解析：检索的最后一个对象

Question

我试着通过递减场“createdAt”检索选择顺序的最后一个对象： 在SQL是这样的：

SELECT * 
FROM Message 
ORDER BY createdAt DESC 
LIMIT 1 

我试试这个，但不起作用。其结果是第一个对象：

let query = PFQuery(className: "Message") 

query.orderByDescending("createdAt") 
query.limit = 1 

var lastMessage = "..." 

if let objects = query.findObjects() as [AnyObject]? 
{ 
    for object in objects 
    { 
     //Execute one time only and don't return the row that i want 
     lastMessage = object["Text"] as! String 
    } 
} 

有了这个懒惰的代码，我得到我想要的东西，但我有循环的所有对象：

let query = PFQuery(className: "Message") 

var lastMessage = "..." 

if let objects = query.findObjects() as [AnyObject]? 
{ 
    for object in objects 
    { 
     //Execute many times and return the row that i want int the last 
     lastMessage = object["Text"] as! Strin 
    } 
} 

Answer 1

var lastMessage = "..."   

let query = PFQuery(className: "Message", predicate: predicate) 

query.orderByDescending("createdAt") 

if let object = query.getFirstObject() as PFObject? 
{ 
    lastMessage = object["Text"] as! String 
} 

Answer 2

避免使用同步调用，如findObjects()不惜一切代价。相反，尝试使用findObjectsInBackgroundWithBlock

let query = PFQuery(className: "Message") 
query.orderByDescending("createdAt") 
query.limit = 1 
query.findObjectsInBackgroundWithBlock { (objects: [AnyObject]?, error: NSError?) -> Void in 
    if (error == nil) { 
     if let firstObject = objects?.first { 
      lastMessage = firstObject["Text"] as! String 
     } 
    } else {   
     // Log details of the failure 
     println("query error: \(error) \(error!.userInfo!)") 
    } 
}