恢复活动后，片段中定义的自定义视图始终为空

Question

我有Activity（称为MonitorActivity），其中包含ViewPager。 MonitorActivity绑定到本地Service并从服务接收Broadcasts。 收到服务广播后，我希望通过调用update方法在ViewPager中update个片段。 一切都行，直到我离开活动一段时间。当我返回MonitorActivity时，应用崩溃，因为片段中的某些观看次数为null。 查看Activity和Fragment生命周期日志：

// First Run
Activity onCreate
Activity onStart
Activity onResume
Fragment onAttach
Fragment onCreate
Fragment onCreateView
Fragment onActivityCreated
Fragment onStart
Fragment onResume
Fragment update :tempView: Not null
Fragment update :tempView: Not null
Fragment update :tempView: Not null
...
// Leaving Activity
Fragment onPause
Activity onPause
Fragment onStop
Activity onStop
// Returning back to Activity
Activity onCreate
Fragment onAttach
Fragment onCreate
Fragment onCreateView
Fragment onActivityCreated
Fragment onStart
Activity onStart
Activity onResume
Fragment onResume
Fragment update :tempView: Null
Fragment update :tempView: Null
Fragment update :tempView: Null
...

我的问题是：重新创建活动会导致在片段内重新创建ViewPager，Adapter，Fragments和Views。 tempView初始化了Fragment's onCreateView。返回null后，为什么此视图为MonitorActivity？

MonitorActivity：

private ViewPager viewpager;
private PagerAdapter pagerAdapter;
private FragmentManager fragmentManager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    Log.d(TAG, "Activity onCreate");
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_monitor);

    fragmentManager = getSupportFragmentManager();

    pagerAdapter = new PagerAdapter(fragmentManager);
    viewpager = (ViewPager) findViewById(R.id.viewpager);
    viewpager.setAdapter(pagerAdapter);

    // ...
    Intent gattServiceIntent = new Intent(this, BluetoothLeService.class);
    bindService(gattServiceIntent, mServiceConnection, BIND_AUTO_CREATE);
}

@Override
protected void onResume() {
    super.onResume();
    Log.d(TAG, "Activity onResume");
    registerReceiver(updateReceiver, makeUpdateIntentFilter());
}

@Override
protected void onPause() {
    super.onPause();
    Log.d(TAG, "Activity onPause");
    unregisterReceiver(mUpdateReceiver);
}

private final BroadcastReceiver mUpdateReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        final Bundle extras = intent.getExtras();
        String deviceAddress = extras.getString(Constants.KEY_EXTRAS_DEVICE_ADDRESS);
        pagerAdapter.getFragment(deviceAddress).update(intent); // <== Calling this method of MonitorFragment instance throws NullPointerException
    }
};

PagerAdapter内部课程：

class PagerAdapter extends FragmentPagerAdapter {

    private HashMap<String, MonitorFragment> fragments;
    private ArrayList<Item> items;

    public PagerAdapter(FragmentManager fm) {
        super(fm);

        fragments = new HashMap<>();
        items = ItemRepository.getInstance().getAll();

        for (Item item : items) {
            String deviceAddress = item.getDeviceAddress();
            fragments.put(deviceAddress, MonitorFragment.newInstance(deviceAddress));
        }
    }

    @Override
    public Fragment getItem(int position) {
        return fragments.get(items.get(position).getDeviceAddress());
    }

    @Override
    public int getCount() {
        return items == null ? 0 : items.size();
    }

    public MonitorFragment getFragment(String deviceAddress) {
        return fragments.get(deviceAddress);
    }

    public HashMap<String, MonitorFragment> getFragments() {
        return fragments;
    }

}

MonitorFragment：

public static MonitorFragment newInstance(String deviceAddress) {
    MonitorFragment fragment = new MonitorFragment();
    Bundle args = new Bundle();
    args.putString(ARG_DEVICE_ADDRESS, deviceAddress);
    fragment.setArguments(args);
    return fragment;
}

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

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.d(TAG, "Fragment onCreate");
    if (getArguments() != null) {
        deviceAddress = getArguments().getString(ARG_DEVICE_ADDRESS);
        item = ItemRepository.getInstance().getByDeviceAddress(deviceAddress);
    }
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    Log.d(TAG, "Fragment onCreateView");
    view = inflater.inflate(R.layout.fragment_monitor, container, false);
    tempView = (TempView) view.findViewById(R.id.tempView);
    // ...
    return view;
}

public void update(Intent intent) {
    // ...
    temp.setValue(Something); // <== Throws NullPointerException
    // ...
}