getSupportFragmentManager（）。即使片段存在，findFragmentById（）也会返回null

我在activity_main.xml中有fragment_main.xml。getSupportFragmentManager（）。即使片段存在，findFragmentById（）也会返回null

片段添加到从Android Studio模板创建的选项卡式活动中。

我想在活动内对片段视图进行更改。这就是为什么我创建了一个界面，允许片段在片段的onStart期间调用活动，当时我知道片段的视图可用于修改。

我确实得到这个工作，如果我通过我的接口回调实际片段实例。但我仍然感到沮丧，因为我不明白为什么这个函数返回null。这与重复使用不同屏幕方向的片段有什么关系？

我已经看过类似的问题在这里SO报道与此函数返回null。有人提到使用import android.support.v4.app.Fragment;而不仅仅是android.app.Fragment可能存在问题。我不认为这是我的情况，因为获得支持 FragmentManager似乎返回正确类型的片段（v4.app）。我不知道为什么v4.app在那里，我对这一切有点新鲜。

无论如何，这里的代码，所有的，包括进口，因为它们可能是相关的。

package org.axonnsd.musicnexus; 

import android.app.Activity; 
import android.content.Context; 
import android.media.MediaPlayer; 
import android.os.Environment; 
import android.os.Handler; 
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.MotionEvent; 
import android.view.View; 
import android.view.ViewGroup; 

import android.widget.MediaController; 
import android.widget.TextView; 

import java.io.FileInputStream; 
import java.io.IOException; 

public class MainActivity extends AppCompatActivity implements OnFragmentReadyListener { 

    /** 
    * 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 static final String TAG = "MyTabbedApplication"; 

    private Handler handler = new Handler(); 

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

    @Override 
    protected void onStop() { 
     super.onStop(); 
     mediaController.hide(); 
     mediaPlayer.stop(); 
     mediaPlayer.release(); 
    } 

    @Override 
    public boolean onTouchEvent(MotionEvent event) { 
     //the MediaController will hide after 3 seconds - tap the screen to make it appear again 
     mediaController.show(); 
     return false; 
    } 


    public void onFragmentReady(PlaceholderFragment fragment) 
    { 
     this.getIntent().putExtra(AUDIO_FILE_NAME,AUDIO_FILE_NAME); 
     audioFile = this.getIntent().getStringExtra(AUDIO_FILE_NAME); 

     PlaceholderFragment fragment = (PlaceholderFragment)(getSupportFragmentManager().findFragmentById(R.id.main_audio_view)); 
       //FRAGMENT IS NULL! 
     fragment.setNowPlayingText(audioFile); 
    } 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 

     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     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.container); 
     mViewPager.setAdapter(mSectionsPagerAdapter); 

     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(); 
      } 
     }); 
    } 

    @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 placeholder fragment containing a simple view. 
    */ 
    public static class PlaceholderFragment extends Fragment { 
     /** 
     * The fragment argument representing the section number for this 
     * fragment. 
     */ 
     private static final String ARG_SECTION_NUMBER = "section_number"; 
     private TextView _txtNowPlaying; 
     private MainActivity _parentActivity; 

     public PlaceholderFragment() { 
     } 

     /** 
     * Returns a new instance of this fragment for the given section 
     * number. 
     */ 
     public static PlaceholderFragment newInstance(int sectionNumber) { 
      PlaceholderFragment fragment = new PlaceholderFragment(); 
      Bundle args = new Bundle(); 
      args.putInt(ARG_SECTION_NUMBER, sectionNumber); 
      fragment.setArguments(args); 
      return fragment; 
     } 

     @Override 
     public View onCreateView(LayoutInflater inflater, ViewGroup container, 
           Bundle savedInstanceState) { 
      View rootView = inflater.inflate(R.layout.fragment_main, container, false); 
      TextView textView = (TextView) rootView.findViewById(R.id.section_label); 
      _txtNowPlaying = (TextView)rootView.findViewById(R.id.now_playing_text); 
      textView.setText(getString(R.string.section_format, getArguments().getInt(ARG_SECTION_NUMBER))); 
      //notifyActivity(_parentActivity); 
      return rootView; 
     } 

     @Override 
     public void onAttach(Activity activity) 
     { 
      super.onAttach(activity); 
      _parentActivity = (MainActivity)activity; 
     } 

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

      if (context instanceof Activity){ 
       _parentActivity = (MainActivity)context; 
      } 
     } 

     @Override 
     public void onStart() 
     { 
      super.onStart(); 
      notifyActivity(_parentActivity); 
     } 

     private void notifyActivity(MainActivity activity) 
     { 
      activity.onFragmentReady(this); 
     } 

     public void setNowPlayingText(String value) 
     { 
      _txtNowPlaying.setText(value); 
     } 
    } 

    /** 
    * 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 PlaceholderFragment.newInstance(position + 1); 
     } 

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

     @Override 
     public CharSequence getPageTitle(int position) { 
      switch (position) { 
       case 0: 
        return "SECTION 1"; 
       case 1: 
        return "SECTION 2"; 
       case 2: 
        return "SECTION 3"; 
      } 
      return null; 
     } 
    } 
}

我粘贴个XML也一样，尽管它们是很好的（正如我所说，它的工作原理，如果我通过实际的片段）。

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?> 
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:id="@+id/main_content" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:fitsSystemWindows="true" 
    tools:context="org.axonnsd.musicnexus.MainActivity"> 

    <android.support.design.widget.AppBarLayout 
     android:id="@+id/appbar" 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:paddingTop="@dimen/appbar_padding_top" 
     android:theme="@style/AppTheme.AppBarOverlay"> 

     <android.support.v7.widget.Toolbar 
      android:id="@+id/toolbar" 
      android:layout_width="match_parent" 
      android:layout_height="?attr/actionBarSize" 
      android:background="?attr/colorPrimary" 
      app:layout_scrollFlags="scroll|enterAlways" 
      app:popupTheme="@style/AppTheme.PopupOverlay"> 

     </android.support.v7.widget.Toolbar> 

    </android.support.design.widget.AppBarLayout> 

    <android.support.v4.view.ViewPager 
     android:id="@+id/container" 
     android:layout_width="match_parent" 
     android:layout_height="match_parent" 
     app:layout_behavior="@string/appbar_scrolling_view_behavior" /> 

    <android.support.design.widget.FloatingActionButton 
     android:id="@+id/fab" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_gravity="end|bottom" 
     android:layout_margin="@dimen/fab_margin" 
     app:srcCompat="@android:drawable/ic_dialog_email" /> 

</android.support.design.widget.CoordinatorLayout>

fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:id="@+id/main_audio_view" 
    android:layout_height="match_parent" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    tools:context="org.axonnsd.musicnexus.MainActivity$PlaceholderFragment"> 

    <TextView 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_gravity="center" 
     android:text="Now playing:" 
     android:textSize="25sp" 
     android:textStyle="bold" 
     /> 
    <TextView 
     android:id="@+id/now_playing_text" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_marginTop="20dip" 
     android:layout_marginLeft="10dip" 
     android:layout_marginRight="10dip" 
     android:layout_gravity="center" 
     android:text="Now playing.." 
     android:textSize="16sp" 
     android:textStyle="italic" 
     /> 

    <TextView 
     android:id="@+id/section_label" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" /> 

</RelativeLayout>

来源

2017-05-15 Axonn

首先，你用错了findFragmentById，通过你的片段根布局id作为参数，当它预期包含您的片段布局的id（不是内部布局一个片段！）

FragmentPagerAdapter是有点棘手，并得到当前片段不直截了当。检查这个答案： Getting the current Fragment instance in the viewpager

替换您：

PlaceholderFragment fragment = (PlaceholderFragment)(getSupportFragmentManager().findFragmentById(R.id.main_audio_view)); 
      //FRAGMENT IS NULL!

与

PlaceholderFragment page = (PlaceholderFragment) 
getSupportFragmentManager().findFragmentByTag("android:switcher:" + 
R.id.container + ":" + mViewPager.getCurrentItem());

来源

2017-05-15 15:07:37

我明白了！谢谢：）。正如你所说，这看起来有点棘手。我想通过函数调用将实例本身作为参数传递会更简洁吗？任何建议反对这样做？为什么有人会用这种方式，而不是将片段的实例传回活动？ – Axonn

将片段实例一次性传递是可以的，但是将该实例保存在字段中是不可靠的，因为'FragmentManager'可能会在屏幕旋转或其他状态更改时重新创建片段。我发布的片段更多的是当您需要更新当前显示的片段时，但您没有它的实例。 –

谢谢:)。就命名约定（“android：switcher”等）而言，findFragmentByTag是否“稳定”在各版本之间是一致的？有没有更好的方法来获取/保留碎片？我想知道为什么我不能通过说类似view_id.fragment_id.view_id来获得片段内的视图（TextView）为什么我不能将ID附加到片段？ – Axonn

我会尝试以下方法，但我敢肯定它不是最好的解决方案。

我会传递section_number而不是PlaceholderFragment的值，并从SectionPageAdapter中获取片段。事情是这样的：

public void onFragmentReady(int position) 
{ 
    this.getIntent().putExtra(AUDIO_FILE_NAME,AUDIO_FILE_NAME); 
    audioFile = this.getIntent().getStringExtra(AUDIO_FILE_NAME); 

    PlaceholderFragment fragment = (PlaceholderFragment)mSectionsPagerAdapter.getItem(position); 

    fragment.setNowPlayingText(audioFile); 
}

来源

2017-05-15 14:56:23 aborocz

getSupportFragmentManager（）。即使片段存在，findFragmentById（）也会返回null

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