>>>><ADMIN> User '" + nickName + "' Connected").async(new PNCallback<PNPublishResult>() {
                                @Override
                                public void onResponse(PNPublishResult result, PNStatus status) {
                                }
                            });
                        }
                    } else if (status.getCategory() == PNStatusCategory.PNReconnectedCategory) {
                        Toast.makeText(getApplicationContext(), "You were reconnected!", Toast.LENGTH_SHORT).show();
                    }
                }

                @Override
                public void message(PubNub pubnub, PNMessageResult message) {
                    final String[] sForm = message.getMessage().getAsString().split(">>>>");
                    String m = "";
                    if (sForm.length > 2) {
                        for (int x = 1; x < sForm.length; x++) {
                            m += sForm[x];
                        }
                    } else {
                        m = sForm[1];
                    }

                    final String mCopy = m;

                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            switch (sForm[0]) {
                                case "chat":
                                    if (chatFragInitialized) {
                                        chatFrag.adapter.add(mCopy);
                                        runOnUiThread(new Runnable() {
                                            @Override
                                            public void run() {
                                                chatFrag.chatListView.setSelection(chatFrag.adapter.getCount() - 1);
                                            }
                                        });
                                    }
                                    break;
                                case "addTask":
                                    taskFrag.adapter.add(mCopy);
                                    connection.publish().channel(groupName).message("chat>>>><ADMIN> Task '" + mCopy + "' added.").async(new PNCallback<PNPublishResult>() {
                                        @Override
                                        public void onResponse(PNPublishResult pnPublishResult, PNStatus pnStatus) {
                                        }
                                    });
                                    break;
                                case "deleteTask":
                                    taskFrag.adapter.remove(mCopy);
                                    connection.publish().channel(groupName).message("chat>>>><ADMIN> Task '" + mCopy + "' deleted.").async(new PNCallback<PNPublishResult>() {
                                        @Override
                                        public void onResponse(PNPublishResult pnPublishResult, PNStatus pnStatus) {
                                        }
                                    });
                                    break;
                            }
                        }
                    });
                }

                @Override
                public void presence(PubNub pubnub, PNPresenceEventResult presence) {
                }
            });
            connection.subscribe().channels(java.util.Collections.singletonList(groupName)).execute();
        }
    }, 100);
}

@Override
public void onDestroy(){
    super.onDestroy();
    connection.publish().channel(groupName).message("chat>>>><ADMIN> User '" + nickName + "' Logged Out.").async(new PNCallback<PNPublishResult>() {
        @Override
        public void onResponse(PNPublishResult pnPublishResult, PNStatus pnStatus) {
        }
    });
    connection.disconnect();
    Toast.makeText(getApplicationContext(), "Logged out", Toast.LENGTH_SHORT).show();
}

//More Methods
}

另外请注意，问题不在于我需要存储 FragmentManager 实例，因为这没有任何作用。

【问题讨论】：

标签： android android-fragments fragment-tab-host

【解决方案1】：

我发现了我的问题。事实证明，每次在 FragmentTabHost 中分页到一个片段时，都会再次调用它的 createView 方法，并且只有那个方法，所以通过在那个视图中将片段中的适配器设置为空，我认为这只是在开始时，我每次都重置适配器。

我通过将适配器内容保留为实例变量列表对象来解决此问题，当我想更改适配器时，我向/从该对象中添加或删除字符串。不要也将字符串放入适配器中，更新列表就足够了。该列表会直接将其添加到适配器中。

另请注意，如果您将初始内容设置在片段之外，它可能不会显示第一次初始化选项卡的时间。请注意您的语句顺序和调用时间。片段构建是一项时髦的业务。

然后，每次调用 createView 方法时，我都会将适配器设置为列表中的任何内容。

【讨论】：