ios - 如何计算 parse.com 类中的对象以获得从中提取随机数的总数？

Question

当我尝试计算我的 parse.com 类的对象时出现错误。我需要计算对象的总数，这样我就可以得到一个随机数并使用它来查询一个对象。我的数据表中有一个名为 Qnum 的列，它为每个对象编号。这是我的功能:

func CallData() {

        var query = PFQuery(className: "QuestionsandAnswers")
        let randomNumber = arc4random_uniform(count)
        var randomNumberCast = Int(randomNumber)
        query.whereKey("Qnum", equalTo: randomNumberCast)
        query.getFirstObjectInBackgroundWithBlock { (object: PFObject!, error: NSError!) -> Void in
            if (error == nil) {
                self.Question = object  ["Question"] as String!
                self.Answers = object  ["Answers"] as Array!
                self.Answer = object  ["Answer"] as String!

                if (self.Answers.count > 0) {
                    self.QuestionLabel.text = self.Question

                    self.Button1.setTitle(self.Answers[0], forState: UIControlState.Normal)
                    self.Button2.setTitle(self.Answers[1], forState: UIControlState.Normal)
                    self.Button3.setTitle(self.Answers[2], forState: UIControlState.Normal)
                    self.Button4.setTitle(self.Answers[3], forState: UIControlState.Normal)

                }


            } else {

                NSLog("Something is wrong, dude.  Sorry.")
            }   
        }        
    }

我的错误出现在函数“let randomNumber = arc4random_uniform(count)”的第二行，我试过 (objects.count) 无济于事；然而，当我只插入对象总数时，它起作用了，(20) 但我想避免对该数字进行硬编码，以防我需要向我的类(class)添加更多对象。对这个新手有什么建议吗？

解决方案: danh 在下面提供了答案，但由于我只需要更改一些东西，而且我不知道如何在评论中发布修改后的代码，我正在编辑这个已回答的问题现在包括解决方案。这是有效的代码:

func CallData() {
    var countQuery = PFQuery(className: "QuestionsandAnswers")
    countQuery.countObjectsInBackgroundWithBlock { (count: Int32, error: NSError!) -> Void in
        if (error == nil) {
            let randomNumber = Int(arc4random_uniform(UInt32(count)))
            var query = PFQuery(className: "QuestionsandAnswers")
            query.skip = randomNumber
            query.limit = 1
            query.findObjectsInBackgroundWithBlock { (objects: [AnyObject]!, error: NSError!) -> Void in
                if (error == nil) {
                    var object: AnyObject = objects[0]
                    self.Question = object  ["Question"] as String!
                    self.Answers = object  ["Answers"] as Array!
                    self.Answer = object  ["Answer"] as String!

                    if (self.Answers.count > 0) {
                        self.QuestionLabel.text = self.Question
                        self.Button1.setTitle(self.Answers[0], forState: UIControlState.Normal)
                        self.Button2.setTitle(self.Answers[1], forState: UIControlState.Normal)
                        self.Button3.setTitle(self.Answers[2], forState: UIControlState.Normal)
                        self.Button4.setTitle(self.Answers[3], forState: UIControlState.Normal)
                    }
                } else {
                    NSLog("Something is wrong with the find request, dude.  Sorry. %@", error)
                }
            }
        } else {
            NSLog("Something is wrong with the count request, dude.  Sorry. %@", error)
        }   
    }
}

Answer 1

只有知道计数才能使用，知道需要调用计数 api。一旦你有了计数，你就可以用它来绑定(bind)一个随机数。

random 可用于基于“Qnum”字段的查找，或用作不合格查询的跳过。跳过不太容易出错，因为它仅取决于计数。

不是说得很快，我尽力拼凑出这条建议的快速形式。以下内容在语法上可能有点错误，但我很确定它的基本思想是正确的...

    var countQuery = PFQuery(className: "QuestionsandAnswers")
    countQuery.countObjectsInBackgroundWithBlock { (count: Int, error: NSError!) -> Void in
        if (error == nil) {
            let randomNumber = Int(arc4random_uniform(UInt32(count))) 
            var query = PFQuery(className: "QuestionsandAnswers")
            query.skip = randomNumber
            query.limit = 1
            query.findObjectsInBackgroundWithBlock { (objects: [AnyObject]!, error: NSError!) -> Void in
                if (error == nil) {
                    var object: AnyObject = objects[0]
                    self.Question = object  ["Question"] as String!
                    self.Answers = object  ["Answers"] as Array!
                    self.Answer = object  ["Answer"] as String!

                    if (self.Answers.count > 0) {
                        self.QuestionLabel.text = self.Question
                        self.Button1.setTitle(self.Answers[0], forState: UIControlState.Normal)
                        self.Button2.setTitle(self.Answers[1], forState: UIControlState.Normal)
                        self.Button3.setTitle(self.Answers[2], forState: UIControlState.Normal)
                        self.Button4.setTitle(self.Answers[3], forState: UIControlState.Normal)
                    } 
                } else {
                    NSLog("Something is wrong with the find request, dude.  Sorry. %@", error)
                }
            }
        } else {
            NSLog("Something is wrong with the count request, dude.  Sorry. %@", error)
        }   
    }