从活动接收数据时碎片崩溃

Question

我有一个问题，我的活动正在进行一个网络调用，当网络调用完成时，它使用从调用中收到的JSON对象的数据对活动进行了一些更改，然后将对象传递给同一活动中的片段。这些碎片在一个台面上。

我有同样的问题，我在这里问了这个问题，但我似乎也有同样的问题，即使在没有改变任何重要的东西后，它有了一点作用。我只是添加了更多我想要更改的字段？

我遇到的问题是，如果我使用system.out.println（），它会打印出正确的数据。当我想用片段中收到的数据设置TextView时，应用程序就会崩溃，出现NullPointer。当我用Android studio中的debug调试它时，我设置的TextView由于某种原因总是为空。

执行初始网络调用的活动代码：

  @Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_listings);
ButterKnife.bind(this);
setSupportActionBar(toolbar);
handleIntent(getIntent());

}


private void handleIntent(Intent aIntent) {
if (aIntent != null) {

  String tradeType = aIntent.getStringExtra("itemType");
  String tradeId = aIntent.getStringExtra("itemId");

  presenter = new ItemPresenterImpl(this, ItemBuyNowActivity.this);   
  presenter.doListingServiceCall(tradeId); // <------- This is the where I send the Trade Id so I can do the network call.
} else {
  System.out.println("Intent is null in " + ItemBuyNowActivity.class.getSimpleName());
}

}

活动与演示者之间的接口：

public interface ItemPresenter {

void doListingServiceCall(String itemId);  //<------- Comes to this Interface

void doToolbarBackgroundImageCall(TradeItem aTradeItem);

}

实现演示者的类：

@Override
  public void doListingServiceCall(String aItemId) {  // <------- This is where the network call starts
    String homeURL = BobeApplication.getInstance().getWsURL() + mContext.getString(R.string.ws_url_item) + aItemId;
    BobeJSONRequest jsObjRequest = new BobeJSONRequest(Request.Method.GET, homeURL, null, this, this);
    VolleySingleton.getInstance().addToRequestQueue(jsObjRequest, "ListingRequest");
  }

  @Override
  public void doToolbarBackgroundImageCall(TradeItem aTradeItem) {
    ImageRequest request = new ImageRequest(aTradeItem.getItem().getImageUrl(),
            new Response.Listener<Bitmap>() {
              @Override
              public void onResponse(Bitmap bitmap) {
                Drawable drawable = new BitmapDrawable(mContext.getResources(), bitmap);
                mItemView.loadBackgroundImage(drawable);
              }
            }, 0, 0, null,
            new Response.ErrorListener() {
              public void onErrorResponse(VolleyError error) {
                mItemView.displayErrorMessage(VolleyErrorHelper.getErrorType(error, mContext) + " occurred downloading background image");
              }
            });
    VolleySingleton.getInstance().addToRequestQueue(request, "ListItemToolbarBackgroundImageRequest");
  }

  @Override
  public void onResponse(Object response) { 
    Gson gson = new Gson();
    TradeItem tradeItem = gson.fromJson(response.toString(), TradeItem.class);
    mItemView.populateListViews(tradeItem); // <------- This is the where I send the Object so the views in the activity can be manipulated
    doToolbarBackgroundImageCall(tradeItem);
  }

方法中处理

  @Override
public void populateListViews(TradeItem aTradeItem) {
  mOverviewPresenter = new OverviewPresenterImpl(new OverviewListItemFragment(), aTradeItem);
  OverviewListItemFragment.setData(aTradeItem);  //<------- This is the where I send the Object to the fragment so i can manipulate the views in the fragment
}
class TabAdapter extends FragmentPagerAdapter {
    public TabAdapter(FragmentManager fm) {
      super(fm);
    }

    @Override
    public Fragment getItem(int position) {
      Fragment fragment = null;

      if (position == 0) {
        fragment = new OverviewListItemFragment();
      }
      if (position == 1) {
        fragment = new DescriptionListItemFragment();
      }
      if (position == 2) {
        fragment = new ShippingListItemFragment();
      }
      if (position == 3) {
        fragment = new PaymentListItemFragment();
      }
      return fragment;
    }

    @Override
    public int getCount() {
      return 4;
    }

    @Override
    public CharSequence getPageTitle(int position) {
      if (position == 0) {
        return "Overview";
      }
      if (position == 1) {
        return "Description";
      }
      if (position == 2) {
        return "Shipping";
      }
      if (position == 3) {
        return "Payment";
      }
      return null;
    }
  }

接收数据的片段：

    public class OverviewListItemFragment extends Fragment implements OverviewView {

  private static TextView mOverViewHeading;

  public OverviewListItemFragment() {
  }

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

  @Nullable
  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.overview_list_item_fragment, container, false);
    mOverViewHeading = (TextView) view.findViewById(R.id.frag_overview_heading_textview);
    return view;

  }

  @Override
  public void populateOverviewViews(final TradeItem aTradeItem) {
    System.out.println("Overview Trade Object title is:" + aTradeItem.getItem().getTradeTitle());  // <------- This is print statement works 100% but when I try setting mOverViewHeading to the text in aTradeItem.getItem().getTradeTitle() I get a Null pointer Exception.

  }

  public static void setData(TradeItem aTradeItem) {
    System.out.println("Overview Trade Object title is:" + aTradeItem.getItem().getTradeTitle());  // <------- This is print statement works 100% but when I try setting mOverViewHeading to the text in aTradeItem.getItem().getTradeTitle() I get a Null pointer Exception.
    mOverViewHeading.setText(aTradeItem.getItem().getTradeTitle());// <------- This is where it crashes and mOverViewHeading is still null at this point.
  }
}

编辑：对不起，我忘记了logcat：

02-05 17:08:21.554 30512-30512/com.example.app E/AndroidRuntime: FATAL EXCEPTION: main
                                                          java.lang.NullPointerException
                                                              at com.example.app.ui.fragments.OverviewListItemFragment.setData(OverviewListItemFragment.java:46)
                                                              at com.example.app.ui.activities.ItemBuyNowActivity.populateListViews(ItemBuyNowActivity.java:95)
                                                              at com.example.app.listing.ItemPresenterImpl.onResponse(ItemPresenterImpl.java:62)
                                                              at com.android.volley.toolbox.JsonRequest.deliverResponse(JsonRequest.java:65)
                                                              at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run(ExecutorDelivery.java:99)
                                                              at android.os.Handler.handleCallback(Handler.java:725)
                                                              at android.os.Handler.dispatchMessage(Handler.java:92)
                                                              at android.os.Looper.loop(Looper.java:137)
                                                              at android.app.ActivityThread.main(ActivityThread.java:5041)
                                                              at java.lang.reflect.Method.invokeNative(Native Method)
                                                              at java.lang.reflect.Method.invoke(Method.java:511)
                                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
                                                              at dalvik.system.NativeStart.main(Native Method)

Answer 1

overviewListItemFragment我假设这不是添加的片段实例，而是类。

我建议进行以下更改：从setdata和您的textview中删除static，如果您确实知道它是如何工作的，请保留它。我认为这没有必要，也不值得推荐。

private OverviewListItemFragment mFrag; //declare globally

mFrag = new OverviewListItemFragment(); 
//if you do not want to add it now, ignore the following line
getSupportFragmentManager().beginTransaction().add(R.id.yourContainer, mFrag, "mFrag").commit();

现在，每次要设置数据时，调用mfrag.setdata。检查mfrag是否为null，然后重新初始化，可能重新添加，或者执行任何您想要执行的操作。

编辑：既然我知道您使用的是viewpager，我建议您执行以下操作：

执行上述操作。我认为在这个context中使用静态方法是不可取的。由于试图访问片段中的TextView,因此会出现错误。这是在ViewPager/PagerAdapter中初始化的，PagerAdapter保存对片段的已用实例的引用。您可以通过以下方式访问已使用的片段

 Fragment mFragment = pagerAdapter.getFragment(0); //frag at position 0

通过一些选角，您将能够找到您的（现在不是静态的）方法：

((OverviewListItemFragment)pagerAdapter.getFragment(0)).setData(YOUR_DATA);

请添加一些try/catch。检查您的片段是否为null，因为您的片段有可能在FragmentPagerAdapter中被回收，因为它达到了偏移量。实现这一点的另一种方法是存储所需的数据，并在每次片段可见时更新它，如本文所述。

编辑2：显然，您需要在适配器中进行一些更改：

我建议在构造html" target="_blank">函数中创建一个包含片段的数组：

//global in your adapter:
private Fragment[] fragments;

public CustomPagerAdapter(FragmentManager fm) {
        super(fm);
        fragments = new GameFragment[4];
        fragments[0] = new MyFragment();
        fragments[1] = new SecondFragment();
        ....
    }
    public Fragment getItem(int position) {
       return fragments[position];
    }

    public Fragment getFragment(int position) {
       return fragments[position];
    }