AsyncCallback優先

Question

我的應用程序中有另一個問題。我用下面的代碼在MainActivity從我ContactAdapter獲取用戶數據：

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; 
} 

而且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; 
} 

當然，我的問題是，Backendless查詢是慢......所以在我的Logcat中，我首先看到一個空的「SPINNER」，第二個「getAllContacts」與來自服務器的用戶數據。由於loadContactData（）的return contacts語句也是空的。

我該如何實現getAllContacts（）完成第一個獲取loadContactData（）中的用戶數據？

Answer 1

你的問題是find調用是異步的（這是好的，意味着在新線程上調用handleResponse()）。因此，當您的return contacts_name運行時，contacts_name仍爲空，因爲您的handleResponse()中的代碼尚未運行，因爲新線程仍在等待數據庫的響應。

對數據庫的調用很慢，因爲它可能包含文件甚至網絡IO。你不應該在與視圖一起工作的主線程上執行此操作，否則應用程序將不再響應，因爲更新UI（並繪製微調器動畫）的主線程正忙於等待數據庫。通過使數據庫請求異步，您的應用保持響應，UI不會「卡住」。

爲了簡單起見，你可以做這樣的：

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

最好的選擇，這裏將是一個監聽器接口連接到ContactAdapter，使MainActivity實現連接的接口。所以當ContactAdapter完成取數據時，它會通知監聽器類，然後你可以從主活動onRecieve回調中調用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); 

}

和接口可能看起來像這樣

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

然後MainActivity應該實現MyListener接口和實現方法，你可以設置微調有內部。

我相信loadContact()可重構爲類似於

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

} 

請注意，您MainActivity實現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)); 

    } 

這應該給你一個頭，我想你應該能夠從這裏找到你的方式。也嘗試重構代碼。