Question

在我的应用程序中，我尝试将书籍与MBaaS Backendless同步。因此，我从Backendless获取它们之后生成一个List以保存所有书籍。我从服务器获得了正确的数据（没有重复），但我的for循环开始得太早，所以没有数据可以迭代。在开始for循环之前，如何轻松完成List的填充？这是我的代码：

final List<String> all_books_without_duplicates = new ArrayList<String>();
// queryId = Id of the user
Backendless.Data.of(Books.class).find(queryId, newLoadingCallback<BackendlessCollection<Books>>(this, getString(R.string.loading_books), true) {
@Override
public void handleResponse(BackendlessCollection<Books> booksBackendlessCollection) {
    Iterator<Books> booksIterator = booksBackendlessCollection.getCurrentPage().iterator();
    while (booksIterator.hasNext()) {
    Books booksonline = booksIterator.next();
    final String book_title = booksonline.getBookTitle();
    // avoid duplictes
    if (!all_books_without_duplicates.contains(book_title)) {
    all_books_without_duplicates.add(book_title);
    Log.d("added book title:", book_title);
    }}}});
    Log.d("for-loop elements: ", String.valueOf(all_books_without_duplicates));
    for (int x=0; x<all_books_without_duplicates.size(); x++){
    final String book_title_value = all_books_without_duplicates.get(x);
    String whereClause = "booktitle LIKE '%" + book_title_value + "%'";
    QueryOptions queryOptions = new QueryOptions();
    queryOptions.setRelated(Arrays.asList("book"));
    BackendlessDataQuery query = new BackendlessDataQuery(queryOptions);
    query.setWhereClause(whereClause);
    // get all book-ids from Backendless where title is x
    Backendless.Data.of(BookIds.class).find(query, new LoadingCallback<BackendlessCollection<BookIds>>(MainActivity.this, getString(R.string.loading_books), true) {
    @Override
    public void handleResponse(BackendlessCollection<BookIds> bookIDsBackendlessCollection) {
    super.handleResponse(bookIDsBackendlessCollection);
    Iterator<BookIds> bookIDsSyncIterator = bookIDsBackendlessCollection.getCurrentPage().iterator();
    while (bookIDsSyncIterator.hasNext()){
    BookIds book_ids = bookIDsSyncIterator.next();
    String book_id_be = book_ids.getObjectId();
    // get all book-ids from SQLite-DB
    DatabaseHelper db=new DatabaseHelper(getApplicationContext());
    final List<String>bookIds_sql = db.getAllBookIds();
    // save new book in SQLite if it lacked so far
    if(bookIds_sql.contains(book_id_be)){
    // book already exists
    }
    else{
    saveNewBookFromBackendless(book_id_be);
    }
    }}});}

Answer 1

正在发生的是您的代码正在执行find方法，该方法异步执行后端调用（在不同的线程中），然后它直接进入Log.d和{ {1}}循环指令，只有当后端响应准备就绪时，它才会调用for方法。

解决此问题的最简单方法是在handleResponse循环之后将for循环移到handleResponse内;取决于您希望如何构建代码，还有其他方法可以通过在方法中编写for循环并从while调用此方法来实现，并且还有一些方法可以选择执行此代码的线程。但问题中没有足够的信息可以为您提供适当的建议。

同步时任务的顺序错误

1 个答案: