ios - 解析查询结果未附加到新数组

Question

我正在使用 Swift，并且尝试将解析查询的结果附加到数组中，但是，该数组仅在查询的 for 循环中打印时才显示附加值。当我尝试在方法返回之前在循环外部打印数组时，我得到一个空数组。如何获取带有附加值的数组以在 for 循环之外显示？

class Task {   

    func all() -> Array<String> {
        var taskArray = Array<String>()
        var query = PFQuery(className:"Task")
        var currentUser = PFUser.currentUser() // this will now be nil
        let userId = currentUser?.objectId
        query.whereKey("userId", equalTo: currentUser!)
        query.findObjectsInBackgroundWithBlock {
            (objects: [AnyObject]?, error: NSError?) -> Void in

            if error == nil {
                // The find succeeded.
                println("Successfully retrieved \(objects!.count) tasks.")

                // Do something with the found objects
                if let objects = objects as? [PFObject] {
                    for object in objects {
//                        println(object.objectId)
                        let title = object["title"]! as! String
                        taskArray.append(title)
                        println("Array: \(taskArray)")
                    }
                }
            } else {
                // Log details of the failure
                println("Error: \(error!) \(error!.userInfo!)")
            }
        }
        println("Outside query array: \(taskArray)")


        return taskArray
    }
}

看起来外部查询是在查询之前被调用的，或者类似的东西。调试控制台:

Answer 1

正如您所说， println("Outside query array:\(taskArray)") 在 println("Array:\(taskArray)") 之前调用，并且对此有一个简单的解释。

findObjectsInBackgroundWithBlock 是一种异步方法，这意味着它启动另一个线程并在该线程上工作。这意味着当您调用 findObjectsInBackgroundWithBlock 时，它会直接继续到 println("Outside query array:\(taskArray)")。

<小时/>

现在，可以从 Parse 同步下载内容(因此它不会直接跳到 println("Outside query array:\(taskArray)"))，但我不会推荐您去做这个。

相反，我建议您实现一个在下载完成时调用的闭包。类似于以下内容(PS，它是用 Swift 2 编写的):

//
//  ViewController.swift
//  ParseFun
//
//  Created by Stefan Veis Pennerup on 21/09/15.
//  Copyright (c) 2015 Kumuluzz. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

    // MARK: - Lifecycle methods

    override func viewDidLoad() {
        super.viewDidLoad()

        all { (tasks) -> Void in
            print("The downloaded tasks are: \(tasks)")
        }
    }

    // MARK: - Download methods

    func all(completionHandler: ([String])->Void) {
        // Creates the query
        var taskArray = Array<String>()
        let query = PFQuery(className:"Task")
        let currentUser = PFUser.currentUser() // this will now be nil
        query.whereKey("userId", equalTo: currentUser!)

        // Starts the download
        query.findObjectsInBackgroundWithBlock {
            (objects: [AnyObject]?, error: NSError?) -> Void in

            if error == nil {
                // The find succeeded.
                print("Successfully retrieved \(objects!.count) tasks.")

                // Do something with the found objects
                if let objects = objects as? [PFObject] {
                    for object in objects {
                        let title = object["title"]! as! String
                        taskArray.append(title)
                        print("Array: \(taskArray)")
                    }
                }

                completionHandler(taskArray)

            } else {
                // Log details of the failure
                print("Error: \(error!) \(error!.userInfo)")
            }
        }
    }
}