尝试从活动接收对象时发生片段崩溃

Question

大家好，

我有一个问题，我的活动正在进行网络调用，当网络调用完成时，它使用从调用接收到的 JSON 对象中的数据对活动进行一些更改，然后将对象向下传递到同一活动中的片段。这些片段位于 TabLayout 中。

我遇到的问题是，如果我放一个 System.out.println() 它会打印出正确的数据。在我想设置一个 TextView 的那一刻，我在 Fragment 中收到的数据会导致应用程序因 Nullpointer 崩溃。当我在 Android Studio 中使用 Debug 进行调试时，由于某种原因，我设置的 TextView 始终为空。

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());
}

}

Activity和Presenter的接口：

public interface ItemPresenter {

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

void doToolbarBackgroundImageCall(TradeItem aTradeItem);

}

实现 Presenter 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);
  }

Activity中处理的方法

  @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
}

接收数据的Fragment：

    public class OverviewListItemFragment extends Fragment implements OverviewView {

  private static TextView mOverViewHeading;

  public OverviewListItemFragment() {
  }

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mOverViewHeading = (TextView) getActivity().findViewById(R.id.frag_overview_heading_textview);


  }

  @Nullable
  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    return inflater.inflate(R.layout.overview_list_item_fragment, container, false);

  }

  @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)

我的想法是，我尝试设置的视图在接收数据时不是“活动”（如果这是正确的话），因为当我在方法处使用断点运行调试器时接收到 Fragment 中的数据时，mOverViewHeading TextView id 为 null，即使我在 onCreate 中有 findViewById，也尝试将其放在 onCreateView() 中，但两次都失败了。我还尝试将 findViewById 放在响应成功时调用的相同方法中，但在我尝试在 TextView 上设置 setText() 之前。

谢谢

Answer 1

您不能在片段 onCreate() 中调用 findViewById。那时还没有 UI。尝试在 onCreateView() 中调用 findViewById。此外，您必须在片段内的视图上使用 findViewById()。您不能为此使用 Activity。

像这样：

@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;
}