AsyncCallback 的优先级

Question

我的应用程序还有另一个问题。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;
}

以及 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"，第二个 "getAllContacts" 包含来自服务器。因此，来自 loadContactData() 的 return contacts 语句也是空的。

我怎样才能让 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是运行的时候，contacts_name还是空的，因为你的代码在handleResponse()还没有运行因为新的线程还在等待响应数据库。

对您的数据库的调用很慢，因为它可能包含文件甚至网络 IO。你不应该在处理视图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（并绘制微调动画）的主线程正忙于等待数据库。通过使数据库请求异步，您的应用程序将保持响应，并且 UI 不会获得 "stuck".

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();

    }
}

Answer 2

此处最好的选择是将侦听器接口附加到 ContactAdapter 并使 MainActivity 实现附加的接口。所以当 ContactAdapter 完成获取数据时，它会通知监听器 class 然后你可以从主 activity onRecieve 回调中调用 loadContactData()。

public void getAllContacts(MyListener listener){
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());}});
listener.done(contacts_name);

}

界面可能是这样的

public interface MyListener {
   void done(List<String> contactList);
}

然后 MainActivity 应该实现 MyListener 接口，在 done 方法中你可以在那里设置微调器。

我相信 loadContact() 可以重构为

private void loadContactData() {
ContactAdapter db = new ContactAdapter(getApplicationContext());
// Spinner Drop down elements
db.getAllContacts(this);        

}

请注意，您的 MainActivity 实现了 MyListener 接口。即

class MainActivity ... implements MyListener {
  @Override
  public void done(List<String> contacts) {
      // 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));

  }

这应该会让您有所了解，我想您应该能够从这里找到自己的路。也尝试重构代码。

AsyncCallback 的优先级

Priority on AsyncCallback

android

thread-priority

backendless