java - 搬运装置旋转

Question

我试图让我的应用程序处理设备旋转，但当我在 mainActivity 的 onCreate 方法上添加以下代码时，它总是崩溃。这是我收到的错误。我该如何解决这个问题？ :

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.clear()' on a null object reference

2019-09-25 06:58:16.743 10444-10444/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplication, PID: 10444
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.clear()' on a null object reference
        at com.example.myapplication.MainActivity$1.onChanged(MainActivity.java:48)
        at com.example.myapplication.MainActivity$1.onChanged(MainActivity.java:44)
        at androidx.lifecycle.LiveData.considerNotify(LiveData.java:113)
        at androidx.lifecycle.LiveData.dispatchingValue(LiveData.java:131)
        at androidx.lifecycle.LiveData.setValue(LiveData.java:289)
        at androidx.lifecycle.LiveData$1.run(LiveData.java:91)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        contactListView = (ListView) findViewById(R.id.contactsListView);
        adapter = new ArrayAdapter<Contact>(this,android.R.layout.simple_list_item_1, contacts);
        contactListView.setAdapter(adapter);
        contactListView.setOnItemClickListener(this);
        setContentView(R.layout.activity_main);
}

Answer 1

您最好将 LiveData 放置在 ViewModel 中，并在 Activity 中观察它，可能是在 onCreate() 方法中。让我向您展示如何在 ViewModel 中正确使用 LiveData，

你的ViewModel

public class YourViewModel extends ViewModel {

    // Create a LiveData with a List of Contact
    private MutableLiveData<List<Contact>> contactList = new MutableLiveData<>();

    // encapsulated with immutable live data
    public LiveData<List<Contact>> getContactList() {
        return contactList;
    }

    // call this method with a list of new contacts whenever you need to refresh your list
    public void updateContactList(List<Contact> list) {
        contactList.setValue(list);
    }

    // Rest of the ViewModel...
}

您的 Activity

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        contactListView = (ListView) findViewById(R.id.contactsListView);

        yourViewModel = ViewModelProviders.of(this).get(YourViewModel.class);

        if(contacts==null) contacts = new ArrayList<>();

        adapter = new ArrayAdapter<Contact>(this,android.R.layout.simple_list_item_1, contacts);
        contactListView.setAdapter(adapter);
        contactListView.setOnItemClickListener(this);

        yourViewModel.getContactList().observe(this, contactObserver);
}


Observer<List<Contact>> contactObserver = new Observer<List<Contact>>() {
        @Override
        public void onChanged(@Nullable List<Contact> newContacts) {
            if(adapter != null && newContacts != null) {
                adapter.clear();
                adapter.addAll(newContacts);
            }
        }
};

通过上述方式，您可以确保 liveData 与 Activity 生命周期相关联，并且它在 onChanged() 方法中获得非 null adapter 。希望我的回答能够帮助您解决您的问题。