[切换片段后Android应用程序崩溃

Question

当我切换片段时，我的应用程序将崩溃。我的聊天主要片段。当我使用bottomNavigationBar切换片段并返回主聊天片段时，我的应用将崩溃

    Process: com.example.uwebeta1, PID: 11210
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.app.Application android.app.Activity.getApplication()' on a null object reference
        at androidx.lifecycle.ViewModelProviders.checkApplication(ViewModelProviders.java:42)
        at androidx.lifecycle.ViewModelProviders.of(ViewModelProviders.java:122)
        at androidx.lifecycle.ViewModelProviders.of(ViewModelProviders.java:85)
        at com.example.uwebeta1.Adapter.ChatAdapter.<init>(ChatAdapter.java:68)
        at com.example.uwebeta1.Views.Fragments.MainActivityFragments.MainMasterFragment$9.onSuccessCallback(MainMasterFragment.java:366)
        at com.example.uwebeta1.Views.Fragments.MainActivityFragments.MainMasterFragment$9.onSuccessCallback(MainMasterFragment.java:362)
        at com.example.uwebeta1.Repository.ChatsRepository$1.onDataChange(ChatsRepository.java:79)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@16.0.5:75)
        at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@16.0.5:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@16.0.5:55)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7073)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)
I/Process: Sending signal. PID: 11210 SIG: 9
Process 11210 terminated.

这是我的适配器类

  public ChatAdapter(Context mContext, List<ChatModel> mChat, String activityName, String city) {
        this.mChat = mChat;
        this.mContext = mContext;
        this.activityName = activityName;
        this.city = city;

        userViewModel = ViewModelProviders.of((FragmentActivity) mContext).get(UserViewModel.class);
        rootRef = FirebaseDatabase.getInstance().getReference().child("Users");
    }  

这是我的方法，我正在获取适配器类的数据

chatsViewModel.getMainChatByRoom(cityName, activityName, new MyFirebaseCallBack<List<ChatModel>>() {
                        @Override
                        public void onSuccessCallback(List<ChatModel> object) {

                            chatAdapter = new ChatAdapter(getContext(), object, activityName, cityName);
                            recycler_view.setAdapter(chatAdapter);
                        }

                        @Override
                        public void onFailureCallback(String message) {

                        }
                    });

Answer 1

好像您通过匿名Fragment将对MyFirebaseCallBack的引用传递到存储库中。如果存储库中的onDataChange()被触发，则会调用MyFirebaseCallback.onSuccessCallback()。由于执行以下行的Fragment实例

chatAdapter = new ChatAdapter(getContext(), object, activityName, cityName);

不再附加到任何Activity，它的getContext()返回null。

[通过（反）模式将回调传递到ViewModel方法中，由您决定getContext()是否返回null。如果是这样，则您不应修改与View相关的任何内容。另请注意，分离的Fragment实例可能永远不会再附加到Activity。

相反，您可以利用ViewModel具有生命周期感知这一事实。让getMainChatByRoom(cityName, activityName)返回LiveData<List<ChatModel>>并在Observer<List<ChatModel>>中注册Fragment.onActivityCreated()，如下所示：

chatsViewModel.getMainChatByRoom(cityName, activityName).observe(getViewLifecycleOwner(), new Observer<List<ChatModel>>(){
    @Override
    onChange(@Nullable List<ChatModel> data){
        chatAdapter = new ChatAdapter(getContext(), object, activityName, cityName);
        recycler_view.setAdapter(chatAdapter);
    }
}

（（Android Studio可能会将其中的大部分交换为lambda表达式，但我想向您展示Observer的设置方式]

另请参见Android应用架构指南，尤其是Observe LiveData objects