对Fragment调用getView返回null

Question

因此，正如您在onCreate方法中看到的那样，我在"fragments"列表中的第0个fragment上调用getView。它返回null。我试着在代码中放入一个计时器，让它每秒运行一次，并检查视图是否为null。在计时器的第二个刻度上，视图不是空的。基本上

主要活动.java

package com.axinite.standapp;
import android.graphics.Color;
import android.os.CountDownTimer;
import android.os.Handler;
import android.support.annotation.Nullable;
import android.support.design.widget.TabLayout;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.widget.EditText;
import android.widget.NumberPicker;
import android.widget.RelativeLayout;
import android.widget.TextView;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;
import java.util.Vector;
public class MainActivity extends AppCompatActivity {
    int i = 0;
    /**
     * 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;
    /**
     * The {@link ViewPager} that will host the section contents.
     */
    private ViewPager mViewPager;
    int standUpSeconds = 4000;
    List<android.support.v4.app.Fragment> fragments = new Vector<android.support.v4.app.Fragment>();
    NumberPicker SUPickerH;
    NumberPicker SUPickerM;
    NumberPicker SUPickerS;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        fragments.add(Fragment.instantiate(this, BlankFragment.class.getName()));
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager(), fragments);
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);
        final TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);
        tabLayout.setSelectedTabIndicatorColor(Color.WHITE);
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
        final int[] ICONS = new int[] {
                R.drawable.walking,
                R.drawable.iris
        };
        for (int i=0; i < tabLayout.getTabCount(); i++)
        {
            tabLayout.getTabAt(i).setIcon(getDrawable(ICONS[i]));
        }//endfor)
        View view = ((BlankFragment) fragments.get(0)).getView();
        if(view != null) {
            TextView tv = (TextView) view.findViewById(R.id.glupost);
            tv.setText("dasdas");
            //Toast.makeText(getApplicationContext(), ((Fragment) fragments.get(0)).toString(), Toast.LENGTH_SHORT).show();
        }
    }
    void Setup() {

    }

    @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_main, 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
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {
        List<Fragment> fragments;
        public SectionsPagerAdapter(FragmentManager fm, List<Fragment> fragments) {
            super(fm);
            this.fragments = fragments;
        }
        @Override
        public Fragment getItem(int position) {
            return fragments.get(position);
        }
        @Override
        public int getCount() {
            // Show 3 total pages.
            return fragments.size();
        }
        @Override
        public CharSequence getPageTitle(int position) {
            return "";
        }
    }
}

BlankFragment.java

    package com.axinite.standapp;
import android.app.Activity;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

/**
 * A simple {@link Fragment} subclass.
 * Activities that contain this fragment must implement the
 * {@link BlankFragment.OnFragmentInteractionListener} interface
 * to handle interaction events.
 * Use the {@link BlankFragment#newInstance} factory method to
 * create an instance of this fragment.
 */
public class BlankFragment extends Fragment {
    protected View mView;
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";
    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;
    private OnFragmentInteractionListener mListener;
    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment BlankFragment.
     */
    // TODO: Rename and change types and number of parameters
    public static BlankFragment newInstance(String param1, String param2) {
        BlankFragment fragment = new BlankFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }
    public BlankFragment() {
        // Required empty public constructor
    }
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_blank, container, false);
        mView = view;
        return view;
    }
    // TODO: Rename method, update argument and hook method into UI event
    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }
    }
    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnFragmentInteractionListener) activity;
        } catch (ClassCastException e) {
        }
    }
    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }
    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p/>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        public void onFragmentInteraction(Uri uri);
    }
}

Answer 1

Activity与其Fragment通信的典型方法如下：

MyFragment frag = (MyFragment) getFragmentManager().findFragmentById(R.id.container_of_fragment);
frag.getMyTextView().setText("New text");

或（通常首选IMO）

MyFragment frag = (MyFragment) getFragmentManager().findFragmentByTag("fragment_tag_string_value");
frag.getMyTextView().setText("New text");

然而，对于ViewPager，这变得更加困难，因为容器的"id"是ViewPager本身，它包含多个片段。此外，没有一个内置的适配器机制（据我所知）可以将标记设置为片段。

我能想到两种方法来规避这一点。首先，您可以使用EventBus，例如GreenRobot的EventBus，或者来自Android API的LocalBroadcastManager。GreenRobot的EventBus也是这个类似问题的推荐技术。

另一种选择是利用Fragment总是引用其主机Activity的事实，这意味着Fragment可以通过interface从Activity请求文本，后者将文本返回给Fragment:

public class MainActivity implements FragmentOne.Callbacks {
    //...
    @Override
    public String requestTextViewString() {
        return "Text to send to Fragment"
    }

在你假设的FragmentOne中：

public class FragmentOne extends Fragment {
    private Callbacks mCallback;
    private TextView mTextView;
    public interface Callbacks {
        String requestTextViewString();
    }
    @Override
    public void onAttach(Activity activity) {
        if (activity instanceof Callbacks) {
            mCallback = (Callbacks) activity;
        }
    }
/*
*Later in your Fragment, when you want to get text from your MainActivity
* i.e. in onCreateView()
*/
    mTextView.setText(mCallback.requestTextViewString());

编辑：还要记住，尝试从Activity中的onCreate()访问Fragment可能会导致NullPointerException。这张图显示了两个生命周期。然而，这不应该是一个问题，因为在Fragment的"设置"生命周期事件中所需的任何东西都可以作为Fragment的newInstance()方法中的参数传递：

public class FragmentOne extends Fragment {
    private static final String KEY = "text_for_text_view_key";
    public static FragmentOne newInstance(String textForTextView) {
        Bundle args = new Bundle();
        FragmentOne frag = new FragmentOne();
        args.putString(KEY, textForTextView);
        frag.setArguments(args);
        return frag;
    }
//And when required...
    mTextView.setText(getArguments().getString(KEY));