Question

我正在学习带数据绑定的MVVM，并且在TaskViewModel中存在空对象的问题。我创建了两个构造函数，其中一个为空，但是Task对象中为null。当我在viewmodel中只有一个构造函数时，viewmodel实例出错。我做错了什么？如果您需要更多代码，请告诉我。

TaskViewModel

public class TaskViewModel extends ViewModel {

private TaskRepository mTaskRepository;
private LiveData<List<Task>> taskMutableLiveData;
private String TAG = "TaskViewModel = ";
private Context context;

public final ObservableField<String> description = new ObservableField<>();
public final ObservableField<String> date = new ObservableField<>();
public final ObservableField<String> time = new ObservableField<>();


public TaskViewModel(TaskRepository taskRepository) {
  mTaskRepository = taskRepository;
}

public TaskViewModel() {
}

public void insert() {
    addTask(description.get(), date.get(), time.get());
    Log.d(TAG, "DATA " + description + " / " + date + " / " + time);
}

public void addTask(String description, String date, String time) {
    Task addTask = new Task(description, date, time);
    mTaskRepository.insert(addTask);
}
}

TaskRepository

public class TaskRepository {

private TaskDao taskDao;
private LiveData<List<Task>> allTasks;


public TaskRepository(Application application) {
    TaskDatabase database = TaskDatabase.getInstance(application);
    taskDao = database.taskDao();
    allTasks = taskDao.getAllTasks();
}

public void insert(Task task) {
    new InsertTaskAsyncTask(taskDao).execute(task);
}

public void update(Task task) {
    new UpdateTaskAsyncTask(taskDao).execute(task);
}

public void delete(Task task) {
    new DeleteTaskAsyncTask(taskDao).execute(task);
}

public LiveData<List<Task>> getAllTasks() {
    return allTasks;
}


private static class InsertTaskAsyncTask extends AsyncTask<Task, Void, Void> 
{
    private TaskDao taskDao;

    private InsertTaskAsyncTask(TaskDao taskDao) {
        this.taskDao = taskDao;
    }

    @Override
    protected Void doInBackground(Task... tasks) {
        taskDao.insert(tasks[0]);
        return null;
    }
}

private static class DeleteTaskAsyncTask extends AsyncTask<Task, Void, Void> 
{
    private TaskDao taskDao;

    private DeleteTaskAsyncTask(TaskDao taskDao) {
        this.taskDao = taskDao;
    }

    @Override
    protected Void doInBackground(Task... tasks) {
        taskDao.delete(tasks[0]);
        return null;
    }
}

private static class UpdateTaskAsyncTask extends AsyncTask<Task, Void, Void> 
{
    private TaskDao taskDao;

    private UpdateTaskAsyncTask(TaskDao taskDao) {
        this.taskDao = taskDao;
    }

    @Override
    protected Void doInBackground(Task... tasks) {
        taskDao.update(tasks[0]);
        return null;
    }
}

}

Answer 1

您无需在ViewModel中使用参数化构造函数，而是在默认构造函数中进行初始化。因为毕竟我们要消耗ViewModel中的ViewModelProviders，{em}会从中调用默认构造函数（如果尚未自定义）。

所以请像下面这样

public class TaskViewModel extends ViewModel {

    private TaskRepository mTaskRepository;
    ...

    public TaskViewModel() {
        mTaskRepository = new TaskRepository(); // This will avoid your null pointer exception
    }

    public void insert() {
        addTask(description.get(), date.get(), time.get());
        Log.d(TAG, "DATA " + description + " / " + date + " / " + time);
    }

    public void addTask(String description, String date, String time) {
        Task addTask = new Task(description, date, time);
        mTaskRepository.insert(addTask);
    }
}

如果您想在Context中包含ViewModel，请改用AndroidViewModel，它将在默认构造函数实现中为您提供应用程序上下文。但不要将其存储在您的ViewModel中。

视图模型中的空对象

1 个答案: