java - 尝试从 Activity 接收对象时 fragment 崩溃

Question

大家好，

我遇到一个问题，我的 Activity 正在进行网络调用，当网络调用完成时，它会使用从调用中收到的 JSON 对象中的数据对 Activity 进行一些更改，然后将该对象传递给同一 Activity 中的 fragment 。这些 fragment 位于 TabLayout 中。

我遇到的问题是，如果我放置 System.out.println() 它会打印出正确的数据。当我想用我在 fragment 中收到的数据设置一个 TextView 时，应用程序会因空指针而崩溃。当我使用 Android studio 中的“调试”对其进行调试时，由于某种原因，我设置的 TextView 始终为 null。

执行初始网络调用的 Activity 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 之间的接口(interface):

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);
  }

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:抱歉我忘记了 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 View 在接收数据时不是“Activity ”(如果这是正确的词)，因为当我在方法上使用断点运行调试器时，接收Fragment中的数据，mOverViewHeading TextView id为空，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 放在响应成功时但在尝试在 TextView 上设置 setText() 之前调用的相同方法中。

谢谢

Answer 1

您无法在 fragment onCreate() 中调用 findViewById。那个时候还没有UI。尝试在 onCreateView() 中调用 findViewById。另外，您必须在 fragment 内的 View 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;
}