Question

我收到以下错误

我正在使用Android架构组件，并试图从LiveData实例化viewmodel和Observe数据。但我坚持到这里请帮助我解决这个问题：

Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observe(android.arch.lifecycle.LifecycleOwner, android.arch.lifecycle.Observer)' on a null object reference

My Fragment是这样的：在调用getCompany（）方法时获取和NullPoint异常错误

    public class CompanyFragment extends LifecycleFragment {
    private CompanyViewModel companyViewModel;

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        companyViewModel = ViewModelProviders.of(this).get(CompanyViewModel.class);
        companyViewModel.initData();
        companyViewModel.getCompany().observe(this, new Observer<CompanyEntity>() {
            @Override
            public void onChanged(@Nullable CompanyEntity companyEntity) {

            }
        });
    }
    @Override
    public View onCreateView(LayoutInflater inflater,
                             @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_company, container, false);
    }
}

我的ViewModel类是这样的：

public class CompanyViewModel extends ViewModel {
    private LiveData<CompanyEntity> companyEntityLiveData;
    private CompanyRepository companyRepo;

    public CompanyViewModel() {}

    public CompanyViewModel(CompanyRepository companyRepo){
        this.companyRepo = companyRepo;
    }

    public void initData(){
        companyEntityLiveData = companyRepo.getCompany();
    }

    public LiveData<CompanyEntity> getCompany() {
        return companyEntityLiveData;
    }

}

错误就在这一行：

   companyViewModel.getCompany().observe(this, new Observer<CompanyEntity>() {
        @Override
        public void onChanged(@Nullable CompanyEntity companyEntity) {

        }
    });

这是我的CompanyRepository类：

public class CompanyRepository {
private WebService webService;

    public CompanyRepository(){

    }

    public LiveData<CompanyEntity> getCompany() {
        final MutableLiveData<CompanyEntity> data = new MutableLiveData<>();

        webService.getCompany().enqueue(new Callback<CompanyEntity>() {
            @Override
            public void onResponse(Call<CompanyEntity> call, Response<CompanyEntity> response) {
                data.setValue(response.body());
            }

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

            }
        });

        return data;
    }

}

Answer 1

您必须观察LiveData的相同实例。在CompanyRepository课程中，每次访问公司时都会创建一个新的LiveData实例，这意味着您的活动正在观察旧实例。

应该将getCompany方法更改为始终返回相同的LiveData实例。此外，您应该将setValue（）更改为postValue（），因为您正在尝试更新后台线程上的值。

final MutableLiveData<CompanyEntity> data = new MutableLiveData<>();
public LiveData<CompanyEntity> getCompany() {

    webService.getCompany().enqueue(new Callback<CompanyEntity>() {
        @Override
        public void onResponse(Call<CompanyEntity> call, Response<CompanyEntity> response) {
            data.postValue(response.body());
        }

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

        }
    });

    return data;
}

Answer 2

更改

public CompanyViewModel(){
    companyRepo= new companyRepo();
}

public LiveData<CompanyEntity> getCompany() {
    if(companyEntityLiveData==null)
        companyEntityLiveData= companyRepo.getSection();
    return companyEntityLiveData;
}

在您的片段中：

    CompanyViewModel companyViewModel= 
    ViewModelProviders.of(this).get(CompanyViewModel .class);
    LiveData<CompanyEntity> s = companyViewModel.getCompany();

    s.observe(this, new Observer<CompanyEntity>() {
        @Override
        public void onChanged(@Nullable CompanyEntity companyEntity) {
            Toast.makeText(getContext(),"Go go go", Toast.LENGTH_LONG).show();
        }
    });

Answer 3

在getCompany方法中，您需要检查companyEntityLiveData对象是否为null，如果是，则调用service中的服务。

public LiveData<CompanyEntity> getCompany() {
    if(companyEntityLiveData == null){
       companyEntityLiveData = companyRepo.getCompany();
    }
    return companyEntityLiveData;
}

Answer 4

问题是您正在尝试使用尚未初始化的内容。 因此，为了不依赖于执行顺序，我们必须在存储存储库返回内容的变量中建立安全的可操作性 private val nullableObj: LiveData<T2>?

有了这个，我们甚至可以在存储库加载属性之前就使用该属性。在以下示例中检查执行顺序：

val aUse: LiveData<T> = MediatorLiveData<T>().apply {
    nullableObj?.let {
    }
}
private val nullableObj: LiveData<T2>? = repository::getNullableObj

在使用存储库#getCompany（）之前回答您的问题，检查可为空性。在科特林：

companyVM.company?.observe(...) { ... }

GL

尝试在空对象引用上调用虚方法'void android.arch.lifecycle.LiveData.observe

4 个答案: