使用回調函數從異步請求返回值

Question

我有一個返回值的HTTPRequest。我用回調函數捕獲這樣的值。

如果用戶名在數據庫中重複，代碼將執行併發出警報。然而，「返回false」不起作用，並且表單無論如何都被提交（saveNewUser）與重複的用戶名。到目前爲止，我所見過的所有例子都只是在回調中停下來，就像在我的代碼中一樣。那麼如何實現返回false在其他情況下停止執行：首先，姓氏和密碼檢查？

非常感謝。

function checkUsername(callbackUsername){ 
    var username = document.getElementById('username_id').value; 
    var ajaxRequest = getXMLHttp(); 

      ajaxRequest.onreadystatechange = function() 
      { 
      if(ajaxRequest.readyState == 4 && ajaxRequest.status==200){ 
       var response = trim(ajaxRequest.responseText); 
       callbackUsername(response); 
      } 
      }; 
     var url = "../admin/check_unique_username.php";  
     var parameters = "username="+username; 
     ajaxRequest.open("POST", url, true); 
     ajaxRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");   
     ajaxRequest.send(parameters); 
    } 

function checkNewUserForm(){ 
    if(document.getElementById('first_name_id').value === ""){ 
     alert("First Name cannot be blank!"); 
     document.getElementById('first_name_id').focus(); 
     return false; 
    } 
    if(document.getElementById('last_name_id').value === ""){ 
     alert("Last Name cannot be blank!"); 
     document.getElementById('last_name_id').focus(); 
     return false; 
    } 
    checkUsername(function(result) { 
     if(result > 0){ 
     alert("ERROR: Username already exist. Please select a different username!!"); 
     document.getElementById('username_id').focus(); 
     return false; 
     } 
     }); 
    re = /[0-9]/; 
    if(!re.test(document.getElementById('password_id').value)) { 
     alert("Error: password must contain at least one number (0-9)!"); 
     document.getElementById('password_id').focus(); 
     return false; 
    } 
    saveNewUser(first_name,last_name,username,password); 
} 

Answer 1

通過return false你只會從callback函數返回。您必須從checkNewUserForm()函數返回以停止保存新用戶。要發生這種情況，您可以使用boolean變量hasDuplicateUsername來檢查是否存在重複的用戶名條目，以便您可以在回調中將其設置爲true。在saveNewUser()之前，你應該檢查它是否是false。

function checkNewUserForm(){ 
    var hasDuplicateUsername = false; 
    if(document.getElementById('first_name_id').value === ""){ 
     alert("First Name cannot be blank!"); 
     document.getElementById('first_name_id').focus(); 
     return false; 
    } 
    if(document.getElementById('last_name_id').value === ""){ 
     alert("Last Name cannot be blank!"); 
     document.getElementById('last_name_id').focus(); 
     return false; 
    } 
    checkUsername(function(result) { 
     if(result > 0){ 
     alert("ERROR: Username already exist. Please select a different username!!"); 
     document.getElementById('username_id').focus(); 
     hasDuplicateUsername = true; 
     } 
    }); 
    re = /[0-9]/; 
    if(!re.test(document.getElementById('password_id').value)) { 
     alert("Error: password must contain at least one number (0-9)!"); 
     document.getElementById('password_id').focus(); 
     return false; 
    } 
    if(!hasDuplicateUsername) { 
     saveNewUser(first_name,last_name,username,password); 
    } 
}