支持MapFragment地图在轮换

Question

我正在使用基于地图的应用程序，该应用程序创建包含地图和Web视图的相册视图。这个问题发生在轮换期间，并且在com.android.support:support-v4:19.1中运行正常，但是一旦我更新到22.2.0就无法正常工作。

地图在初始视图中按预期显示，但在旋转后，地图不再显示。我验证了GoogleMap，容器视图（包含地图）和SupportMapFragment都是有效的。

在调试时，我决定检查SupportMapFragment的视图，看看是否可能是原因。旋转之后，对SupportMapFragment上的getView（）的调用返回null（我怀疑这是为什么地图没有出现，但我不明白为什么它最初是有效的并且在旋转后为null）。

我很遗憾，因为据我所知，我有一个有效的SupportMapFragment，一个有效的GoogleMap和一个具有适当大小的容器视图（在本例中为680x800）。正如我上面所说，使用之前的版本我使用的是19.1，这段代码按预期工作。

以下是代码的相关摘要：

SupportMapManager创建在EventReportAlbumActivity类

中

public class EventReportAlbumActivity extends FragmentActivity
{
   @Nullable protected SupportMapFragment m_mapFragment;

   @NotNull
   public SupportMapFragment getMapFragment ()
   {
     if( m_mapFragment == null )
     {
        GoogleMapOptions options = new GoogleMapOptions();
        options.compassEnabled( false );
        options.rotateGesturesEnabled( false );
        options.scrollGesturesEnabled( false );
        options.tiltGesturesEnabled( false );
        options.zoomControlsEnabled( false );
        options.zoomGesturesEnabled( false );

        m_mapFragment = SupportMapFragment.newInstance( options );
        m_mapFragment.setRetainInstance( true );
     }
     return m_mapFragment;
  }
}

显示地图和网络视图的实际Fragment实例

public class EventAlbumItemFragment extends Fragment
{
   @Nullable
   protected SupportMapFragment m_mapFragment;

  public static EventAlbumItemFragment newInstance ( )
  {
     final EventAlbumItemFragment fragment = new EventAlbumItemFragment();
     fragment.setRetainInstance( true );

     return fragment;
  }

    @Override
    public View onCreateView ( @NotNull LayoutInflater inflater,
                               @NotNull ViewGroup container,
                               @Nullable Bundle savedInstanceState )
    {
       final View view = inflater.inflate( R.layout.event_album_item, container, false );

       View mapContainer = view.findViewById( R.id.mapview_container );

       // make map fragment view invisible until we configure it so the coordinates will be correct
       mapContainer.setVisibility( View.INVISIBLE );

       // other code removed for brevity...
   }

  /**
   * Setter for the map fragment and its UI
   *
   * @param mapFragment The map fragment which should be added to this fragment's view, or null to remove the current fragment
   */
  public void setMapFragment ( @Nullable SupportMapFragment mapFragment )
  {
     final FragmentManager fragmentManager = getChildFragmentManager();

     if( m_mapFragment != null && m_mapFragment.isAdded() )
     {
        // Remove the existing map manager
        fragmentManager.beginTransaction()
           .remove( m_mapFragment )
           .commit();
     }

     if( mapFragment != null )
     {
        // Add the map view to the current view
        fragmentManager.beginTransaction()
           .add( R.id.mapview_container, mapFragment, MAP_FRAGMENT_TAG )
           .commit();
     }

     fragmentManager.executePendingTransactions();

     m_mapFragment = mapFragment;
     configureMap();
  }

  protected void configureMap ()
  {
     if ( m_mapFragment != null && m_eventMapFeature != null && m_eventMapFeature.haveValidLocation() == true )
     {
        final EventAlbumItemFragment localThis = this ;

        m_mapFragment.getMapAsync( new OnMapReadyCallback()
        {
           @Override
           public void onMapReady( GoogleMap googleMap )
           {
              final View mapContainer = getView().findViewById( R.id.mapview_container );
              final int mapWidth = mapContainer.getWidth();
              final int mapHeight = mapContainer.getHeight();

              // we have our map and the coordinates should be correct. Make map visible and proceed
              mapContainer.setVisibility( View.VISIBLE );

              // This line below crashes because view is NULL after rotation. 
              // Left here to illustrate what I believe to be the root cause of the issue
              // m_mapFragment.getView().setVisibility( View.VISIBLE );

              // Remove all overlays from the map
              googleMap.clear();

              final DisplayMetrics displayMetrics = getResources().getDisplayMetrics();

              final float zoom = (float)GeoUtils.getZoomForMetersWide( MAP_MINIMUM_METER_SPAN,
                      mapWidth / displayMetrics.scaledDensity,
                      m_eventMapFeature.getCoordinate().latitude );

              // Change the map bounds
              CameraUpdate cameraUpdate;

              // draw some items on the map
              final LatLngBounds bounds = m_eventMapFeature.getBounds();

              cameraUpdate = CameraUpdateFactory.newLatLngZoom( bounds.southwest, zoom );

              googleMap.moveCamera( cameraUpdate );
           }
        } );
     }
  }

  /** Assign the map fragment variable on resume. This is necessary for orientation change events */
  @Override
  public void onResume ()
  {
     super.onResume();

     // If the map fragment hasn't been set, but this is the current item, get the map fragment from the parent
    final EventReportAlbumActivity activity = (EventReportAlbumActivity)getActivity();

      if( m_mapFragment == null && this == activity.getCurrentItemFragment() )
      {
         setMapFragment( activity.getMapFragment() );
      }

     //
     configureMap();
  }

}

UPDATE：上面的setMapFragment也会在有多个页面时崩溃，并且用户在轮播后浏览相册

经过一些测试后，我还在相册中有多个页面时测试了此代码。没有旋转，我可以滚动浏览专辑，没有任何问题。但是，如果我旋转设备并尝试滚动相册，则应用程序崩溃。同样，使用支持库的第19版，此代码按预期工作。这仅在升级到版本22后才开始发生。

在EventAlbumItemFragment：

中的方法setMapFragment（）中

fragmentManager.executePendingTransactions();

我得到以下堆栈跟踪：

java.lang.IllegalStateException: Could not execute method of the activity
    at android.view.View$1.onClick(View.java:4020)
    at android.view.View.performClick(View.java:4780)
    at android.view.View$PerformClick.run(View.java:19866)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5254)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.reflect.InvocationTargetException
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at android.view.View$1.onClick(View.java:4015)
    at android.view.View.performClick(View.java:4780)
    at android.view.View$PerformClick.run(View.java:19866)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5254)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.ArrayList.set(int, java.lang.Object)' on a null object reference
    at android.support.v4.app.FragmentManagerImpl.makeInactive(FragmentManager.java:1192)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1099)
    at android.support.v4.app.FragmentManagerImpl.removeFragment(FragmentManager.java:1235)
    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:710)
    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1501)
    at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:490)
    at crc.carsapp.fragments.MapAlbumItemFragment.setMapFragment(MapAlbumItemFragment.java:78)
    at crc.carsapp.listeners.OnMapAlbumScrollListener.onPageSelected(OnMapAlbumScrollListener.java:46)
    at crc.carsapp.listeners.OnEventAlbumViewScrollListener.onPageSelected(OnEventAlbumViewScrollListener.java:34)
    at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1786)
    at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:568)
    at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:552)
    at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:513)
    at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:505)
    at crc.carsapp.activities.AlbumActivity.scrollToPrevious(AlbumActivity.java:133)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at android.view.View$1.onClick(View.java:4015)
    at android.view.View.performClick(View.java:4780)
    at android.view.View$PerformClick.run(View.java:19866)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5254)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Answer 1

因此，上述代码存在2个问题。首先（也是最关键的），Activity（EventReportAlbumActivity）负责创建地图片段（及其相关片段保留对它的引用）。在轮换期间，活动（当然）被破坏并重新创建。因此，后续调用getMapFragment将生成一个新的SupportMapFragment。同时，显示地图的片段仍然具有对先前地图片段的引用。

每当在setMapFragment中调用remove时，它都会引用旧的SupportMapFragment，从而引发异常。

这也导致地图在旋转后无法正常显示。

修复是在setMapFragment中取出要删除的调用（ViewPager处理从onPageSelected中的前一页删除）并添加＆＃39; new＆＃39;根据需要在setMapFragment中使用SupportMapFragment。

在升级到版本22之前原始代码如何工作是一个谜，因为这似乎是一个错误，无论支持版本如何。并且，解决方案仍然感觉不太正确，因为SupportMapFragment将其setRetainInstance设置为true，这意味着我们希望它在片段的整个生命周期中存在，但是当在旋转期间重新创建活动时，活动会覆盖它。所以，如果有人有更优雅的解决方案，我很乐意听到它。

相关更新代码如下：

public class EventAlbumItemFragment extends Fragment
{
   public void setMapFragment ( @Nullable SupportMapFragment mapFragment )
  {
     final FragmentManager fragmentManager = getChildFragmentManager();

     // The working assumption is that our map fragment, if already added, is added to
     // our current fragment.
     if ( mapFragment != null && mapFragment.isAdded() == false )
     {
        // Add the map view to the current view
        fragmentManager.beginTransaction()
           .add( R.id.mapview_container, mapFragment, MAP_FRAGMENT_TAG )
           .commit() ;
        fragmentManager.executePendingTransactions();
     }

     m_mapFragment = mapFragment;
     configureMap();
  }

  @Override
  public void onResume ()
  {
     super.onResume();

     // We can get here in (at least) 3 different ways:
     // 1. The first time this fragment is created.
     //       In this case, we will not have a local reference to a map fragment and will will add
     //       it if we are the currently displaying fragment.
     // 2. After rotation
     //       Because the actual map fragment is maintained by our activity, the map fragment will
     //       have been recreated and our reference to it will no longer be valid.
     // 3. After a pause event (such as the device going into power saving mode).
     //       We should still have a reference to the current map fragment and it should be
     //       valid. We can test that by checking if the map fragment is attached. This is done
     //       in the setMapFragment() method.
     //
     final AlbumActivity activity = (AlbumActivity)getActivity();

     Fragment currentFragment = activity.getCurrentItemFragment() ;

     if ( m_mapFragment == null && this == currentFragment )
     {
        setMapFragment( activity.getMapFragment() );
     }
     else if ( m_mapFragment != null )
     {
        setMapFragment( activity.getMapFragment() );
     }

     configureMap();
   }
}