为什么片段中的 getContext() 有时会返回 null? [英] Why getContext() in fragment sometimes returns null?
问题描述
为什么 getContext() 有时会返回 null?我将上下文作为参数传递给 LastNewsRVAdapter.java.但是 LayoutInflater.from(context) 有时会崩溃.我在 Play 控制台上收到了一些崩溃报告.以下是崩溃报告.
Why getContext() sometimes returns null? I pass context to LastNewsRVAdapter.java as an argument. But LayoutInflater.from(context) sometimes crashes. I'm getting a few crash reports on play console. Below is crash report.
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里面的代码onCreateView
This is the code onCreateView inside Fragment
@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refresh方法
This is refresh method inside Fragment
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) {
}
});
推荐答案
首先，正如您在此link，片段生命周期中的 onCreateView() 方法在 onAttach() 之后，因此此时您应该已经有了上下文.你可能想知道，为什么 getContext() 会返回 null 呢?问题在于您创建适配器的位置:
First of all, as you can see on this link, the method onCreateView() inside the fragment's lifecycle comes after onAttach(), so you should have already a context at that point. You may wonder, why does getContext() return null then? the problem lies on where you are creating your adapter:
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) {
}
});
尽管您在 onCreateView() 中指定了回调，但这并不意味着该回调中的代码将在该点运行.网络调用完成后，它将运行并创建适配器.考虑到这一点，您的回调可能会在片段生命周期中的那个点之后运行.我的意思是，在网络请求完成(并且回调运行)之前，用户可以进入该屏幕(片段)并转到另一个片段或返回到前一个片段.如果发生这种情况，那么如果用户离开片段(onDetach() 可能已被调用)，则 getContext() 可能返回 null.
Though you are specifying a callback in onCreateView(), that does not mean the code inside that callback will run at that point. It will run and create the adapter after the network call is done. With that in mind, your callback may run after that point in the lifecycle of the fragment. What I mean is that the user can enter that screen (fragment) and go to another fragment or return to the previous one before the network request finishes (and the callback runs). If that happens, then getContext() could return null if the user leaves the fragment (onDetach() may have been called).
此外，您也可能会出现内存泄漏，以防活动在您的网络请求完成之前被破坏.所以你有两个问题.
Besides, you can have memory leaks also, in case the activity is destroyed before your network request finishes. So you have two issues there.
我对解决这些问题的建议是:
My suggestions to solve those issues are:
为了避免空指针异常和内存泄漏，应该在调用fragment内部的onDestroyView()时取消网络请求(retrofit返回一个可以取消请求的对象:链接).
另一个防止空指针异常的选项是将适配器 LastNewsRVAdapter 的创建移到回调之外，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
Another option that will prevent the null pointer exception is to move the creation of the adapter LastNewsRVAdapter outside the callback and keep a reference to it in the fragment. Then, use that reference inside the callback to update the content of the adapter: link
这篇关于为什么片段中的 getContext() 有时会返回 null?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
