android – 为什么片段中的getContext()有时会返回null？

为什么getContext()有时会返回null？我将上下文传递给LastNewsRVAdapter.java作为参数.但LayoutInflater.from(context)有时会崩溃.我在游戏机上收到一些崩溃报告.以下是崩溃报告.

java.lang.NullPointerException
com.example.adapters.LastNewsRVAdapter.<init>

java.lang.NullPointerException: 
at android.view.LayoutInflater.from (LayoutInflater.java:211)
at com.example.adapters.LastNewsRVAdapter.<init> (LastNewsRVAdapter.java)
at com.example.fragments.MainFragment$2.onFailure (MainFragment.java)
or                     .onResponse (MainFragment.java)
at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run 
(ExecutorCallAdapterFactory.java)
at android.os.Handler.handleCallback (Handler.java:808)
at android.os.Handler.dispatchMessage (Handler.java:103)
at android.os.Looper.loop (Looper.java:193)
at android.app.ActivityThread.main (ActivityThread.java:5299)
at java.lang.reflect.Method.invokeNative (Method.java)
at java.lang.reflect.Method.invoke (Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run 
(ZygoteInit.java:825)
at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:641)
at dalvik.system.NativeStart.main (NativeStart.java)

这是LastNewsRVAdapter.java构造函数.

public LastNewsRVAdapter(Context context, List<LatestNewsData> 
    latestNewsDataList, FirstPageSideBanner sideBanner) {
    this.context = context;
    this.latestNewsDataList = latestNewsDataList;
    inflater = LayoutInflater.from(context);
    this.sideBanner = sideBanner;
}

这是Fragment里面的onCreateView代码

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment

    final View view = inflater.inflate(R.layout.fragment_main_sonku_kabar, container, false);
    tvSonkuKabar = view.findViewById(R.id.textview_sonku_kabar_main);
    tvNegizgiKabar = view.findViewById(R.id.textview_negizgi_kabar_main);
    refresh(view);

    final SwipeRefreshLayout swipeRefreshLayout = (SwipeRefreshLayout) view.findViewById(R.id.mainRefreshSonkuKabar);
    swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
        @Override
        public void onRefresh() {
            refresh(view);
            swipeRefreshLayout.setRefreshing(false);
        }
    });
    setHasOptionsMenu(true);
    return view;
}

这是Fragment中的刷新方法

private void refresh(final View view) {
    sideBanner = new FirstPageSideBanner();

    final RecyclerView rvLatestNews = (RecyclerView) view.findViewById(R.id.recViewLastNews);
    rvLatestNews.setLayoutManager(new LinearLayoutManager(getContext(), LinearLayoutManager.VERTICAL, false));
    rvLatestNews.setNestedScrollingEnabled(false);
    App.getApiService().getLatestNews().enqueue(new Callback<LatestNews>() {
        @Override
        public void onResponse(Call<LatestNews> call, Response<LatestNews> response) {
            if (response.isSuccessful() && response.body().isSuccessfull()){
                adapter = new LastNewsRVAdapter(getContext(), response.body().getData(), sideBanner);
                rvLatestNews.setAdapter(adapter);
                tvSonkuKabar.setVisibility(View.VISIBLE);
            }
        }

        @Override
        public void onFailure(Call<LatestNews> call, Throwable t) {

        }
    });

解决方法:

首先,正如您在此link上看到的那样,片段生命周期内的onCreateView()方法位于onAttach()之后,因此您应该已经有了一个上下文.您可能想知道,为什么getContext()会返回null呢？问题出在您创建适配器的位置：

App.getApiService().getLatestNews().enqueue(new Callback<LatestNews>() {
        @Override
        public void onResponse(Call<LatestNews> call, Response<LatestNews> response) {
            if (response.isSuccessful() && response.body().isSuccessfull()){
                adapter = new LastNewsRVAdapter(getContext(), response.body().getData(), sideBanner);
                rvLatestNews.setAdapter(adapter);
                tvSonkuKabar.setVisibility(View.VISIBLE);
            }
        }

        @Override
        public void onFailure(Call<LatestNews> call, Throwable t) {

        }
    });

虽然您在onCreateView()中指定了一个回调,但这并不意味着该回调中的代码将在该点运行.它将在网络调用完成后运行并创建适配器.考虑到这一点,您可以在片段的生命周期中的该点之后运行回调.我的意思是用户可以进入该屏幕(片段)并转到其他片段或在网络请求完成之前返回到前一个片段(并且回调运行).如果发生这种情况,那么如果用户离开片段,则getContext()可以返回null(可能已经调用了onDetach()).

此外,如果活动在网络请求完成之前被销毁,您也可能会发生内存泄漏.所以你有两个问题.

我解决这些问题的解决方案是：

>为了避免空指针异常和内存泄漏,您应该在调用片段内的onDestroyView()时取消网络请求(retrofit返回一个可以取消请求的对象：link).
>另一个阻止空指针异常的选项是在回调之外移动适配器LastNewsRVAdapter的创建,并在片段中保留对它的引用.然后,在回调中使用该引用来更新适配器的内容：link

标签：android,android-fragments,fragment-lifecycle
来源： https://codeday.me/bug/20190926/1822270.html