Livedata观察到未运行

Question

我尝试使用这个实现ViewModel。但观察者从未被调用。

基本上，这个应用程序在SAMPLE_URL上发出网络请求，将JSON转换为列表，并通过BookView显示列表。这个应用程序在没有ViewModel的情况下工作得很好。使用ViewModel，应用程序会运行，但Observer从未被调用，也没有显示任何数据。

我在这里做错了什么？

BookActivity类：

 public BookAdapter bookAdapter;
    ListView bookView;
    public final static String SAMPLE_URL = "https://www.googleapis.com/books/v1/volumes?q=search+terms";
    public ArrayList<Book> books = new ArrayList<>();
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activitymain);
        bookView = findViewById(R.id.list);
        bookAdapter = new BookAdapter(BookActivity.this,books);
        BookViewModel bookViewModel = new ViewModelProvider(this).get(BookViewModel.class);
        bookViewModel.getBooks().observe(this, books -> {
            Log.d("INSIDE", "observe");
            bookAdapter = new BookAdapter(this,books);
            bookView.setAdapter(bookAdapter);
            bookAdapter.notifyDataSetChanged();
        });
    }

BookViewModel类：

public class BookViewModel extends ViewModel {
    public MutableLiveData<List<Book>> books;
    public LiveData<List<Book>> getBooks(){
        if (books == null) {
            books = new MutableLiveData<>();
            loadBooks();
        }
        return books;
    }
    private void loadBooks() {
        thread.start();
    }
    Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
            URL url = QueryUtility.createURL(BookActivity.SAMPLE_URL);
            try{
                assert url != null;
                String JSONResponse = QueryUtility.ReadFromStream(QueryUtility.MakeHTTPRequest(url));
                books = new MutableLiveData<>(QueryUtility.extractBooksFromJSON(JSONResponse));
            }
            catch (IOException | JSONException ioException){
                ioException.printStackTrace();
            }
        }
    });
}

Answer 1

线程为books创建了一个新的MutableLiveData实例。您可能观察到的原始懒散加载实例从未更新。您应该更新现有实例的值。

books.postValue(QueryUtility.extractBooksFromJSON(JSONResponse));