android - AsyncCallback 的优先级

Question

我的申请中还有另一个问题。我在 MainActivity 中使用以下代码从我的 ContactAdapter 中获取用户数据:

private List<String> loadContactData() {
    ContactAdapter db = new ContactAdapter(getApplicationContext());
    // Spinner Drop down elements
    List<String> contacts = db.getAllContacts();        
    // Creating adapter for spinner
    ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_spinner_item, contacts);
    // Drop down layout style - list view with radio button   
 dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);        
    Log.d("SPINNER", String.valueOf(contacts));
    return contacts;
}

和 ContactAdapter:

public List<String> getAllContacts(){
    final List<String> contacts_id = new ArrayList<String>();
    final List<String> contacts_name = new ArrayList<String>();
    Backendless.Data.of(BackendlessUser.class).find(new AsyncCallback<BackendlessCollection<BackendlessUser>>() {
        public void handleResponse(BackendlessCollection<BackendlessUser> users) {
            Iterator<BackendlessUser> userIterator = users.getCurrentPage().iterator();
            while (userIterator.hasNext()) {
                BackendlessUser user = userIterator.next();
                String user_mail = user.getEmail();
                String user_id = user.getUserId();
                contacts_name.add(user_mail);
                contacts_id.add(user_id);
            }
            Log.d("getAllContacts: ", String.valueOf(contacts_name)); }
        public void handleFault(BackendlessFault backendlessFault) {
            System.out.println("Server reported an error - " + backendlessFault.getMessage());}});
    return contacts_name;
}

当然，我的问题是 Backendless 查询速度很慢...所以在我的 Logcat 中，我首先看到一个空的“SPINNER”和第二个“getAllContacts”，其中包含来自服务器的用户数据。结果，来自 loadContactData() 的 return contacts 语句也为空。

如何实现 getAllContacts() 先完成以获取 loadContactData() 中的用户数据？

Answer 1

您的问题是 find 调用是异步的(这很好，意味着 handleResponse() 是在新线程上调用的)。所以当你的 return contacts_name 运行时， contacts_name 仍然是空的，因为你在 handleResponse() 中的代码还没有运行，因为新线程还在等待数据库的响应。

对数据库的调用很慢，因为它可能包含文件甚至网络 IO。永远不要在处理 View 的主线程上执行此操作，否则应用程序将不再响应，因为更新 UI(和绘制微调动画)的主线程正忙于等待数据库。通过使数据库请求异步，您的应用程序可以保持响应，并且 UI 不会“卡住”。

为了简单起见，您可以这样做:

public class MyActivity extends AppCompatActivity {

   private List<String> mContactIds = new ArrayList<>();

    private Spinner mSpinner;

    private ProgressDialog mProgressDialog;

    @Overrride
    public void onCreate() {

        this.setContentView(...);
        this.mSpinner = ...;

        this.showLoadingDialog();

        Backendless.Data.of(BackendlessUser.class).find(new AsyncCallback<BackendlessCollection<BackendlessUser>>() {
            public void handleResponse(BackendlessCollection<BackendlessUser> users) {

                List<String> mails = new ArrayList<>();
                List<String> ids = new ArrayList<>();
                Iterator<BackendlessUser> userIterator = users.getCurrentPage().iterator();
                while (userIterator.hasNext()) {
                    BackendlessUser user = userIterator.next();
                    String mail = user.getEmail();
                    String id = user.getUserId();
                    mails.add(user_mail);
                    ids.add(user_id);
                }

                Log.d("getAllContacts: ", String.valueOf(contacts_name)); 
                applyData(mails, ids);

            } 

            public void handleFault(BackendlessFault backendlessFault) {
                Log.e("error: ", "Something went wrong");

            }
        });
    }

    private void applyData(List<String> mails, List<String> ids) {
       Log.d("getAllContacts: ", String.valueOf(contacts_name)); 
       this.hideLoadingDialog();
       this.mContactIds = ids;
       this.mSpinner.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, mails);

    }

    private void showLoadingDialog() {
        this.mLoadingDialog = new ProgressDialog(this, null, "Loading contacts...", true);
        this.mLoadingDialog.dismiss();

    }

    private void hideLoadingDialog() {
        if(this.mLoadingDialog != null) {
            this.mLoadingDialog().dismiss();

        }
    }

    private void showErrorDialog() {
        this.hideLoadingDialog();
        new AlertDialog.Builder(this)
            .setMessage("Something went wrong, try again")
            .setPositiveButton("Ok", null)
            .show();

    }
}